one of the three governments into which Prussian Silesia is divided, comprising the N.W. part of Upper Silesia, and that part of Upper Lusatia which now forms part of Prussia. It is bounded on the N., N.E., and N.W., by the provinces of Posen and Brandenburg, S.E. by the government of Breslau, and S.W. by Bohemia. Area 53,000 square miles. The surface is rugged and mountainous in the S., and traversed by ridges of the Riesengebirge, several of which exceed 4000 feet in height. It then slopes rapidly northward to the great plain of the Oder, which flows from S.E. to N.W. through the government, and forms part of its northern boundary. Besides the Oder, the chief rivers are its affluents, the Katzbach, Bober, and Neisse. The soil is generally sandy, and the corn produced is not sufficient for the wants of the inhabitants. Large numbers of sheep and cattle are reared. Pop. (1849) 921,002.

Life-Preservers.

Although it too frequently happens that an accident which materially endangers the life of an individual, deprives him, in the meantime, of that presence of mind which alone would enable him to take proper measures for his safety; yet to have meditated, in an interval of leisure, upon the best method of proceeding in case of emergency, must tend greatly to diminish the embarrassment and confusion that commonly accompany the accident, even if it should not be thought necessary to provide any particular apparatus for the purpose of escaping the danger. There are also many ways in which those who are not immediately involved in the disaster may contribute to the preservation of life, whether actuated by interest, or by humanity only; and the modes of relief will therefore be naturally divided into the internal and the external, whether relating to fires or to shipwrecks.

Internal Fire-Escapes.—Whenever a family establishes itself in a residence not detached from others, it becomes of importance to ascertain what facilities the house affords for ascending to the roof, and for passing to those of the neighbouring houses. It is scarcely possible that a conflagration should extend at once to the contiguous houses on each side, before the inhabitants of the house in question have had time to escape. But in a detached house, if there are not two or more staircases remote from each other, and even in a house contiguous to others, when there is no facility of communicating by the roof, it becomes highly expedient to provide some internal means of escaping through the windows in case of fire, and to have on every floor a strong rope, with a hook or a loop at the end, by which it may be fastened to a bed-post, so as to enable an active person to descend by its help out of the window, finding from time to time a partial footing in the inequalities of the wall. This process will be greatly facilitated by having the rope knotted at intervals of about a foot throughout its length; the knots being nearly as convenient as the blocks or clips that are sometimes made for the purpose of regarding the descent, by holding them, and regulating the friction by the pressure of the hand; unless the clip be attached to a strong cross bar, on which a person may sit, while he regulates the position of the clip by its handles, and allows himself to descend with more or less velocity at pleasure. The arrangement for this purpose may be made by a roller, or pipe, sliding on the rope, and pushed down so as to open the handles of the clip and tighten its teeth.

1 Emerson's Mechanics, figs. 228, 229; Leopold's Theatrum Machinarum, plate liv. Life-Preservers.

When the person holds by the roller and draws it down (fig. 1); and, on the contrary, the clip may be opened by pressing on the handles with the other hand, or with the thighs; or any other simple mode of regulating the clip may be adopted, provided that it be not liable to be misunderstood, or misapplied, in a moment of confusion. After all, a rope-ladder would perhaps be preferable, as not being liable to be disarranged; it is often kept ready made in the shops; and, in the absence of any other rope, a common bed-cord will generally be found strong enough to support the weight even of a stout man; for a quarter-inch rope may be safely trusted with 2 cwt., and ought indeed to support three times as much, if new and of good quality.

In speaking of the duty of providing internal means of escape, Mr Baddley remarks, that "egress can sometimes be made at the top of a house, either by a door or by an opening made in the roof with a poker for the purpose. Sheets and blankets tied together and fastened to the bed-post, or the bed-cords attached in the same way, afford the means of descending; the feather-bed, &c., thrown out serve to break the fall when jumping from the window as the last alternative. With a little contrivance, women and children may be lowered by means of the bed-clothes. Upon these occasions, all depends upon the persons in danger retaining so much presence of mind as will enable them to avail themselves of the best means in their power; and it often happens that pressing danger develops a great deal more ingenuity and intrepidity in individuals than they have previously taken credit for."

External means of escape from fire.—The external means to be employed in cases of conflagration must be provided by the managers of fire-offices, or by other public officers; and every ingenious workman whom they may employ will be able, at his leisure, to devise such apparatus as he can the most conveniently execute, and to give it a full trial in the absence of all danger; it will therefore only be advisable that he should compare for himself the particular inventions which have been suggested for this purpose, and that he should choose from among them such as he thinks most likely to do him credit; and he may, indeed, very possibly find means of improving on any of them.

In Leopold's Theatrum Machinarum (plates liv. lv.), we find the representation of a chair calculated to be drawn up or down by means of pulleys. Mr Varcourt obtained, in 1761, the approbation of the Parisian Academy of Sciences for his invention of a hollow mast, fixed in a wagon, and supporting a stage, with the means of ascending and descending. (Hist. p. 158.) In the beginning of the present century, a fire-escape of Mr Audibert was approved by the Parisian Institute. (Mém. Inst. iv.) A committee was also appointed for examining several similar inventions at the Lyceum of Arts, and a medal was awarded by it to Mr Daujon, for his apparatus, which consists of a platform carried on wheels, supported by three frames, with brass wires, on which boxes are made to slide up and down for the conveyance of persons or of furniture. (Annales des Arts. Repertory ii., vol. i., p. 459.) Mr Collin's invention of pipes raised by ropes, and affording a centre to a long lever, is described in the fourth volume of the American Transactions, and in the Repertory (vol. xv., p. 35). In the thirty-first volume of the Transactions of the Society of Arts for 1813 (p. 244), we have an account of a fire-escape invented by Mr Adam Young, for which he received a Life-Preserver medal from the society. It appears to constitute by far the most portable of ladders, consisting of cross bars or rounds connected by ropes, and having their ends fitted together, so as to form a pole, which is readily elevated to the window; and the rounds being separated, and the hooks at the end properly fixed to the window-frame, the whole forms itself into a very convenient ladder of a mixed structure. The thirty-fourth volume, for 1816 (p. 227), contains a description of Mr Braby's fire-escape, consisting of a car made to slide on a strip of plank fixed to a pole, and governed by a rope, which is eased with iron, to protect it, in case of necessity, from the effect of the fire.

The fire-escape in use in the metropolis is founded on the invention of Mr John Davies, submitted to the Society of Arts in 1809. It consists of three ladders connected to, and sliding upon, each other by means of ropes worked by a small windlass; a second windlass raises and lowers a cradle, which is supported by ropes passing over pulleys at the top of the uppermost ladder. This machine is mounted upon a low four-wheeled truck, and can be drawn by a horse, or by six men. As improved by Mr Gregory, the three ladders sliding on each other, when lowered, are balanced horizontally on a frame, mounted on a light four-wheeled carriage, so as to be run under low gateways, &c. The ladders being brought into a nearly vertical position, are raised by a small windlass in front of the machine to any height between 10 and 40 feet; when the ladders are inclined towards the window, and the top one made to rest against the sill. A greater elevation than 40 feet may be obtained by means of joints fixed at the top, and a cradle is also used for those who are too timid to descend ladders.

A great many other forms of fire-escape might be noticed, for it is one of those subjects which readily appeal to the ingenious mechanic in a large city where fires are numerous; and every form of fire-escape must at times fail. Hence there has been a sort of competition among uneducated inventors, who have displayed some mechanical ingenuity, but have not, as far as we are aware, developed any new principle.

The modes of extinguishing fires are not precisely the object of the present inquiry. The engines employed for the purpose are described under Hydrodynamics. Within the last few years, an apparatus called a fire-annihilator has attracted some attention; its object is to generate a large amount of gaseous matter, which will not support combustion, and which being injected into the burning mass, is to extinguish the fire by displacing the air. The non-supporting gases are generated by means of chlorate of potash and sugar contained in a perforated cylinder, and ignited, when required, by crushing a small glass vessel containing sulphuric acid. There is also a contrivance for saturating the gases with moisture.

Many attempts have been made to prevent fires by rendering houses fire-proof, either by coating the timber with some uninflammable substance, or by rejecting combustible substances altogether in building. The Earl of Stanhope's method of fire-proofing consisted in the use of non-combustible materials, with, among, and between the timbers forming the frame-work of a house. Many other methods have been proposed for coating the timbers with plaster, covering the houses with fire-proof paint, soluble glass, &c. We do not attach much importance to these inventions, for if the house contain goods and furniture of an inflammable kind, and the fire once obtain an ascendency over them, the house will scarcely escape destruction. In the construction of floors and roofs, Messrs Fox and Barrett propose to substitute for timber joists of iron, and successive layers of incombustible materials, such as mortar, concrete, &c., the floors being finished with a smooth and uniform

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1 Hebert's Engineers' and Mechanics' Encyclopædia. surface of lime and sand well trowelled up to a face, and coated with linseed oil; while the roofs are finished with a coating of coal-tar, paper, and sand, instead of the ordinary timber and slate. Mr Frost proposes for the floors of rooms to use hollow earthenware tubes, embedded in cement, and so combined as to cover the whole floor. Mr London recommends that the floors be formed of flat tiles and cement. Mr Payne proposes to render timber fire-proof by means of a solution of sulphuric acid or of calcium, forced into the timber by appropriate apparatus, and then adding an acid or a solution of some substance, such as sulphate of iron, which unites with the barium or calcium, and sets free the sulphur. Iron joists and girders have been largely used in the construction of fire-proof buildings; but it has been shown by Mr Bradwood, that a fire originating in the goods stored in these so-called fire-proof warehouses sometimes produces a temperature nearly equal to the melting point of cast-iron; while the jets of water from the fire-engines crack and bring down beams and columns of cast-iron which are used in supporting the upper floors, so as really to cause a greater destruction of life and property than timber beams, which resist the fire for a considerable time, and permit the inmates to escape, and the firemen to penetrate the building with their branches and water-hose, which they are sometimes forbidden to do if the building is known to have iron-girders. A better plan for fire-proof buildings seems to be the use of groined brick arches, supported by pillars of brick laid in cement.

Internal Means of Escape from Shipwreck.—The means of escaping from shipwreck may be similarly divided into internal and external, or into the precautions to be taken by the ship's company, and the measures to be adopted by persons on shore. The internal means depend either on enabling the individuals to swim or float, or establishing a connection with the shore by ropes; and of the former, we may first consider those which require no particular preparation before the occurrence of the accident that calls them into action, and which are, therefore, the most universally applicable.

Of such expedients, the most effectual appear to be those which depend on the employment of empty water-casks for assisting the ship's company to drift on shore. 1. A paper on the arrangement of water-casks, to serve as floats in case of shipwreck, appears in the publications of the Society for the Improvement of Naval Architecture, dated in 1796 (vol. ii., p. 51). 2. In 1818, Mr Grant of Ildeford obtained a gold medal from the Society of Arts, for the invention of a life-preserve, consisting of a thirty-six gallon cask, with some iron ballast fixed on a wooden bed, and lashed to the cask, and with ropes round it for the men to hold; and it was found that ten men were supported by it with convenience in tolerably smooth water, the bung of the cask being well secured by cork (vol. xxxvi., p. 63). The ballast could be of very little use, and a cask simply tied round with a rope, like a common parcel, would probably answer the purpose equally well. It would, indeed, be prudent for every ship in a storm, on a lee shore, to have a few of her casks well emptied and stopped, and tied in this manner, before the actual occurrence of imminent danger. 3. In the thirty-seventh volume of the Transactions of the Society (p. 110), there is an account of Mr Cook's life-raft, consisting of a square frame with canvas nailed across it, supported by a cask at each corner, for which the gold medal was voted to him. 4. It is followed by a description of Lieutenant Rodger's life-raft (p. 112), which obtained a similar compliment. This raft has the advantage of requiring only such materials as are usually found on board of every ship; capstan-bars, boat-masts, yards, or any other spars of moderate dimensions, which are tied together so as to make a sort of waggon frame, with a large cask fixed on each side; it appears to afford a very convenient support to the men, but it can scarcely possess any great strength for resisting the force of the breakers.

Mr J. Bremner, a clergyman in the Orkneys, had received a medal from the Society in 1810, for his method of converting any ship's boat into a life-boat by putting into it three or four casks lashed to the keel, which is to have ring-bolts fixed in it for receiving the ropes by which the casks are fastened; he gives particular directions for making all the necessary arrangements, in the twenty-eighth volume of the Transactions (p. 134); he particularly advises that no use should be made of the natural buoyancy of the cavity of the boat, but that the bottom should be perforated without hesitation, wherever the hole would afford any additional facility for fixing a rope. Captain Manby's jolly-boat, fitted as a life-boat, "at the expense of three pounds," seems to be comprehended among those preparations which are to be made previously to the voyage.

The buckling a soldier's canteen on his breast as an assistance to enable him to float, belongs to those temporary expedients which may occasionally be employed with advantage. Tying a hat in a pocket-handkerchief, and holding it as a float, has been recommended by Mr Lawson in the Philosophical Magazine (vol. xx., p. 362); he advises that the crown of the hat should be held downwards, and observes that a stick may be employed, to enable us to use two or four hats at once; but this method can only be adopted when the accident occurs in very still water.

The first and most obvious preparation for enabling a person to float, is the learning to swim. It is well known that swimming is scarcely ever sufficient to enable a seaman to reach the land from a ship that has been wrecked, without some assistance; and many have certainly been drowned from depending too much on their own strength; but for a momentary support, and to afford courage and presence of mind to seek for other aid, there is no question but that the faculty of swimming possesses an invaluable advantage. A boy generally learns to swim by the help of his schoolfellows better than by any general rules, and more agreeably than in a school of natation; but it may be of some use to observe, that the act of diving to the bottom and reascending, in tolerably shallow water, is much more easily performed by a beginner than that of simply supporting himself on the surface; and when he has thus acquired the feeling of the immediate effect of his arms in propelling and sustaining him, he soon finds out the means of employing his feet in their assistance. The art of swimming has, however, been systematically treated by Bachstrom, Kunst zu Schwimmen; 8vo, Berlin, 1742; by Thévenot, Art de Nager, Paris 1711; and by Bernardi, Arte Ragionata del Nuoto, 2 vols. 4to. Naples, 1794.

It is easy to convince ourselves, by trials in a warm bath, without reference to Robertson's experiments (Phil. Trans., 1757), that a substance possessing a very small degree of buoyancy is sufficient to enable the human body to float without effort. In fact, when the chest is fully expanded, the thinnest and most bony person will commonly float in sea-water; but the effort of keeping the chest expanded is as fatiguing as any other muscular exertion; and when the chest collapses, the fattest people may be in danger of sinking, unless they have learned to swim. Sir William Hamilton, indeed, tells us that, in 1783, "a woman of Scilla, four months gone with child, was swept into the sea by the wave" accompanying the earthquake, "and was taken up alive, floating on her back at some distance, nine hours after; she had been used to swim; her anxiety and suffering, however, had arrived at so great a pitch, that just at the time that the boat which took her up appeared, she was trying to force her head under water, to put a period to her miserable existence." In China, a frame of bamboo surrounding the person is used for a float, and the lightness and strength of this substance must well adapt it for the purpose; sometimes also a gourd is tied to a child, to secure its floating in case of accident. The inflated goat-skins used from time immemorial by the Arabs, or the seal-skins employed by the Chilians, have the disadvantage of being easily rent or torn by a rock or a spar; an objection which is also more or less applicable to all substances containing air; for example, to the air-jackets described in Leupold's *Theatrum Pontificum*, published about 1724. A float of a semicircular form was recommended by Ozanam, the author of the *Recreations*; and Bachstrom, in his *Art of Swimming*, proposed to float a troop of cavalry, by fixing cork to the saddles. The cork jacket of Gelaszy is described in the *History of the Parisian Academy of Sciences* for 1757, and Lachapelle's *Scaphander*, which is considered an improvement on it, in the volume for 1765. In the year 1764 the attention of the British public was particularly called to the floating powers of cork, by some experiments which were made with cork jackets on the Thames, together with some comparative experiments on air-jackets; and Dr Wilkinson, in the *Philosophical Transactions* for 1765, describes some experiments by which he ascertained that about a pound of cork was amply sufficient to enable a man of ordinary size and make to float without effort. It is almost superfluous to enumerate the multitude of trifling variations that have been made in the arrangements of cork jackets and air-jackets, apparently for the purpose of exciting a momentary interest, though possibly from the best motives.

Mr Bosquet advised a bag of cork shavings to be kept in readiness by each person; the *Seaman's Friend* was composed of two pieces of cork, united by straps; the *Collinetta* was a hollow vessel of copper, divided into cells; a "marine spencer" has been described by Mr Spencer, in the sixteenth volume of the *Philosophical Magazine*, consisting only of a number of old corks, arranged so as to form a giraffe; and in 1806, Mr T. C. Daniel obtained a gold medal from the Society of Arts, for the invention of an apparatus of waterproof leather, surrounding the body, which, according to the testimonials he produced, had saved the lives of some persons who had been sailing in a pleasure-boat on a river. In smooth water, it has been suggested that throwing a foot-ball, with a small weight tied to it, to the person immersed, would often afford sufficient assistance; and, with respect to floating, there is no doubt that any of the assistances which have been proposed would be sufficient if they were at hand; but there is another object, to which it is necessary to attend, in cold, and even in temperate climates, that of supporting a temperature compatible with life and health, if the immersion is likely to be of long duration; and an additional provision of worsted stockings, jackets, and trousers, will be almost as essential, in such cases, as the means of obtaining buoyancy.

The invention of India-rubber cloth led to the introduction of inflated belts, the advantages of which, compared with cork, and other forms of belt, are their greater buoyancy compared with their bulk, and their greater portability, for, when emptied of air, they can be folded up, and packed into a small space. The objections to them are their liability to get punctured or torn, and to decay, from being put away damp; the metal valves by which they are inflated may also get out of order; during the hurry and confusion of a wreck they are liable to be only partially inflated, and the valves to be only half screwed up, so as to allow of the escape of the inclosed air.

Commander J. R. Ward, R.N., inspector of life-boats to the National Life-Boat Institution, has invented a belt with four compartments, which admit of being separately inflated, thus mitigating the danger arising from puncture or injury to the inflating valves; it has a buoyancy equal to 30 lb., and should two of its compartments be disabled, the remaining two would be sufficient to float the wearer.

For the rough purposes of ordinary boat-work, Commander Ward insists on the advantages of cork as a material for life-belts, and he has invented a form of belt, which has been selected by the National Life-Boat Institution for the use of its life-boats' crews. The buoyant power of each belt is from 20 to 24 lb.; the cork is uncovered, so that its quality can be seen, and it is divided into numerous narrow pieces, each of which is sewed separately to a strong linen or dark belt, which covers the body from the arm-pits to below the hips. The pieces of cork are distributed in two rows, one above, the other below the waist, and the belt is secured closely about the body by means of strings passed round the waist, between the two rows of cork. It is further secured by other strings, crossed over the shoulders. By this arrangement the trunk of the body is enveloped in cork, attached so as to be quite flexible, and to allow of the usual movements of the body without inconvenience, while it protects the body against injury from blows, and is a warm covering in cold weather.

Various forms of buoyant mattress have been contrived by Mr Laurie and others. As manufactured by Mr Silver, numerous waterproof tubes are partly distended with horse-hair, woollen flocks, or cocoa-nut fibres, so that, should one or more of the tubes fail, the others may suffice to sustain the required weight on the water. The tubes are made up into mattresses, pillows, and floats,—the last to be placed under the thwarts of boats. A mattress weighing 17 lb. sustains in the water 284 lb. A pillow sustains 28 lb. A mattress for emigrant vessels, sold at 9s., was proved at the Great Exhibition. It sustained 96 lb. in the water during five days, without being injured. Floating mattresses are also made, filled with cork shavings. In the Great Exhibition, Mr Rhind had various models of deck seats and benches for steamers, so constructed as to be readily formed into rafts, each of which was capable of sustaining eight persons.

For the second object which is desirable to a ship in distress, that of obtaining a safe communication with the shore, it has been usual of late years to rely principally on the humane exertions of persons who may be on the coast, and who may have made preparations for this purpose; and with this view, some instructions for properly co-operating in the measures to be adopted with Captain Manby's apparatus have been liberally distributed to all ships when they received their papers from some of the British customhouses. There are, however, some simple expedients which may be adopted for this purpose by persons on board of the ship; for example, the making a kite with a pocket-handkerchief stretched over a hoop, and causing it to carry a cord to the lee shore, by means of which a stronger line, and at last a hawser, may be drawn by persons standing on the beach. A line may also sometimes be carried on shore by a cask, allowed to drift before the wind; and a bag has been recommended to be attached to such a cask, or to a buoy, in order to act as a sail, and to insure its crossing the surf. Mr Cleghorn was also rewarded, in 1814, by the Society of Arts, for the invention of a buoyant line, having a heart of cork, to obviate the inconvenience which would arise from its sinking and being dragged on the stones under the breakers; but he observes, that in heavy storms there is generally a current along shore which renders the method almost impracticable. (Transactions, xxxii., p. 181.)

A Mr Wheatley assures us, in Captain Manby's Essay, that his own life, and that of eight other persons, was saved, in 1791, by a lead line, which was carried on shore by a Newfoundland dog that he happened to have on board, when two good swimmers had been drowned in the attempt to swim on shore. It had occurred to Lieutenant Bell, in 1791, that a rope might be thrown from a ship which had struck; by means of a mortar carrying a heavy shot, and upon the principle of the gun harpoon; and he showed the practicability of the suggestion by an actual experiment, in which a deep-sea line was carried to a distance of about 400 yards. (Trans. Soc. Arts, xxv., p. 136.) He recommended that every ship should be provided with a mortar capable of carrying such a shot, and observed that it might be placed on a coil of rope to be fired, instead of a carriage. The line was to be coiled on handspeaks, which were to be drawn out before the mortar was fired. In 1792 he received a premium of fifty guineas from the Society of Arts (Transactions, x., p. 204); and he obtained his promotion in the Ordnance as an acknowledgment of his merits. The shot was to weigh about 60 lbs. or more, and the mortar 5 or 6 cwt. The experiments of the French artillery at Laferre were subsequent to those of Mr Bell, though they have sometimes been quoted as the first of the kind.

It has, however, generally been thought impracticable to manage a mortar with effect under the circumstances of actual shipwreck; and Mr Tregrouse has preferred a rocket, as more easily fired, and as having a smaller initial velocity than a shot, so that the rope would be less in danger of being broken by the impulse. He found that a rocket of 8 oz. carried a mackerel line 180 yards, and a 1 lb. rocket 212; and, in some experiments made under the inspection of the Society of Arts, a rocket 1½ inch in diameter carried a cord across the Serpentine River in Hyde Park. The musket is provided with a valve, to prevent the escape of the materials of the rocket, and it is to be fired with a little powder, without wadding. The whole apparatus is packed in a chest, containing from eight to twelve rockets, the musket, a life-spencer, a chair to traverse on a rope, a canvas bag, and a ball of wood to throw to a person swimming. Mr Tregrouse was complimented with a medal from the Society of Arts in 1820. (Vol. xxxviii., p. 161.)

External Means of Escape from Shipwreck.—The means to be employed by persons on shore, in cases of shipwreck, depend either on projecting a line over the ship, or on the use of a life-boat. Mr Bell had cursorily observed that a line might be carried over a ship from the shore by means of his mortar; but for the actual execution of this proposal, in a variety of cases, we are indebted to the meritorious exertions of Captain G. W. Manby, whose apparatus, according to the report of a committee of the House of Commons, dated in March 1810, appears “to be admirably adapted to its purpose, and to have been attended with the fullest success in almost every instance.” In consequence of this report, Captain Manby was thought worthy of a parliamentary reward; and he afterwards published a description of his inventions, under the title of An Essay on the Preservation of Shipwrecked Persons, 8vo, London 1812. He had previously received a gold medal from the Society of Arts in 1808 (Transactions, xxvi., p. 209). His success makes it expedient to extract from his Essay a detailed description of the apparatus, and it will be easy to make it somewhat more intelligible by a slight alteration of the order of arrangement.

“The method of affixing a rope to a shot, for the purpose of effecting communication, when projected from a piece of ordnance over a stranded vessel, was at length succeeded in, by introducing a jagged piece of iron, with an eye at the top, into a shell, and securing it by filling the hollow sphere with boiling lead; and in another way, by drilling a hole through a solid ball, and passing a piece of iron, with an eye to it, as before described, to the bottom, where it should be well secured by riveting.

“To produce the means of connecting a rope to a shot, and prevent its being burnt, and rendering it ‘irresistible’ to the powerful inflammation of gunpowder, was the labour of infinite time, and the number of experiments to accomplish it is beyond all possible conception. Chains in every variety of form and great strength breaking, proved that it required not only an elastic, but a closer connected body. At length, some stout platted hide (fig. 2), woven extremely close to the eye of the shot, about 2 feet in length beyond the muzzle of the piece, and with a loop at the end to receive the rope, happily effected it.

“This method is certainly desirable, as the rope may, immediately [as] it is required, be affixed to the loop, and applied in service. The form of the platted hide may likewise be woven by twisting it in the manner that the lashes of whips or ropes are spun; there is another method, by passing the rope through a case of leather, taking the greatest care that it is so well secured at the eye of the shot, as to leave no room for the slightest play, as is represented by the annexed barbed shot (fig. 3).

“When the crews of the distressed vessel are incapable of availing themselves of the benefits arising from communication, they having previously lashed themselves in the rigging to prevent being swept away by the sea, which is repeatedly breaking over them, and when, from long fatigue and the severity of the storm (on which occasions it too frequently occurs), they totally lose the use of their limbs, and are rendered incapable of assisting themselves in the slightest degree—the advantages of this shot are, that, on its being projected over the vessel, and the people of the shore hauling it in, it firmly secures itself on some part of the wreck or rigging, by which a boat can be hauled to the relief of the distressed objects; and by the counterbars it is rendered impossible [that it should] give up its hold, or slip, while that part of the wreck remains to which it has secured itself.

“Among the many that have been saved by this shot, the following are testimonials of a few of the cases:—We, the crew of the brig Nancy of Sunderland, do hereby certify that we were on board the said vessel when she was stranded on the beach of Yarmouth, on Friday morning, the 15th of December 1809, and compelled to secure ourselves in the rigging to prevent being swept away, the sea running so high over the vessel. And we do further declare and certify that Captain Manby, firing a rope with a hooked shot, securely holding on the wreck, enabled a boat to be hauled from the shore over the surf to our relief, otherwise we must inevitably have perished.” This certificate is attested by six signatures.

“Facilitating communication is at all times of importance; but when the stranded vessel is in momentary danger of going to pieces, this point becomes a consideration of extreme urgency. I feel a persuasion that this particular service can only be carried into effect by a small and light piece of ordnance, the range of which is consequently very inconsiderable, when compared with that of a large and heavier piece, as it is weight alone that conveys the rope. In order, therefore, to increase the powers of a shot projected from a small mortar, its natural form must be varied, so as to give it additional ‘preponderance.’ The annexed shape, in the form of a pear (fig. 4), has been used with the greatest success; for, by the increased weight, the shot’s momentum and power over the line is in consequence considerably augmented in its range; and when made to fit the piece as close as possible, a great increase of velocity is likewise produced from that decrease of windage. "Portability in the construction of a piece of ordnance (as just described) is the very essence of this service; and communication with the stranded vessel or wreck may be effected with a cord, by which cord a rope can be conveyed, and by that rope a hawser or cable sent to the distressed vessel; for this purpose the annexed was constructed" (fig. 5).

"A person completely equipped with every necessary apparatus to effect communication with a vessel driven on a lee-shore...the horseman, fully equipped, travelled a mile and a half, the howitzer was dismounted, and the line projected 153 yards, in six minutes.

"The application of a small piece of ordnance likewise offers particular advantages, capable of being employed from a boat to go to the assistance of a vessel grounded on a bar when running for a harbour, the necessity of which repeatedly occurs, and was twice witnessed at Blakeney on the 10th of November 1810, when boats endeavoured to go to their relief, and were enabled to get out of the harbour on the ebb tide, within 20 yards of the vessel; but it was found impossible to approach them nearer. Had such boats been provided with a piece of this description, and the same firmly secured on a stout piece of plank, by the holes left at each corner of the iron bed, they might have projected a small rope, coiled in a crate or basket made to the form of the bow of the boat; and the persons in the boat, so provided, would not have remained the distressed spectators of the untimely end of their fellow-creatures, without being able to afford them the smallest relief, although so little was then wanted for that desirable purpose.

"Although advantages have been pointed out in the use of these small mortars, it is necessary to be kept in remembrance, that they are produced for particular services; as the nature of the coast, and circumstances attending the distressed vessel, will direct what piece is best adapted to the undertaking. To enable the mind to form a judgment of what can be effected by other pieces, the following are the minutes of experiments made with a 5½-inch brass mortar, stating the quantity of powder used, and distance the ropes were projected against a strong wind, at the angle of 17° (elevation): weight of the mortar and bed about 300 lb.:

| Ounces of Powder | Yards of inch and half Ropes | Yards of Deep-sea Line | |------------------|-------------------------------|------------------------| | 4 | 134 | 148 | | 6 | 159 | 182 | | 8 | 184 | 215 | | 10 | 207 | 249 | | 12 | 232 | 290 | | 14 | 250 | 310 |

"With a short 8-inch mortar, the weight of which and bed was supposed to be about 700 lb.; the angles of elevation uncertain:

| Ounces of Powder | Yards of Deep-sea Line | Yards of two-inch patent Sunderland Rope, capable of hauling the largest Boat from a beach | |------------------|------------------------|------------------------------------------------------------------------------------------| | 32 | 439 | ... | | 32 | 479 | ... | | 32 | ... | 338 |

"Directions for using the Apparatus.—When the rope (which should be pliant and well stretched) is brought on the beach or cliff opposite to the stranded vessel, the most even spot, and free from projecting stones, should be selected to lay it on, and great care be taken that no two parts of it whatever overlay or even touch each other, nor must it be laid in longer lengths than of two yards. But to project a small line or cord, it will be necessary, if it is required, to contract the faler to half a yard at most, to avoid the jerk received at the end of each right line. The best method, with such a description of cord, is to lay it on the ground in the most short and irregular windings, to relieve it from this powerful impulse. To prove the effect of the impulse on a rope, if it is faked in lengths of 10 or 15 yards, it will break each time, as it then becomes a most powerful pendulum. These precautions are absolutely necessary to the success of the service.

"The following has, after various trials, been found a certain method of laying the rope, and placing it into compartments." (French Faking, fig. 6.)

"A particular attention to this mode will never fail with a good rope, when the impediments are removed that might otherwise obstruct its rapid flight. Its advantages are, that it will allow the eye rapidly (yet correctly, just before firing, which is absolutely necessary) to pass over the different compartments, and at once discover if any fake has been displaced by the storm, or by any other casualty or accident come in contact with another part, which would destroy its application by the rope breaking.

"It may likewise be coiled in the manner used in the whale fishery, whale lair (fig. 7); and in the method called chain faking (fig. 8):

"It is, however, necessary to add, that great attention is required in laying it agreeably to the two latter methods, arising not only from the arm being liable to get under certain parts of the rope, and thereby displace it, but from the great anxiety of mind natural on these occasions, where the lives of fellow-creatures are literally dependent on the correctness with which the rope is laid; it is therefore extremely difficult, in a moment of agitation, to determine whether any overlay has taken place, an error that would infallibly destroy every endeavour, and occasion even the fate of those whose lives we might be exerting ourselves to preserve. Could persons in the performance of this service be always collected, the two latter methods would have a decided advantage over the first mode of faking, they being laid in a much less space of time.

"As all these methods of laying the rope occupy time to place it with the care necessary; and as it has repeatedly happened that vessels, very soon after grounding, have gone to pieces, and all hands perished; it was necessary to produce a method of arranging the rope, so that it could be immediately projected as soon as it arrived at the spot; and..." none proved so effectual as when brought ready in a basket (fig. 9).

"In this case, the rope should be most carefully laid in alternate tiers or fakes, no part of it overlying, and it should be well secured down, that in travelling it be not displaced; but, above all, no mistake must happen in placing the basket properly. For example, that the end of the basket, from which the shot hangs in the above figure, should be previously marked, and must be placed towards the sea or wreck, that the rope be delivered freely, and without any chance of entanglement. It will be scarcely necessary to add, there will be several tiers of the rope when laid. The utmost care and attention are required in laying the rope in tiers with strict regularity, to prevent entanglement."

"The next is the application of the mortar. If the wind is sideways to the shore, it must be pointed sufficiently to windward to allow for the slack of the rope lighting on the object, as the rope will, of course, be considerably borne to leeward by the effect of a strong wind, and by its being laid at a low elevation insures the rope falling against the weathermost part of the rigging. While this service is performing, great care should be taken to keep the mortar dry; nor should it be loaded until everything is ready; when that is done, it should be primed; but as it would be impossible to do it with loose powder in a storm, a tube is constructed in the simplest manner of common writing paper (the outer edge being cemented with a little gum) in this form (fig. 10). It is filled with meal gunpowder, made into paste with spirit of wine; when in a state of drying, run a needle through the centre, and take care the hole is left open, for, on the tube being inflamed, a stream of fire darts through the aperture with such force as to perforate the cartridge. The mortar should then instantly be fired; and in order to lessen a difficulty that has often occurred in performing this service, a pistol may be used, having a tin box over the lock, to exclude the effect of wind or rain on the priming; and the muzzle being cut obliquely, dilates the inflammation, so as to require but little exactness in the direction of the aim.

"We will suppose the communication to be secured, although it is scarcely necessary to offer any other assistance than that of a rope, as the inventive genius of a sailor will supply everything else; yet I could expect the people on shore to get a boat ready for meeting the vessel when driven on a beach; it is the promptest and most certain method of relief, as well as the most easy to be accomplished; for by hauling her off with the rope projected, the boat's head is kept to the waves, and not only insures safety by rising to the surge, but prevents her upsetting."

"When the rope attached to the shot (not having barbs to it), is fired over the vessel and lodges, let it be secured by those on board, and made fast to some firm part of the rigging or wreck, that they may haul off a boat by it; but should there not be any boat, then haul on board by the projected rope a larger one, and a tailed block, through which a smaller rope is rove. Let the large rope be made fast at the mast-head, between the cap and the top of one of the lower masts, and the tailed block a little distance below it; but, if the masts should have been cut or carried away, then it must be made fast to the loftiest remaining part of the wreck. When this is done, there will be supplied from the shore a cot, hammock, netting, basket, hoop, or any of the numerous resources of seamen, which will run on the larger rope, and be worked by the people on shore. If a cot be used, the men may be so securely fastened to it as to preclude all possibility of falling out, and then be brought from the wreck, one by one, in perfect safety."

"While communication is gaining, three stakes should be driven into the ground in a triangular position, so as to meet close at the heads to support each other. As soon as communication has been effected by the crew of the vessel, and they have secured the line attached to the shot, made fast to these stakes, the crew will haul on board by it a large rope and a tailed block, through which a smaller rope is to be rove, both ends of which (the smaller rope) are to be kept on shore. When they have secured these on board, and the larger rope is rove through the rollers, let a gun-tackle purchase be lashed to it, then lash the purchase to the stakes. By the means of the purchase the larger rope may be kept at a fit degree of tension; for, if care be taken to slacken the purchase as the ship rolls out to sea, the danger of the rope being broken will be guarded against; and, on the other hand, if the purchase be gathered in as the ship rolls toward the shore, the slackness of the rope, which would prevent the cot (fig. 11) traversing as it ought to do, and plunge it in the water more than it otherwise would, will be avoided."

"Supposing neither boat nor cot apparatus at hand, first cast off the shot from the projected rope, and with a close hitch, let it be put over the head and shoulders of the person to be saved, bringing it close under each arm, drawing it tight, observing particularly the knot is on the breastbone; for, by having the knot in that position, on the people of the shore hauling the person from the wreck, he will naturally be on his back, consequently, the face will be uppermost to seize every moment for respiration, after each surf has passed over the body. If circumstances compel recourse to this method, care must be taken to free the rope from any part of the wreck, and to jump clear away; but should there be more than one on board, each man should make himself fast in the same way, about four feet from the other, and join hands, all attending to the same directions."

"To give Relief to Vessels Stranded on a Lee Shore in a Dark and Tempestuous Night.—It will be requisite, first, to devise the means of discovering precisely where the distressed vessel lies, when the crew are not able to make their situation known by luminous signals; secondly, to produce a method of laying the mortar for the object, with as much accuracy as in the light; thirdly, to render the flight of the rope perfectly distinguishable to those who project it, and to the crew on board of the vessel, so that they cannot fail of seeing on what part of the rigging it lodges, and consequently have no difficulty in securing it.

"To attain the first object, a hollow ball was made to the size of the piece, composed of layers of pasted cartridge paper of the thickness of half an inch, having a lid on the top to contain a fuze (fig. 12), and it was then filled with about fifty luminous balls of star composition, and a sufficient quantity of gunpowder to burst the ball and inflame the stars. The fuze fixed in the ball was graduated, to set fire to the bursting powder at the height of 300 yards. Through the head of the fuze were drilled..." holes, at equal distances, to pass through them strands of quick match, to prevent the possibility of any accident from the match falling out, or from its not firing the fuze. On the stars being released, they continued their splendour, while falling, for near one minute, which allow ample time to discover the situation of the distressed vessel. During the period of the light, a stand, with two upright sticks (fig. 13), painted white, to render them more discernible in the dark, was ready at hand, and pointed in a direct line to the vessel.

"A shell affixed to the rope, having four holes in it to receive a like number of fuzes (headed as before described), and filled with the fiercest and most glaring composition, which, when inflamed at the discharge of the piece, displayed so splendid an illumination of the rope, that its flight could not be mistaken."

"To get a Boat from a Beach over the Surf.—The importance of going to the relief of ships in distress at a distance from the land, or for taking off pilots, was viewed as of the highest consequence by the elder brethren of the Trinity House, and offered to my particular attention by several distinguished characters. After numerous experiments to accomplish it in various ways, the mode following was most approved:—About forty fathoms of 2½-inch rope, made fast to two moving anchors, was laid out parallel with the shore, at a distance beyond the sweep of the surf; to the centre of this rope was made fast a buoy, of sufficient power to suspend the great rope, and prevent it from chafing on the sand, rock, or stones, as well as embedding, a circumstance that has rendered it impossible, on a sandy or shingly coast, to heave out an anchor with a rope to it from the shore. As this service should be performed in fair weather (to be prepared for the storm), it may be regulated with the greatest exactness, and should take place at the top of high-water, that the upper part of the buoy may be at the full stretch of its power, and only seen at that time. Should the shore be extremely flat, it will be desirable to place another set at a sufficient distance beyond the first, to insure the operation of this method in any state of the tide.

"The royal mortar being brought to the spot, is to be pointed in the direction for the buoy, and should be laid at a very low elevation, but such as to insure the range; for the more it is depressed, the less slack of rope there will be from the parabola formed in the shot's flight; the basket with the rope ready laid (having a barbed shot to it) is to be placed in the front of the mortar; on its being fixed, instantly haul the slack of the rope in, to prevent the grappling with the great rope; when that is caught and hooked, a power will be acquired fully adequate to the service.

"As a cast-iron anchor appears particularly adapted to this method, and would be much cheaper than hammered, fig. 14 is a plan of one which the Honourable the Navy Board approved, and allowed me to cast at their expense, for the purpose of making the experiment."

"When a vessel is in that extreme and perilous situation, driven under a rugged and inaccessible cliff, and in danger of going soon to pieces, the most prompt method I should suggest is, by lowering to the crew a rope with stiff loops spliced into it (fig. 15), at the distance of a foot and a half from each loop, of sufficient size to contain the foot, by which they can ascend as a ladder.

"This rope-ladder is capable of being projected; and one of an inch and half rope was thrown from a mortar 194 yards. It might also, from the simplicity of its structure, be extremely useful in escaping from a house on fire. By making one end fast to the leg of a bed or a table, the person would come down from the window in safety, and with much less difficulty, and quicker, than with the common rope-ladder, which is heavier and more unwieldy. It has great advantages when employed in saving shipwrecked men in situations just described, when, from extreme cold, and almost benumbed limbs, it would be impossible for them to climb up a rock, or ascend it even by the aid of a common rope. The holds, thus spliced in, will support both hands and feet."

The Report of the Committee of the House of Commons contains also a paper of instructions for the managers of Captain Manby's apparatus on shore, which are somewhat more minute than the directions published in his Essay. For example:

"If the wind be sideways to the shore, the mortar must be pointed sufficiently to windwards to allow for the slack of the rope lighting on the object, as the rope will, of course, be borne considerably to leeward by the effect of a strong wind.

"The distance your judgment decides the vessel to be from the shore should regulate the charge of powder as stated in the scale, taking just a sufficient quantity to clear the object; an attention to this will be more certain of your effecting communication, and guarding against the danger of the rope breaking, or any other circumstance that might prevent the successful performance of the service. The elevation of fifteen degrees is to be preferred, particularly if the wind is sideways, pointing the mortar sufficiently to windward, as the rope would then fall against the weathermost part of the rigging of the stranded vessel.

"When a vessel is driven on shore in the night, you will flash gunpowder as often as convenient on your way; this will animate the crew, and denote to them you are coming to their assistance. On getting to the spot where you have reason to suspect the vessel lies, as you are not able to discover her from the extreme darkness, and if the people on board cannot [make known] their situation by luminous signals or noises (which they will be directed to make if possible), you will lay the mortar at a very high elevation, and fire a light ball.

"Just before you fire (the rope) it would be advisable to let off a blue light to put the crew on their guard, to look out, and be ready to secure the rope. The service can be performed with a carronade."

In chap. iv. we have a copy of directions to persons on board vessels stranded on a lee-shore, proposed to be delivered to the masters at the custom-house. It is observed, that even snapping a pistol, when the powder is wet, may sometimes afford a signal visible on shore, from the sparks of the steel alone. The other parts of the directions will be supplied by those who understand the principles of the proposed mode of relief.

Rockets have of late years been much employed instead of the mortar, in Manby's apparatus for throwing a line to a ship in distress. "Dennett's Rocket Apparatus" is supplied to many stations along the coast. The only advantage which the rocket has over the mortar is its greater portability; for, being much lighter, it can be used with greater facility amongst rocky cliffs, and in positions difficult of access. The disadvantages of rockets are, that they are somewhat uncertain, sometimes exploding as soon as ignited, to the danger of the bystanders; and they are also liable to deteriorate from the effects of damp or of age. Moreover, being expensive, they cannot be often employed in trials, so as to keep up the practice of the people employed in using them. The range of a shot from a 24-lb. mortar, which is the ordinary size, is about the same as that of a 12-lb. rocket, which is the largest in use. As the management of the mortar and rocket apparatus is much better understood by the officers and men of the coast-guard service than by ordinary boatmen and fishermen, it has been almost entirely left in their hands, and is provided by the Board of Customs. There are in England 132 mortar and rocket stations; in Scotland, 15; in Ireland, 22.

Several inventions, or variations, in the Manby apparatus may be just glanced at. M. G. Delvigne uses a howitzer instead of a mortar, while a portion of the line to be carried is contained in the projectile. Mr Greer has a method of discharging a rocket, with a line attached, from a light harpoon gun. When discharged, the rocket ignites, and is said to prolong the range to a greater distance than if the gun or the rocket were alone employed. Captain Jerningham, R.N., has an anchor of a particular form, which he proposes to fire from a Manby's mortar, in sufficient numbers to afford the means of hauling a life-boat through the surf. Mr A. G. Carte employs a war-rocket instead of a Dennett's rocket.

The last description of the inventions to be considered, with regard to the preservation of lives in cases of shipwreck, is that of life-boats, which are of such a construction as to be incapable of sinking, even when filled with water. The occasional adaptation of the common boats of the ship to such purposes, by means of empty casks, has been already noticed. But the boats now in question are supposed to be kept on shore at proper stations, and manned by active persons, who are in the habit of exerting themselves for the relief of seamen in distress.

Mr Henry Greathead, of South Shields, received a gold medal and fifty guineas from the Society of Arts, in 1802, and a parliamentary reward of L1200, besides further remunerations from the Trinity House, and from Lloyd's Coffee-House, for his invention of a life-boat, which is described in the Transactions of the Society, vol. xx., p. 283. The length of this boat is 30 feet, its breadth 10, and its greatest depth about 3; besides a general curvature, which nearly doubles the depth, as reckoned from the ends; the convexity below being intended to give it a greater facility of turning, and a greater power of mounting on the waves without submersion of the bow, which would increase the resistance, though it would not sink the boat; the breadth is also continued farther than usual fore and aft, in order

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1 This society, founded in 1824, is under the patronage of her Majesty, and the presidency of his Grace the Duke of Northumberland. The object is to assist every wrecked person in the kingdom, by such means as the establishment of life-boats and rocket-mortars at all the dangerous parts of the coast; to assist in the formation of local committees at the chief ports; to confer rewards in the form of medals, votes of thanks, or pecuniary remuneration to all persons risking their lives for the sake of others; and also to encourage the invention of new or improved life-boats, belts, rocket apparatus, buoys, and other means of saving life. This admirable society is dependent on voluntary subscriptions for its existence and support. That the society has worked with some success, may be judged of from the fact, that since its establishment it has been instrumental in saving the lives of 9682 persons; it has granted 79 gold medals, and 555 silver medals, besides pecuniary rewards, amounting to L9551. society is a Wreck Chart of the British Islands, originally published by the Admiralty. A vessel wrecked on our coasts is indicated by a black spot \( \bullet \), while a vessel so seriously damaged as to require to discharge her cargo is indicated by \( + \); and the number of such marks at any one spot indicates the annual average of wrecks, which may be large because the coast is dangerous, or because the traffic is great. Thus, the mouth of the Tyne shows a larger number of black dots and crosses than any other place; the mouth of the Tees and the mouth of the Weir occupy the next places of distinction in this dismal chart; these three rivers being the outlets of the district by which London is supplied by sea with three million tons of coal every year, giving employment to several thousand collier ships, which sail to and fro, and greatly add to the otherwise large trade of the Northumberland and Durham ports. The coast of these two counties indicates per annum 180 wrecks, sinkings, and serious collisions. The mouth of the Humber, the coast of Suffolk between Yarmouth and Southwold, the sandy shoals off the mouth of the Thames, the Goodwin Sands, the Scilly Isles, Barnstaple Bay, and Liverpool, rank as the next dangerous portions of the English coast. The Welsh coast is also dangerous, especially Glamorgan, Pembroke, and Anglesea. Scotland, except near the Firth of Forth, is comparatively free from wrecks, the western coast remarkably so, probably from being less exposed to the winds, which tend to drive ships ashore on the eastern coast. In Ireland the E. and S. coasts present about an equal number of wrecks, the smaller number being on the northern and western. In the year 1855 no less than 1141 wrecks occurred on the coasts of the United Kingdom, about one-half of that number belonging to the E. coasts of Great Britain. The loss of life from shipwreck during that year was comparatively small, being only 469, or less than one-third of the loss of the preceding year, the average loss per annum being between 600 and 700 lives.

Passing over a great variety of proposals for life-boats, we proceed to notice the boat which the Life-Boat Institution recommends and supplies to its stations. Its history is interesting. A few years ago, a lamentable accident occurred to a South Shields life-boat, whereby twenty pilots were drowned. This induced the Duke of Northumberland to offer a reward for the best model of a life-boat. This offer was responded to by boat-builders and others from various parts of the kingdom, as well as from France, Holland, Germany, and America, so that 280 models and plans were sent in. About fifty of the best of these were exhibited by his grace in the Great Exhibition of 1851; and he expressed his intention of placing the best life-boats, with their subsidiary apparatus, on all the exposed points of the coast of Northumberland. He also caused a report to be prepared, accompanied by plans and drawings, with a view to elicit the best form of life-boat; for although the prize of L100 was assigned to Mr Beeching of Great Yarmouth, it was considered that a better boat might still be produced. Accordingly, Mr James Peake, assistant master-shipwright in H.M. Dockyard at Woolwich, and a member of the Life-Boat Committee appointed by the Duke of Northumberland, was requested to furnish a design for a life-boat which might combine as many as possible of the advantages, and have as few as possible of the defects, of the best of the models examined by the committee. A boat was accordingly designed by Mr Peake, and built at the public expense in Woolwich Dockyard. Some modifications were from time to time made in her, in consequence of various experiments, and a trial of her capabilities made in a gale of wind at Brighton. The boat, with others of the same design, built at the cost of the Duke of Northumberland, was placed on the Northumberland coast in the autumn of 1852. In the course of the following winter these boats were taken afloat on trial by the Society's inspector of life-boats, some of them in heavy seas and gales of wind, and the result of the trials was considered to be highly satisfactory. Other boats were therefore built on the same plan, and we may therefore consider this as the model life-boat. These boats have been for the most part of two sizes, viz., 27 and 30 feet in length, with 7\(\frac{1}{2}\) to 8 feet beam, and rowing from eight to twelve oars, double-banked; their weight averaging two tons. But as such boats have been found too heavy to be managed in some localities, where boatmen are few, boats of less beam and weight, rowing six oars single-banked, but on the same design in other respects, have been built under the denomination of second-class life-boats. The former class of boats have also been somewhat modified since the description of the boat was first published, so as to be reduced somewhat in beam, and to have less height, and greater sharpness of bow and stern, to enable them to be rowed with greater speed against a head gale and a heavy sea. They are also built of fir, upon the diagonal principle of double planking without timbers, whereas the earlier boats were of elm, and clunched, or clinker-built.

The accompanying figures show the general form and the nature of the fittings and air-chambers of one of these boats, 30 feet in length, and 7 feet 6 inches in breadth. In figs. 16 and 17, corresponding to the elevation and deck-plans, the general exterior form of the boat is seen, showing the sheer of gunwale, length of keel, and rake, or slope of stem and stern posts. The dotted lines of fig. 16 show the position and dimensions of the air-chambers within beaded, and of the relieving tubes. A represents the deck, B the relieving tubes, 6 inches in diameter, C the side air-cases, D the end air-chambers. In fig. 18 the exterior form of transverse sections, at different distances, from stem to stern is shown. Fig. 19 represents a mid-ship transverse section, A being sections of the side air-cases, B the relieving tubes, bored through solid massive chocks of wood of the same depth as the space between the deck and the boat's floor; C, C are spaces beneath the deck filled up, over 6 feet in length, at the mid-ship part of the boat, with solid chocks of light wood, or boxes of cork, forming a portion of the ballast; D is a section of a tier below the deck, with a moveable hatch or lid, in which the boat's cable is stowed, and into which all leakage beneath the deck is drained through small holes with valves fixed in them.

In some of the later boats a small draining-tap only is placed, having a pump in it, by which any leakage can be pumped out by one of the crew whilst afloat. The festooned lines in fig. 16 represent exterior life-lines, attached round the entire length of the boat, to which persons in the water may cling until they can be got into the boat; the two central lines are festooned lower than the others, to be used as stirrups, so that a person in the water by stepping on them may climb into the boat.

The chief peculiarity of a life-boat is its incapability of being sunk, in consequence of its being fitted with watertight air-cases, or compartments. One of the difficulties of life-boats has been to decide as to the amount and distribution of such air-cases. The necessary space for rowing and working the boat, and for the stowage of shipwrecked persons being secured, the spare space along the sides within-board should be entirely occupied by buoyant cases, or compartments, because, on the boat's shipping a sea, the water, until got rid of, is confined to the midship parts of the boat, where it serves to a great extent as ballast, instead of falling over to the lee-side, and destroying the equilibrium of the boat. Hence, barrels or casks, which do not conform in shape to the sides of the boat, are not well adapted to serve as air-cases. In Mr Peake's life-boat there is a water-tight deck at the load-water-line, and detached air-boxes along the sides, closely conforming to their shape from the thwarts to the deck. Extra buoyancy is also derived from large end air-cases, built across the bow and stern, and occupying from 3 feet to 4½ feet in length, from the stem and stern posts to gunwale height. These cases are chiefly intended to provide self-righting power; but in the event of the boat being stove in, and the space below the deck being filled with water, these air-cases alone have sufficient buoyancy to float the boat.

The second peculiarity of a life-boat is its power of discharging, in a few seconds, any water which may be shipped by the breaking over of the sea, or by the boat being suddenly thrown on her beam-ends. This property does not belong to all life-boats, for, in certain cases (the Norfolk life-boats, for example), the plugs which stop up certain holes in the floors are taken out during a gale of wind, or a heavy sea, so as to let the water into them until it is at the level of the sea. The water thus let in is confined by the wide side-cases to the mid-ships of the boat, where it serves as a loose ballast, and the boatmen consider that it is safest to go off under sail with the boat deeply immersed. The Liverpool life-boats have no relieving holes, so that when filled by a sea, the water must be got rid of by baling. In Mr Peake's boats there is a water-tight deck at the load-water-line, and a number of large open tubes, opening at the surface of the deck, and passing through the space between the deck and the floor; the bottom orifices being furnished with self-acting valves opening downwards, so as to allow any water shipped to escape through them. The deck being placed at or above the load-water-line, any water which is above it will be above the outside level of the sea, so that the water escapes from the deck by its own weight, and disappears in a few seconds.

As a life-boat has very great buoyant power, it is important to her stability and safety to attend to the ballasting. The Greathead life-boats have usually no ballast, their great breadth of beam being relied on for stability; but some of them have a tank in the midships beneath the deck which can be filled with water. Beeching's life-boats are similarly ballasted; but accidents, with loss of life, have arisen from a difficulty in filling the tanks, and preventing the escape of the water when full; hence solid ballast is to be preferred. Mr Peake's life-boats are ballasted with heavy iron keels, and with solid wood and cork ballast, stowed under the decks; and should these be stowed in, and the space beneath be filled with water, the wood and the cork would supply extra buoyancy.

A life-boat ought to be self-righting if upset, a property which, however, belongs only to Mr Peake's and Mr Beeching's boats, some boat-builders considering that stability is sacrificed thereby. The fact, however, has been established in the Life-Boat Journal, that the means employed to produce self-righting add to the stability of a boat, and improve her in other respects. The self-righting power is thus attained:—

1st. The boat is built with considerable sheer of gunwale, the bow and stern being from 1 ft. 6 in. to 2 ft. higher than the sides of the boat at her centre, and the space within the boat at either extremity, to the distance of from 3 to 4½ ft. from the stem and stern posts to gunwale height, is then inclosed by a sectional bulk-head and a ceiling, and so converted into a water-tight air-chamber, the cubical contents of which, from the thwarts upwards, are sufficient to bear the whole weight of the boat when she is placed in the water in an inverted position, or keel upwards.

2nd. A heavy iron keel (from 4 to 8 cwt.), is attached, and a nearly equal weight of light wood or cork ballast is stowed betwixt the boat's floor and the deck. No other measures are necessary to be taken in order to effect the self-righting power. When the boat is forcibly placed in the water with her keel upwards, she is floated uneasily on the two air-chambers at bow and stern; whilst the heavy iron keel and other ballast being then carried above the centre of gravity, an unstable equilibrium is at once effected, and the weight of the iron keel falling over on one side, immediately restores the boat to her proper position; in other words, she self-rights" (The Life-Boat, No. 22).

Lateral stability or stiffness, being the tendency to preserve an upright position in the water, with proportionate resistance to upsetting, is obtained by breadth of beam or by ballast—as in Mr Peake's boats, by an iron keel and other solid ballast, and by flatness and length of floor, with moderate beam only. The other qualities to be required in a good life-boat are speed, strength, and stowage-room, all of which seem to have been well-considered in Mr Peake's boats.

A new description of life-boat, invented by the Rev. E. L. Berthon, M.A., of Fareham, and known as the Fareham Life-boat, has been made the subject of a patent. Its novel feature is, that it is collapsible, so that it combines the pro- property of the life-boat, with facility of stowage in a small space. Hence, it is well adapted for the use of ships, especially large steamers, emigrant vessels, and troop-ships. Its frame-work is of wood, all the timbers extending the whole length of the boat, there being no transverse timbers or ribs. The timbers, four on each side of the stem and keel-piece, are thin, flat, and deep, something like a thin slice of melon; they are made without studding, by bending plank over plank till the required thickness is attained. They are jointed together at their ends, and to the tops of the stem and stern posts by a kind of chain hinge. When the boat is collapsed, these timbers stand side by side in vertical planes, like the leaves of a closed book; but when expanded, they stand apart in radial planes, somewhat like the segments of an orange. Attached to the edges of all the timbers are the water-proof coverings, of which there are two, the outer skin being secured to the outer edges, and the inner skin to the inner edges of the timbers, by which means the whole body of the boat is divided into eight separate longitudinal cells or compartments, which become filled with air on expanding the boat. This is effected and maintained by the bottom boards and thwarts, which being jointed along the middle line, are made to stand up at an acute angle when the boat is collapsed, and fall down to straight lines when open. The inventor compares the principle of extension to that of a carriage-head, the frame of which may be compared to the boat's timbers, and the joints to the thwarts and stretchers of the bottom boards; and as the leather covering of the carriage shuts in when the head is down, so the coverings of the boat shut in between the timbers. The boat has rather a deep keel, besides two bilge pieces on each side, and in every other salient point the covering is protected by wood or copper. The boat is lowered by the following contrivance:—Inside the bulwarks is a large, flat, deeply-grooved sheave about 2 ft. 6 in. in diameter; it has two deep, narrow grooves cut nearly to its axis, and in these are wound separately the ends of the two falls. From this sheave is a projection on which a friction-strap with a powerful lever is made to work. This being placed flat against the bulwarks, the falls are brought to it fore and aft by small sheaves set in the top-rail; thus the friction of the strap when the boat is up is enough to prevent motion; but by slackening the lanyard by which the lever is secured, it may be allowed to descend rapidly or slowly, according to the pressure applied to the break. Rising and falling derricks are substituted for davits.

The average size of the Fareham life-boat is 32 by 10 ft., it has eight thwarts, besides seats round the stern, and will pull, if required, twelve oars double-banked.

Captain Maundy's proposal for throwing ropes from ship to ship in cases of accidents may easily be understood from the methods which he employs for saving lives in shipwrecks. The life-boat by Lieutenant Cook, R.N., F.R.S., Professor of Fortification at Addiscombe College, is related to the same class of inventions; its object is to preserve the life of a person falling overboard in the night, by means of a floating light; and it obtained him a gold medal from the Society of Arts in 1818. (Transactions, xxxvi., p. 121.) He observes that a ship may often have to run half a mile before she can get about and lower a boat, so that it becomes highly desirable to afford a temporary support to the sufferer. The machine consists of two copper spherical air-vessels, with a square tapering tube through each, made water-tight, and united together by a cross piece of wood, in which are two brass conducting tubes through which is fixed a perpendicular tubular-staff, with a brass ferule at each end, and a copper sliding rod, nearly its own length, within it. Attached to the lower end of the rod is a flat circular balance-weight, bearing a chain by which the life-buoy is suspended, and a link which, when hooked to a stud in the lower ferule, bears up the rod and the balance-weight, but which, when unhooked, allows the weight to draw the rod about two-thirds out of the staff. To the head of the perpendicular staff is attached at night a fuse, on a brass fuse-plate, the shank of which is secured into a socket by a thumb-screw. The buoy is secured to the ship by the chain only, the ring of which hangs on the hook of the sheave of the trigger-plate. Attached to the stern of the vessel are two iron rods cased with copper tubing, together with the screw-bolts, from which they are suspended; just above the forked stay which keeps the rods parallel, at a proper distance from the stern, is the trigger-plate, and the brass fuse-case which covers and protects the fuse on the head of the staff. There is also a brass case for the lock or percussion hammer, placed so as to communicate with the fuse-case, by means of the horizontal tube; all these, together with the pulleys and guard-iron are, firmly attached to the stern of the vessel, inside of which, immediately opposite to the pulleys, are fixed the cups and handles, the one for firing the lock and lighting the fuse, the other for raising the trigger-bolt and disengaging the buoy from the ship. As soon as the trigger-bolt is raised, the sheave revolves, the stop turns round, and the life-buoy slides off the rods into the water, bearing on the head of the staff a brilliant flame. The balance-weight, when no longer held up by the chain, drops upwards of 3 feet below the cross-piece, prevents the buoy from upsetting, and affords a place for the man to stand on. This apparatus admits of being lighted and let down into the water in the short space of five seconds. Lieutenant Cook is also the inventor of a plan for converting boats used for ordinary purposes into life-boats at pleasure.

Mr Miller's safety-poles for skaters, and Mr Prior's mode of preventing accidents in descending mines, are mentioned in the Transactions of the Society of Arts (vols. xxxii. and xxxvi.) Apparatus of the latter kind has been introduced at different times with various modifications. In coal-pits, or coal and iron pits, where the men are raised and lowered in a rectangular iron frame called a cage, the rope or chain may break, or the cage may be overwound by drawing it over the framing at the pit's mouth. Mr Robert Blee of Redruth has introduced what he calls a safety-bucket, and Messrs White and Grant of Glasgow have a safety-cage. These inventions depend upon some such arrangements as the following:—Two pairs of eccentrics are attached to the ends of two parallel shafts, which extend across the top of the cage; the edges of the eccentrics are toothed, and when the cage is in motion they are free of the vertical wooden rails which steady the cage in its motion up and down the pit. Should the rope break, two volute springs bring round the thick sides of the eccentrics to bear against the guides, and hold the cage securely. To prevent over-winding, the holdfast which connects the rope to the cage is secured by a curved bolt, kept in place by a strong spring; this bolt moves on a fulcrum, and is continued as a lever beyond the holdfast; across the framing at the mouth of the pit is a bar so arranged that, when the lever comes in contact with it, the bolt becomes disengaged, the cage by the action of the eccentrics becomes fixed, and the rope only is drawn up over the pulley. In Mr Blee's safety-cage the catches allow it to move freely so long as there is a vertical strain on them; but should this cease by the breaking of the rope, the catches become liberated and attached to the iron staves of the ladders placed on either side of the shaft.

A sketch of the expedients which have been recommended for the preservation of mariners, published in a work entitled Shipwrecks and Disasters at Sea (vol. iii., p. 459, Edinb. 1822, 8vo), contains a few further historical details relating to some of the inventions which have been described. The nature and properties of the agent on which vision depends have been objects of philosophical disquisition from ancient times. The earliest speculations which have reached us on this subject are those of Pythagoras, who considered vision as produced by particles continually emanating from the surfaces of bodies, and entering the pupil of the eye. The Platonists, on the other hand, conceived that vision was the consequence of the emission of something from the eye meeting with certain emanations from the surfaces of things; yet, with this very gratuitous hypothesis, they appear to have detected several properties of Light, especially its propagation in right lines, and the equality of the angles of incidence and reflection when it falls on bright and polished surfaces. The effects of the concentration of the sun's rays by concave specula were certainly known to the ancients. Antiquaries have accordingly supposed, that, in this manner, the Romans kindled their sacred fire; and thus also it has been alleged that Archimedes destroyed the Roman fleet at the siege of Syracuse.

Aristotle regarded light but as a mere quality of matter; and he has some ingenious speculations on the rainbow, and on other luminous meteors. Ptolemy the geographer wrote a treatise on optics, which has perished; but, from some fragments preserved by other authors, he appears to have had distinct ideas on the subject of atmospheric refraction.

A long interval of darkness succeeded his era of speculation, until the Arabs began to cultivate the learning of the Greeks, and several of their philosophers treated of optics. But the earliest Arabian work which has reached our times, is the celebrated treatise of Alhazen. In it we find a description of the eye, and of the uses of its different parts. The author details many experiments on refraction, both as exhibited in the atmosphere, and as regards the modifications of light in passing from one medium to another of different density. He likewise notices the magnifying power of segments of spheres of glass; a hint from which it has been supposed that the important invention of spectacles originated. We also owe to him the idea that single vision, with two eyes, is produced by images painted on corresponding points in each retina; and that stars may be seen by refraction, when they are actually below the horizon; remarkable speculations for the twelfth century.

The work of Alhazen was in 1270 commented on by Vitellio, a native of Poland, who added a considerable number of observations on the refractive power of air, water, and glass, which he reduced into a tabular form. He made some ingenious attempts to explain the phenomena of refraction; and he seems to have conceived the true idea of burning lenses.

Roger Bacon, the contemporary of Vitellio, was undoubtedly acquainted with the magnifying property of segments of spheres, and recommended that small segments should be preferred for such purpose; adding, et video hoc instrumentum est utile semibus et habentibus oculos debiles. This so plainly indicates the invention of spectacles, that we cannot doubt that it had then been made. We know that they became common in the thirteenth century, and are described by Spina of Pisa in 1318, although we have no absolute certainty as to the person who first constructed them.

After the revival of letters, one of the earliest cultivators of mathematics was Maurolycus of Messina, who made optics his study. He proved that the crystalline lens of the eyes of animals converges the rays of light which enter that organ, and transmits them to the retina, in or near which the foci of the lenses are situated. Hence also he inferred, that in persons who are short sighted, the defect is owing to the too sudden convergence of the pencil of rays before the retina; and that in those who are long sighted, the foci are placed behind that expansion of the optic nerve. Maurolycus, however, did not discover that the images of objects are painted on the retina.

Baptista Porta, the author of Magia Naturalis, a Neapolitan of rank, was much addicted to philosophic research; to him we owe the first description of the camera obscura, and its application to the delineation of objects. His work contains many observations on light, some of which are accurate; and though some are now found to be erroneous, his remarks are always ingenious. This subject also engaged the attention of Lord Bacon, who complained that the form and origin of light had been too much neglected; but his labours in other branches of philosophy diverted his powerful mind to different objects.

The true theory of the rainbow was first given by Antonio, bishop of Spalatro, although he could not satisfactorily explain the cause of the colours.

The next optical discovery of importance was the telescope, for which we are indebted to Zacchia Jansen, a spectacle-maker of Middleburg, in Walcheren, in 1590; and this important invention was quickly applied by Galileo to physical astronomy, with brilliant success, crowned by the discovery of the satellites of Jupiter, the structure of the Via Lactea, the phases of Venus, the ring of Saturn, the spots on the sun's disk, and a knowledge of numerous stars unknown to former observers.

The study of light was improved under the auspices of Kepler, who gave an explication of the effect of lenses on the rays, and suggested the form of the telescope now called astronomical. He treated of refraction, and discovered, that when light falls within glass at an angle a little above 42°, it is wholly reflected. But his theory of vision is much more important; for he showed that images of external objects are painted on the retina, and appear there inverted, a fact also ably illustrated by Scheiner. The invention of the compound microscope seems also due to Jansen, and dates from the same period.

Perspective was first scientifically treated by Pietro del Borgo, Baldassarre Perussi, and Guido Ubaldi. To the second of these is due the detection of the distance points, to which all lines forming an angle of 45° with the ground line are drawn; to the third, the convergence of all parallel lines inclined to the ground line, in a point in the horizontal line, and also that a line drawn from the eye, parallel to them, will pass through this point. These principles are the foundation of perspective, which afterwards received improvements from Gravescande, and were completed by Brooke Taylor. The true law of refraction is undoubtedly due to Willebrod Snell, or Snellius, professor of mathematics at Leyden. He experimentally showed that the co-secants of the angles of incidence and refraction are always in the same ratio. This discovery, we are assured, on the authority of Huygens, was appropriated by Descartes, who had consulted the papers of Snell, but gave it as his own, under a somewhat different form. The successive labours of Descartes, Kircher, Grimaldi, De la Hire, Hooke, and Huygens, gave to the study of optics a profound scientific character; and the interesting discoveries of that century were crowned by the important researches of our immortal Newton concerning the optical properties of light. During the last century, our knowledge of this subject has been steadily progressive, by the labours of a multitude of philosophers, so numerous, that we can afford space for little more than to record the names of some of the most successful inquirers into the mysteries of this subtle agent. Amongst these, Mairan, Dufay, Mariotte, Boscowich, Euler, Mitchell, Melville, Canton, Bennet, and Lagrange, stand conspicuous; nor must we omit the important fact, illustrated by the labours of Bradley and Roemer, that the velocity of light, from whatever source derived, whether from the sun, the fixed stars, the planets or their satellites, is equal, or that its velocity before and after reflection is the same; a formidable objection to the theory of emission. During the present century, the progress of discovery in this field has been no less brilliant. Very early in it, Dr Thomas Young illustrated the principle of the interference of the rays of light, founded on some facts observed by Grimaldi, but first distinctly stated in Dr Young's Memoir in the Philosophical Transactions for 1803, entitled Experiments and Calculations relative to Physical Optics; and his conclusions have been demonstrated beyond all doubt by the researches of Fresnel and of Sir John Herschel. The splendid talents of Laplace, of Poisson, Biot, Arago, Pouillet, Cauchy, Ampère, and Fresnel, amongst continental philosophers, have especially illustrated the phenomena and theory of light; and in our own country, Sir William Herschel, Young, Brewster, the younger Herschel, Airy, and Whewell, have pursued these delicate investigations with singular ability and success. Above all, we must record, as amongst the most signal triumphs of modern science, the detection and explanation of the polarization of light, and the singular confirmation thereby afforded of the theory of its propagation by undulations. See Sixth Dissertation.

Sect. II.—Nature of Light.

Notwithstanding this long list of splendid discoveries, the nature of light is still in some degree enigmatical. It is admitted, that the phenomena of vision depend upon the agency of a subtile, extremely attenuated matter, set in motion by the sun and other luminous bodies. Its materiality is inferred from its deflection from its rectilinear course, in passing near various bodies; from its being arrested by certain substances, though it passes freely through others; from its reflection by polished surfaces; from its capability of condensation and dispersion, in passing through certain media; from its producing chemical changes in certain compounds; and from its apparently entering into the composition of some bodies, from which it may be again extracted.

Thus far the majority of philosophers are agreed; but two opposite theories have been advanced respecting its propagation, and the mode in which it manifests itself to our senses.

Some maintain that light is a peculiar matter, which is projected in all directions from luminous bodies in a rapid succession of material particles. This theory is sustained by the illustrious name of Newton, and has been very generally received; but of late, certain difficulties in the explanation of the recently-discovered properties of light, especially its polarization, have tended to revive the doctrine maintained by Descartes, Huygens, and Euler, viz., that all the phenomena of light depend on the undulations of a highly attenuated fluid or ether, universally diffused throughout space, which, while at rest, is insensible by our senses, but, when acted on by luminous bodies, is thrown into a succession of waves. Luminous bodies are thus supposed to act on the universally diffused fluid somewhat in the same manner that sonorous bodies do on air in the production of sound.

It is true, that all the known facts regarding light may be explained upon either hypothesis. It must be owned, however, that the remarkable coincidence of fact with theory, and the facility of explanation, favour the theory of undulations, whilst it scarcely can be said to involve any greater assumption than the doctrine of direct transmission. Both assume the existence of a subtile fluid; both admit the influence of luminous bodies; and it does not seem more difficult to conceive them acting, by causing an undulation in the matter of light, than by projecting it in a rapid succession of particles, the minuteness and velocity of which almost elude the grasp of our imagination.

Whichever hypothesis we adopt, the propagation of light is a process of astonishing rapidity. Astronomers have found, from observations on the eclipses of Jupiter's satellites, that planetary light requires about fourteen minutes to cross the earth's orbit; or, if we adopt the more recent and probably more accurate determination of Bradley, that the light of the sun requires about eight minutes to reach our earth; and, if we reckon the mean distance of the sun to be 94,879,956 English miles, it follows, whether we regard it as an emanation or an undulation, that light must travel with a velocity of about 200,000 miles per second.

It is difficult to form any adequate idea of such enormous velocity; but we may approximate it, by comparing this with the ascertained velocity of a cannon ball. A twenty-four pounder, with the common charge of powder, according to Robins, discharges its ball with an initial velocity equal to 1600 feet per second; yet, if such a ball were to continue this velocity undiminished, it would require about ten years to traverse a space which the light of the heavenly bodies pervades in eight minutes.

This prodigious rate, and the ease with which light can penetrate many solid bodies, have been adduced as arguments against the doctrine of the successive emanation of particles from luminous surfaces; but did not the undulatory theory afford an easier solution of certain recently-discovered properties of light, we should not regard such arguments as conclusive. Now, however, the undulatory theory has been shown to correspond so exactly with known facts, and has even enabled us to predict so exactly what experiment has since confirmed, that it has received the sanction of the greatest names in modern science. See Chromatics.

Sect. III.—Properties of Light.

Whichever hypothesis be adopted, light must be considered as a material substance, possessed of certain properties, detected by observation and experiment.

1. Light is given out by luminous bodies in all directions, and from every point of the luminous surface. This is proved by its being equally seen from every point of observation.

2. It is divisible into homogeneous, independent portions, like air or water; the smallest portion we can separate is termed a ray; and several rays form a pencil of light.

3. Light appears to be absorbed by certain bodies, and is again given out by them spontaneously. This property is well seen in the diamond, which, after being exposed to the sun's rays, continues for a short time to shine in the dark. Various artificial phosphorescent bodies, such as the Bolognian stone, calcined oyster shells, and the like.

4. All solids give out light when heated between 700° and 800° of Fahrenheit's thermometer, and are then said to be incandescent. All liquids that can be heated to that point are luminous, as melted metals; and, if the elasticity of their vapours be repressed, other liquids appear capable of incandescence, yet the phenomena attending their sudden condensation, when they enter into chemical union, Light shows that they contain light. Thus, when a mixture of oxygen and hydrogen is suddenly and strongly compressed, the gases unite to form water, and both light and heat are extricated.

5. Some bodies have the property of arresting the progress of light, and are termed opake; others transmit it, and are said to be transparent. Yet probably no substance is either perfectly opake or perfectly transparent. Thus, gold is one of the most condensed and opake bodies in nature; yet if we enclose gold leaf between two plates of glass, and examine it by transmitted light, it appears of a decidedly greenish hue, showing the transmission of some light through the metal. On the other hand, the most transparent glass, when viewed in a thick plate, and the most limpid water, when in a deep column, appear greenish.

6. When the rays of light fall obliquely on the surface of all bodies, whether transparent or opake, solid or fluid, they are more or less reflected. Smooth and shining surfaces reflect most light, but the degree also depends on the nature of the reflecting substance. The reflected rays are returned from the surface at an angle equal to the angle of incidence; and if the reflecting surface be a plane, the parallel rays that fall on it are reflected parallel to each other.

When the rays are reflected from a concave surface, the reflected rays are more inclined to each other than the incident rays; and if that concave surface be the segment of a sphere, the parallel incident rays will converge to a point in the axis of the mirror half way between its surface and the centre of the sphere of which it is a segment; and this point is termed its principal focus, or focus of the parallel rays. If the incident rays be converging, they will meet the axis between the principal focus and that centre; if the incident rays be diverging, they will meet the axis in a point between the principal focus and the surface of the mirror. From the immense distance of the sun, all the incident rays may be considered as parallel; and therefore his rays will be condensed into the principal focus of such a mirror, which will in this instance also be that of greatest heat. This is the principle of burning mirrors. When the incident rays fall on a convex mirror, they are all reflected more divergently, or are dispersed.

The properties of reflected light form the object of the science of Catoptries. See Optics.

We are not, however, to imagine that all the light incident on bright surfaces is reflected. Many curious experiments were made on this subject by M. Bouguer. The quantity of light returned differs with the inclination of the rays to the reflecting surface. It is generally strongest at small angles of incidence; and the difference becomes excessive when the rays impinge on the surface of transparent fluids with different degrees of obliquity. Metals, from their opacity and splendour, form the best reflecting surfaces; but even pure mercury, perhaps the most perfect of reflectors, does not reflect more than three fourths of the whole incident light.

7. When the rays of light fall on transparent bodies, they are differently affected, according to the angle of incidence. When a ray passes from one transparent medium to another, in a direction perpendicular to their touching surfaces, that ray will pass through them in a straight line; but when the ray passes in a direction oblique to their touching surfaces, that ray will be bent, or will form an angle at their junction.

When the density of a medium is uniform, the rays of light traverse it in straight lines; but in a medium varying in density, like columns of liquids, or the atmosphere, in which density increases with the superincumbent pressure, the passage of the rays will form curves.

When a ray passes obliquely from a dense to a rarer medium, it is bent or deflected from a line perpendicular to their contiguous surfaces. When passing obliquely from a rare to a denser medium, they are bent toward the perpendicular. In such instances the light is said to be refracted. In the first instance, the angle of refraction is always greater than the angle of incidence; in the latter it is always less. The study of the properties of refracted light constitutes the science of Dioptries. (See Optics.)

8. The refractive power of different media is unequal; and when the rays of light pass from one medium to another, it may be measured by the ratio between the sines of the angles of incidence and refraction; and the number expressing the ratio between the first and the last is the exponent or index of the refractive power of that substance.

The refractive power of different substances appears to be nearly in the ratio of their density; but with inflammable bodies, or those containing an inflammable principle, the refractive power is in a ratio greater than their density. It was this law which led Newton to his happy conjecture that water and the diamond might contain an inflammable principle; speculations which have been verified by modern chemistry. The same opinion as regards the diamond was long before maintained by Boetius de Boodt. He says that unctuous and fiery bodies are easily united, but will not mix with watery substances; and because the diamond readily adheres to resins, which are of a fiery nature, and because, like amber, another fiery body, the diamond, when rubbed, attracts light bodies, the diamond itself must be of an inflammable or sulphureous nature; an argument which he considers as confirmed by that gem being "produced in a hot, sulphureous climate." It is obvious that the deduction of Newton differs widely in its principles from the hypothesis of De Boodt; but the latter must be regarded as a curious instance of a true conclusion derived from unsound premises.

9. If a pencil of light be admitted by a small hole in the window-shutter of a darkened room, through a triangular prism of any transparent substance, the white light will be found to undergo a remarkable change. The rays will be separated in the prism; their image will be enlarged; and, if received on a white screen, they will be seen variously coloured. The colours will assume a certain determinate order of juxtaposition; and this appearance has been termed the prismatic spectrum. This coloured spectrum will then be seen divided into colours, of which Newton enumerates seven; red, orange, yellow, green, blue, indigo, violet. These, it is evident, may be resolved into red, yellow, and blue; for the boundaries of the colours are not well defined, and the compound colours which lie between these may be considered as made up of the intermixture of contiguous rays. These rays are not in equal proportions in the spectrum. If we consider it as divided into 360°, the red occupies 45°, the orange 27°, the yellow 48°, the green 60°, the blue 60°, the indigo 40°, the violet 80°; and it is worthy of remark, that this division of the scale of colour is a striking approximation to the divisions of a chord that would give the musical intervals of the octave. But as these colours are not bounded by defined lines, but graduate into each other, it is very difficult to determine their relative extent with tolerable precision. The cause of their separation is the difference of their refrangibility by the prism; the red being the least, the violet the most refrangible, that is, turned from the line of the incident pencil of light. The green ray will be found in the centre of the prismatic spectrum; and hence its index of refraction is considered as the mean refraction of the substance of which the prism consists. Newton employed in his experiments prisms of different substances; but he seems to have taken it for granted, that when the mean refraction was the same, the length of the spectrum was also equal, or that the dispersive power of the bodies in that case was equal. He considered that prisms and lenses of every kind of glass, Light and of all bodies, whether solid or fluid, with the same mean refraction, possessed also the same dispersive power, or formed spectra proportional to their mean refraction. Hence he was led to conclude that "the improvement of the refracting telescope was desperate." This error has, since his day, been detected; and the principle discovered forms the basis of Dollond's admirable invention of the achromatic telescope, in which the error of refraction in one species of glass is ingeniously remedied by a correction derived from the different dispersive power of another kind of glass, so adapted to the first as to form with it one object-glass. The difference in dispersive power has now been ascertained in a considerable number of diaphanous bodies; and tables of this difference have been formed from the observations of many philosophers, particularly of Sir David Brewster. See Chromatics.

The illuminating power of the different rays of the spectrum is different. Sir William Herschel found, that with a prism of flint-glass, the greatest illumination is towards the middle of the spectrum; the yellow rays affording most light, whilst the illuminating power, diminishing towards each end of the spectrum, is least in the violet ray. A series of experiments on this subject by Fraunhofer, a late celebrated instrument-maker of Munich, showed, that with the best made prisms, when other light is carefully excluded, the most luminous point is nearer the red than the violet end of the spectrum, in the proportion of one to four; and he states the mean refrangibility to be between the blue and indigo rays. But one of the most curious discoveries of this ingenious inquirer is, that the solar spectrum is traversed by numerous dark lines of unequal thickness, perpendicular to the length of the spectrum, and parallel to one another. These lines require a fine prism for their exhibition, a microscope for their detection, and the exclusion of light, except that of the coloured ray under examination. He counted 590 of these lines in the spectrum; the greatest number of them being towards the most refrangible end of the spectrum.

It is well known that the rays of the sun communicate heat as well as light; but the heating power of the coloured rays is very different. Herschel discovered that the red ray raises the thermometer most; and that the effect diminishes as we approach the other end of the solar spectrum. This is sufficiently striking; but in pursuing his investigations he made another singular discovery, that the point of greatest heat is fully half an inch beyond the red end of the prismatic spectrum. Delicate thermometers were placed in the different rays, and gave the following results:

| In the blue ray in 3 minutes | it = 56° F. | |-----------------------------|------------| | Green | 3 | = 58° | | Yellow | 3 | = 68° | | Middle of the red | 2½ | = 72° | | Outer confines of red | 2½ | = 73° 5' | | Half an inch beyond the red | 2½ | = 79° |

These curious experiments were confirmed by Sir Henry Englefield and Sir Humphry Davy. The inference from them is, that light and heat are unequally refracted.

The prisms used by these philosophers appear to have been of flint-glass; but Dr Seebeck has since found that the position of the point of greatest heat varies with the nature of the refracting prism. Seebeck found, as Herschel did, that with flint-glass the greatest heat was beyond the red; with plate-glass, in the middle of the red; with sulphuric acid in a hollow thin glass prism, in the orange; with water, in the yellow. The sun's rays would appear to be still more complex. Early in this century, Ritter of Jena found that the rays of the solar spectrum possessed different chemical powers. He found that the salts of silver became soonest black a little beyond the violet end of the spectrum, a little less so in the violet, and still less so in the blue; and Seebeck, in repeating the experiments, found that beyond the violet ray muriate of silver became reddish brown; in the blue, bluish gray; and in the yellow it retained its white colour, or at most had a yellowish tint; it became reddish in the red ray, and even when placed beyond it. These changes might have been attributed to the influence of heating or illuminating power, had not the greatest deoxidating effect been observed where the heat and illumination are the least. Dr Wollaston, who observed these facts about the same time with Ritter, considers the sunbeams as compounded of calorific and deoxidizing as well as luminous rays, all with different degrees of refrangibility. But if the experiments of Morichini be confirmed, the sunbeams have also the property of magnetizing steel. In the year 1813 he announced this discovery; it was repeated by several persons without success, but Mrs Somerville appears to have succeeded. She covered one half of small sewing needles with paper, in Morichini's method, and exposed the naked half to the violet rays for two hours, when she found that the needle had thus acquired a north pole. The indigo ray produced nearly the same effect, but the effect was feeble in the blue and the green; whilst, though exposed in the orange, yellow, and red, for two successive days, no magnetism was induced. Similar effects followed when one half of the needle was enveloped in white paper, and the other half, exposed to the rays, was covered with blue or green glass, or with silk of those colours.

10. The facts already noticed respecting the bending of the rays toward the perpendicular, when they pass from a rare to a denser medium, lead to the inference that the disposition of the surfaces of the refracting medium must materially influence the direction of the rays of light which enter and pass through. Accordingly, it is found that if one or both surfaces be convex, the rays are bent toward the axis of the medium. If the medium be spherical, or a segment of a sphere, the ray, falling perpendicularly on its centre, will pass straight through; but all those that fall obliquely on the spherical surface will emerge from the medium in a direction inclined to the central ray (which may be considered as the axis of the medium), and will cut this axis in some point, which is termed the refracted focus of those rays. When the medium has its surfaces forming segments of spheres, it is called a lens; and lenses are divided into convex and concave, plano-convex and plano-concave, double convex and double concave, according to the form of their surfaces.

When a lens is nearly or really spherical, optical principles will show, that all the emergent rays will not meet in the same point, and that those farthest from the axis will meet first. But if the lens be a thin segment of a sphere, with one or both of its surfaces convex, this error will not be very conspicuous, and the emergent rays will meet nearly in one focus.

When both surfaces of a lens are concave, or when one is concave and the other plane, the emergent rays will be bent from the axis. The mathematical demonstration of these facts, their application to practical purposes, and to the explanation of natural appearances, belong to Optics; to which article attention is directed.

11. Some crystallized bodies have the property of dividing the rays of light which permeate them into two distinct portions, one of which passes in the ordinary direction, whilst the other pencil undergoes an extraordinary refraction, passing at some distance from the other. Hence, when any body is viewed through such a crystal in a certain direction, both sets of rays become apparent by giving a double image of the object. This curious property was first detected by Erasmus Bartholin, in calcareous spar brought from Iceland. But the subject was first philosophically investigated with his usual sagacity by Huygens, Light who proved that the property of double refraction was not confined to calcareous spar; and it has, since his time, been shown, that all crystals, the primitive form of which is neither a cube nor a regular octahedron, possess this property. Newton attempted to explain this double refraction; but his explanation was not happy. He ascribed it to an original difference in the rays of light, by which some are refracted in the usual manner, whilst others undergo unusual refraction. Huygens discovered, that when the ray of light was received through the Iceland crystal in any direction but one, it was always divided into two rays of equal intensity; but he remarked with surprise, that when he received the divided rays through a second crystal of Iceland spar, the two portions into which each of them was now subdivided were no longer equally intense; that their relative brightness depended on the position of the second rhomb with regard to the first; and that there were two positions of the second, in which one of the rays vanished altogether. This Newton supposed to depend on the rays having different sides, possessed of different properties, each of which "answers to or sympathizes with that virtue or disposition of the crystal, as the poles of two magnets answer to one another."

This idea was followed up by Malus. He conceived that the molecules of this modified light have all their homologous sides in the same direction; and he expressed this modification of light by the term polarization, as he compared the effect produced to the influence of a magnet, which directs the poles of a series of needles all to the same side; an hypothesis which Biot modified by supposing that each molecule of light had one axis, similarly placed in each, and all turned in one direction, in a polarized ray; whilst the molecules were conceived to have a free motion round such axes, by which they could assume different positions according to the attractions and repulsions they experience at the surface of each new medium they traverse. The term polarization is not certainly very happy, and it is to be regretted that one more appropriate and less hypothetical had not been employed.

If the rays, thus divided into two pencils by calc spar, be received by a rhomb of the same substance, whilst the axes of both crystals are in the same direction, no new division of the rays takes place; but if, whilst the first crystal remains at rest, the second be turned round, by the time it has made one eighth of a revolution the rays will be again subdivided, and four images will be produced. By continuing the motion until the crystal has described one fourth of a revolution, the subdivision will again disappear.

Malus discovered that an analogous effect was produced by reflection. If a pencil of rays fall on a polished surface of glass at an angle of $35^\circ 25'$, it is reflected at any angle equal to the angle of incidence. If we now place another plate of glass in such a position that the rays reflected from the first shall fall on the second also at an angle of $35^\circ 25'$, or when the plane of both reflections coincide, the rays will also be reflected from the second plate; but if the second plate be turned round one quarter of a revolution, so as to make the plane of the second reflection perpendicular to the plane of the first, the whole of the rays will now be transmitted through the second plate. When this plate has described half a revolution, the rays will be reflected as at first; and when it has made three quarters of a revolution, they will again be transmitted, that is, when the planes of reflection are parallel, light is reflected, but when they are perpendicular, it is transmitted; or light in such circumstances can permeate glass in one direction, but not in another. Sir David Brewster, soon after Malus, began a vast series of experiments to determine the angles of polarization of different media, and to investigate the general law which regulates polarization by reflection from transparent bodies, which was crowned with the beautiful discovery that "the tangent of the angle of polarization is equal to the refractive index," or that when a ray is entirely polarized by reflection, "the angles of incidence and refraction are complementary." In this sketch it would be impossible to do justice to the investigations and beautiful theoretical deductions of Fresnel, which have combined the whole into an inductive science. We must direct the reader to the article on Optics for his important labours, as well as for the profound researches of Airy, Poisson, Biot, Arago, and Cauchy.

Newton rejected the explanation of double refraction offered by Huygens, because he considered the apparent polarization of the rays of light as inconsistent with motions "propagated through a fluid medium;" but this arose from his limiting his ideas of luminous vibrations, as entirely analogous to those of producing sound in air, in which they are propagated in the direction of the advance of the undulations. We owe to the late Dr Thomas Young the first idea of the vibrations being transverse to the direction of the luminous wave; an hypothesis which he illustrated by the propagation of the vibrations of a stretched cord put in motion at one of its ends. This happy idea has been shown to be a necessary consequence of the phenomena of the interference of polarized light, if we admit the theory of luminous waves. The subsequent investigations of Arago and of Fresnel have confirmed the speculations of the English philosopher, which have connected and elucidated those brilliant discoveries that have conferred lustre on the names of Malus, Fresnel, and Brewster. See Optics.

**SECT. IV.—COLOUR OF OBJECTS.**

The discovery by Newton of the colours produced by the decomposition of the sun's rays, naturally turned the attention of that profound philosopher to the cause of colour in different objects; and he has delivered a theory of colours, of which we shall now exhibit an outline.

1. Newton regards the colour of natural objects, not as produced by any modification which light undergoes from refraction or reflection at their surfaces, but as something inherent in the rays according to their different degrees of refrangibility. The same degree of refrangibility invariably gives the same colour; and when the rays are fully separated from each other by the prism, he found it impossible to change the colour. Thus he refracted the red ray with prisms, but found its tint unaltered; he reflected it from bodies which in daylight had other colours, but still it remained red; he transmitted it through coloured media of different tints, he passed it through the coloured rings produced by pressing together plates of glass, but he was unable to convert it into another colour. By condensation or dispersion he could render it stronger or fainter, but still it remained red. Similar experiments on the other rays were attended with similar results.

2. He found, however, that by mingling the different rays of the coloured spectrum, he could produce a sort of intermediate tint. Thus the intermixture of the yellow and red rays formed an orange, and that of the yellow and blue, a green. But this effect was only distinctly produced by the intermixture of contiguous rays; if they were far removed from each other in the spectrum, no such effect was produced. Thus the orange and indigo rays do not produce an intermediate green.

3. The intermixture of all the rays reproduced white light. Newton reflected a pencil of rays through a prism into a dark room, and then interposing a lens of three feet radius, about four or five feet distance from the aperture admitting the light, he collected the convergent rays upon a paper screen, and obtained an intense spot of white light. By moving the paper he could easily find the point of perfect whiteness; and, by drawing it farther from the lens, he could reproduce the coloured spectrum in an inverted order, as the crossing rays diverged farther from each other.

If any of the coloured rays were cut off before their convergence by the lens, the image upon the paper exhibited colour; and if either of them were made to predominate, that tint was rendered perceptible. Newton endeavoured to show the same with mixtures of coloured powders; and though this method presents mechanical difficulties not easily overcome, and the mixtures only afford a gray shade, yet when these were strongly illuminated by concentrated solar light, they became of a dazzling white; whilst all coloured objects appeared most splendid in the prismatic rays of their own colour. From these facts he considered the colour of objects to depend upon the predominance of the coloured rays they reflect. Thus minium, or red lead, appears red, because it reflects principally the least refrangible rays; a violet appears of the colour so denominated, because it chiefly reflects the most refrangible rays; and what we denominate the colour of an object is merely the hue of the rays which it most copiously returns to the eye.

On the other hand, transparent bodies which have colour, when held between the eye and the light, appear so by transmitting most copiously that ray. Thus, too, we see why a body not quite transparent may sometimes appear of different colours by transmitted and reflected light. Such a body may transmit most copiously the blue rays, and reflect the green ones; as we often find in coloured liquids, and sometimes observe in the crystals of fluor spar, and other mineral substances. This fact is a confirmation of the Newtonian theory of colour; for, were the colour inherent in the substance itself, it ought to appear equally by either mode of viewing it.

In transparent coloured liquids the shade often varies with the thickness of the column through which the light is transmitted. Thus a clear red liquid in a conical wine glass appears below of a pale yellowish hue; higher up it seems orange; and only has its full red hue when the column is of considerable thickness. This is owing to the most refrangible rays never being able to penetrate the liquid at all. The remaining part of these rays gives the yellowish colour to the thin film at the bottom of the glass; the separation of part of the yellow rays gives an orange tint to the next film of liquid; and, when the yellow rays are wholly stopped by the thicker column, the red, or least refrangible, come undiluted to the eye of the observer, and give their colour to the body of the liquid in the glass.

See Chromatics.

Sect. V.—Relation of Light and Heat.

It is well known that light and heat are intimately mixed in the beams of the sun; that some bodies give out both light and heat during combustion; and that a high temperature causes the extrication of light in all bodies, the gases excepted. This intimate relation between light and heat has induced some philosophers to consider them as mere modifications of each other. Certain it is that they have many properties in common. They are capable of reflection, of refraction, of concentration, of dispersion, and of polarization; they radiate between distant objects with great celerity, they penetrate solid bodies very readily, they are absorbed by dark and rough surfaces, are generally reflected by smooth surfaces, and they are capable of subverting some chemical combinations.

These properties show very striking analogies; and the phenomena of the polarization of heat, so well illustrated by Professor Forbes of Edinburgh, have undoubtedly rendered this analogy still more apparent: yet in the present state of our knowledge, it would be rash to pronounce that they are absolutely identical. Their total separation, at least as far as our means of detection extend, in some instances; the very different substances which permit or retard their progress; and the different manner in which they affect our sensations, have led some inquirers to the opposite conclusion; and though it may still be true that they are modifications of the same kind of matter, it is safest to content ourselves with pointing out those circumstances in which they differ, as well as their general agreements.

1. Light and calorific are not intercepted by the same substances. If we interpose a plate of thin transparent glass between the face and a bright blazing fire, the intensity of the light has no apparent diminution, but the calorific rays seem to be immediately arrested; and if we make the experiment with a thin diaphanous plate of ice, they seem absolutely intercepted. Some recent experiments of Professor Forbes, with thin plates of ice, afforded an almost microscopic effect on the galvanometer of Melloni's apparatus; but supposing there was no minute hole in the ice, the difference of the transmission of light and heat through diaphanous ice is sufficiently striking. The same takes place with all other species of terrestrial light, though, as we shall presently see, the calorific rays of the sun instantaneously pervade ice.

2. The rays of calorific are more powerfully reflected from a metallic mirror, even of an imperfect shape, than from the best glass mirror; whereas the latter very powerfully reflects light. Dark and dense solids are very readily penetrated by calorific, though they are totally impervious to light.

3. Light affects the organs of vision in a peculiar manner, without producing inconvenience to that most delicate organ, the eye; but a radiation of heat without light, as from a vessel of boiling fluid, though the rays entering the eye may be so powerful as painfully to affect that organ, does not produce any thing analogous to vision.

In the sun's rays, however, heat and light are so intimately blended, that we cannot entirely separate them. The difference in their refractive power causes a partial separation in the coloured spectrum; but both the heat and light of the sun's rays seem to pervade glass or ice with equal facility. Leslie's photometer, placed behind a sheet of diaphanous ice, is immediately affected by the direct rays of the sun, and a lens of transparent ice will concentrate them, so as to fire combustibles; as was long ago observed by Jan Metius and Descartes, and has more recently been proved by Scoresby. The difference between the calorific influence of the sun and of artificial fires has been attributed to the different initial velocity imparted to the calorific emanations in both cases. This is not improbable; but some have considered the calorific influence of the sun's rays as an effect of the condensation or fixation of light. This was the idea of the late celebrated Sir John Leslie, and is the principle of his elegant photometer.

Sect. VI.—Measures of Light.

Various methods have been proposed for affording comparative measures of light. The principles of these depend either on the illumination, as ascertained by the distance at which we can distinctly perceive small objects, such as printed letters of a certain size; the comparative depths of the shadows of an opaque object; or the heat excited by the luminous emanations of the bodies compared.

1. The distance at which the same eye can read a particular printed paper forms certainly a good criterion of the comparative degree of light given out by two or more luminous bodies, at the moment of comparison; but as it must greatly vary with the goodness of eye, it obviously cannot afford the basis of a general scale of illumination, by which the same individual can compare his observations at distant periods, or render the experiments of one person comparable with those of another. Still it is a convenient method, and requires but a very simple apparatus; a tube to admit the light in an uniform manner to the paper, and a graduated sliding rule to ascertain with ease the distance of the paper from the eye.

2. The comparison of shadows, which appears first to have been employed by Bouguer amongst several other ingenious contrivances, was the mode recommended by Count Rumford, who, in the year 1794, read a paper on this subject to the Royal Society of London. It is reprinted in a volume of his Philosophical Papers, published in 1802. This instrument, though well suited to the object in view, is cumbersome, and somewhat complicated. The photometric part is a box eight inches wide; its back a plate of glass, covered by tissue paper, on which the shadows are projected. It is supported at a convenient height by a tripod stand. The table consists of two narrow arms c d, resting on b at one end, and kept horizontal by feet at the other, intended to support the moveable brackets e e, on which are placed the lights to be compared. The arms are divided into decimals of an inch; and are here represented on a smaller scale than the rest of the instrument.

Similar results may be obtained by the following contrivance, proposed, we believe, by Dr W. Ritchie. It consists of a rectangular box of brass, a, b, c, d, three inches long, and 2½ inches wide. In its centre are two plane glass mirrors, g g, two inches square, cut from the same plate, and placed accurately at angles of 45° to the base of the box, as in the diagram. Each end e f of the box is open, and has, at equal distances from the mirrors, two cylindrical wires of brass, h, h, 0·2 inch in diameter, fixed vertically in the centre of the box. The top of the box consists of two thin plates of glass, i i, on which is pasted tissue paper. The inside of the box and the wires are blackened. When the two lights to be compared are placed before the ends of the box, and in its axis, the shadows of the wires will be reflected from the inclined mirrors on the tissue paper. The adjustments of the lights to the machine may be conveniently made by sliding brackets, placed on a long and steady table. If one of the lights be fixed, the other is to be moved backwards or forwards, in the line of the axis of the machine, until both shadows of the wires shall be of equal intensity. Thus, as the intensity of the light, in such cases, is inversely as the square of the distance of the luminous body, the difference between the position of each light, ascertained either by a graduated fillet on the table, or by a common Gunter's scale, will afford a numerical value of the comparative intensity of each light. The method appears sufficiently accurate for such experiments, but, like the former mode, is not susceptible of a fixed scale, unless we could find some uniform unvarying light to be considered as a standard.

3. The calorific influence of luminous matter was proposed as the measure of the light by Lambert, and was adopted by Sir John Leslie as the principle on which he constructed his photometer. It is the author's differential thermometer, with one of its balls made of black enamel, whilst the other is of clear glass. An instrument so prepared, when exposed to a heating cause, has its balls unequally heated. To prevent the influence of currents of air, the whole is covered with an air-tight case of transparent glass. The black ball absorbs the calorific rays which impinge on it; the air within it expands, and raises... the coloured liquor in the opposite stem of the instrument, to which a scale of equal parts being attached, each equivalent to \( \frac{1}{36} \)th of a degree of the centigrade thermometer, affords a numerical result; and if we were sure that the intensity of the light is always in proportion to the calorific effect, the instrument would be a perfect photometer. But, unfortunately, we now know that this is not the fact, especially when we compare different kinds of light by means of this instrument. Thus the influence of a fire, so dull that it is impossible to distinguish a letter of a printed page, will affect this photometer at the distance of several feet, more than the diffused light of day sufficient to enable one to read the same book with facility; and it is more affected by radiation from a piece of iron scarcely incandescent in the dark, than by the intense light of phosphorus burning in oxygen gas. Even with the light of the sun refracted by the prism, the photometer does not indicate the point of the maximum of light. The greatest illumination is in the yellow rays; but this photometer rises highest when in or just beyond the confines of the red.

But if we employ the instrument for the purpose chiefly in the view of its ingenious inventor, the measure of the intensity of solar light, this beautiful instrument appears to us the most elegant and useful photometer hitherto proposed. Its delicacy is such, that when freely exposed, in our climate, to the light of the sky, without being acted on by the direct solar rays, it generally ranges in summer from 30° to 40°, and in winter from 10° to 15°. Exposed freely to the sun-beams at noon in summer, it usually mounts to between 80° and 90°; and in the depth of winter is generally about 25°. In the glowing language of its inventor, "the photometer exhibits distinctly the progress of illumination from the morning's dawn to the full vigour of noon, till evening spreads her sober mantle. It marks the growth of light, from the winter solstice to the height of summer, and its subsequent decay through the dusky shades of autumn; and it enables us to compare, with numerical accuracy, the brightness of distant countries—the brilliant sky of Italy, for instance, with the murky air of Holland."

**SECT. VII.—EVOLUTION OF LIGHT WITHOUT APPRECIABLE HEAT.**

The most familiar instance of this phenomenon is in the rays of the moon, planets, and fixed stars, in the beams of which the most delicate instruments, even the thermomagnetic combinations of Melloni, have been unable to detect any calorific effect. In the beams of the moon and planets, the greatest portion of the incident heat would probably be absorbed by the dark nucleus of those celestial bodies; and if any heating rays were emanated from them towards us, they probably are far too attenuated to produce sensible effects at our planet. The light of the fixed stars, though probably like that of the sun, radiates through too enormous a distance to become sensible to any instrument for measuring heat hitherto contrived. The luminous meteors, too, that belong to our atmosphere, have in general no sensible heat; if we except meteoric stones and condensed electricity or lightning, which has occasionally fired combustibles. We must not confound the effect of the aurora borealis on the magnetic needle with heat; it appears to be altogether magnetic, not calorific. We find, also, that certain terrestrial bodies have the power of emitting light, in some instances largely, without a corresponding degree of heat; and such are usually termed *phosphorescent*. Some of these have the property of absorbing light when exposed to it, and again visibly emitting it. Some become phosphorescent when slightly heated; others give out light during their spontaneous decomposition. Phosphorescence has been examined by Bartholomii, Fabricius ab Aquapendente, Bayle, Algarotti, Reaumur, Father Beccari, Father Bourges, Abbé Haller, Leroy, and Canton.

1. Many bodies, when exposed to light, particularly that of the sun, absorb it, and emit it immediately on being removed into a dark place. When a diamond of some size is thus exposed, it has been observed to give out flashes of light in the dark for a short period, and it recovers this property on a fresh exposure. Several other precious stones, some calcareous minerals, almost all animal and vegetable substances, when very dry, or after solution in nitrous acid, and even snow, are stated by Beccari to possess the same property in a greater or less degree. Several artificial compounds, when carefully calcined, have the same effect. This is particularly the case with the Bolognian stone, and with Canton's phosphorus. The former is a calcined sulphate of baryta, found at the foot of Monte Paterno, near Bologna. Its properties were first discovered by Vincenzo Casciarolo, a shoemaker of that city, who, from its weight, mistaking it for a metal, attempted its reduction. The inventor kept the process secret, but it appears to have at length transpired. According to Kircher, the stone was reduced to a fine powder, beaten up with whites of eggs or linseed oil, and formed into a paste, which was repeatedly baked in a furnace. The Bolognian stone, as the preparation was called, has a very powerful phosphorescence, of a reddish colour; and the Italian preparation generally has this quality in a higher degree than the imitations prepared elsewhere; but it has been quite eclipsed by the phosphorus of Canton. Canton recommends oyster shells which have been long worn on a sea-beach, as the materials to be employed. They are to be calcined in a good coal-fire for half an hour. The purest parts are then to be collected, and reduced to a fine powder. Three parts of this, with one of sulphur, are to be rammed into a crucible about \( \frac{1}{2} \) inch deep, till nearly full. Place it in the midst of the fire, where it must be kept red hot for at least one hour, and then allowed to cool, when the contents are to be removed from the crucible. The fine portions of this, which will be quite white, are to be scraped off, and immediately enclosed in a bottle with a well-ground stopple.

When this bottle is exposed for a short time to the light of day, to any artificial light, or, better, to the direct rays of the sun, it will be luminous for some minutes in the dark; and its light will be renewed by a fresh exposure to the sun. At one time it was a subject of controversy, whether or not these substances emitted only the light they had imbibed by exposure, as it was conceived to be a ready mode of deciding the dispute between the followers of Newton and Descartes respecting the nature of light. Galeazzo, Zanotti, and Algarotti of Bologna, tried whether, when exposed to the different rays of the prism, the Bolognian phosphorus would only show the colour of that ray to which it had been exposed; and they thought that its light was reddish, to whichever ray it had been previously exposed. But in these experiments its light was very feeble, and therefore not satisfactory. Afterwards, however, Father Beccari of Turin, by exposing pieces of more powerful phosphori in tubes of different coloured glass, found that, in the dark, they only emitted the colour of the light to which they had been exposed.

Van Helmont appears to have discovered another powerful phosphorus; and Baldwin of Misnia, in 1677, found that the residuum of a solution of chalk in aquafortis, after distillation, formed a phosphorus of considerable power, but inferior to that of the Bolognian. Du Fay, in 1734, found that similar properties resided in gypsum, marble, and topaz. The emerald, diamond, and many other precious stones, he found to have the same property, without calcination, and by mere exposure to light. From the experiments of Margraaf, all the earthy sulphates have this property when calcined; but he thought that neither metals, metallic ores, nor agates, possess it. The analysis of topaz shows that it contains fluoric acid; and we may now generalize the observation, and state, that all substances capable of becoming phosphorescent by calcination contain some fixed acid, and probably all minerals containing such acids are capable of becoming, in like manner, phosphorescent.

The experiments of Canton are the most complete on this subject (Phil. Trans. lviii.). When his phosphorus was, for a short time, exposed to the light of a candle, the moon, or the diffused light of day, it shone for a considerable time in the dark. When exposed to the direct rays of the sun, it gave out light for two hours, at the common temperature of the air. When it had ceased to shine in the dark, the application of heat renewed its luminousness for a short time. If the glass containing the phosphorus be placed in boiling water, its luminousness will be stronger than in the cold, but will last a shorter time. When it has ceased to give light in hot water, it will again give out light on being placed on a hot iron between 400° and 700° Fahrenheit.

2. Some natural bodies become phosphorescent by a gentle heat. Thus, some kinds of fluor spar, particularly the coloured varieties, give out a pure greenish or a bluish light by being heated; and this is finely exhibited by the green varieties. The mineral called phosphorites, which is a fibrous phosphate of lime, found principally in Spanish Extremadura, gives out much light when heated. Some marbles, some ores of metals, coal, wax, butter, oil, and several other mineral and vegetable substances, so treated, become more or less luminous. In some the light is momentary, in others it lasts several minutes. It soon attains its maximum brightness, and then fades away. A stream of cooler air extinguishes the light for a moment, but it re-appears on the ceasing of the cool current. Analogous to this class of bodies in some degree are those substances which give out light on percussion; such as siliceous minerals, either with one another or with steel, hard porcelain, or the like; but they also, in such collisions, emit heat as well as light.

3. Mineral and vegetable bodies, during their decomposition, often give out light. Fish, mutton, and rotten wood, are the best known instances of phosphorescent bodies of this class.

The luminousness of fish is well known; and Dr Hulme has shown that the light of herrings and mackerels begins to appear whilst the fish is still eatable, and soon arrives at its maximum, but begins to decrease when they pass to putrefaction. To produce this change, the fish should be kept in a dark and cool place. It is not confined to the skin of the animal, for if cut into pieces, the surface of each piece becomes luminous; and it is often seen within the mouth of the fish. The luminous matter easily rubs off, and may be transferred to the hands of him who touches the fish. This light is not attended by any perceptible heat. When scraped off, it forms a gelatinous liquid, that will shine for several days, if preserved in a phial. The addition of fresh water, lime water, water impregnated with carbonic acid, of vegetable acids and alkalies, extinguishes it, as do neutral salts, infusions of pepper, and camphor, when strong; yet the same substances, in a weak solution, seem to promote it, and even render it more durable; but sea-water increases its splendour. This luminous property is also found in lobsters and in testaces, especially in the *Photos Dactylus*, and its congeners. This animal is luminous when quite fresh, and is mentioned by Pliny as rendering the mouths of those who eat it luminous. The light is readily imparted to milk and sea-water, but it is extinguished by spirit, wine, or vinegar. Sea-water, thus rendered luminous, increases in brightness by a gentle heat; but when heated to 133° Fahrenheit, it is suddenly extinguished, and cannot again become luminous. This luminous matter, when narrowly examined, sometimes appears to give out a sort of lambent flame, which closely resembles that of a solution of common phosphorus in oil; and it smells of phosphorus or of phosphuretted hydrogen. The flesh of *Mammalia* undergoes similar changes during its decomposition. This has often been seen in mutton, beef, and veal. This light has sometimes been observed on corpses, much to the terror of the vulgar; and in vaults where dead bodies have been deposited, it has sometimes been observed in a glairy matter adhering to the vault. This last matter, however, probably is the produce of some cryptogamian plant; and it is well known that rotten wood is sometimes highly luminous. A light of this kind is stated to have been observed round the body of a woman at Milan, but it flitted from the bed on the approach of the reporter. This appearance has been more frequently seen around graves, and has obtained in Scotland the name of *elf-candles*.

The light from corrupting animal and vegetable matter requires oxygen in some form for its continuance. It is soon extinguished in the exhausted receiver of the air-pump, as Mr Boyle long ago observed; but it would seem that the small quantity of air which is contained in water is sufficient to sustain its luminousness. No perceptible heat is extracted in any of these kinds of phosphorescence.

Analogous to the light given out by decaying organized bodies, is the curious meteor termed *ignis fatuus*, or *will-o'-the-wisp*. Its ordinary appearance is like the faint flame of a taper; sometimes it resembles the light of a torch, or a faggot; but it usually recedes as it is approached, and can rarely be observed near at hand. The colour of the light is usually pale bluish, and seems brightest when most distant. It is most frequent in marshy grounds, in churchyards, or where a considerable mass of animal and vegetable putrefaction is going on. Dr Derham once observed an ignis fatuus playing round the head of a dead thistle; and, by cautiously approaching, he got within two or three yards of it, when a slight movement of the air made it fit; and when he pursued it, he was unable to overtake it.

A remarkable appearance of ignis fatuus was, about a century and a half ago, common in the vicinity of Bologna, which has been well described by Beccari (Phil. Trans. vii.). He estimated that two, which at that time appeared almost every dark night, one to the east, the other to the north, of the city, gave light equal to an ordinary faggot. One of them accompanied a friend of Beccari for a mile along the road to Bologna, giving as much light as that of the torch carried before him. Sometimes these meteors divided into several parts, or floated like waves of flame, dropping small scintillations.

Dr Shaw, the author of *Travels in the Holy Land*, describes a remarkable one which accompanied him, for upwards of an hour, in one of the valleys of Mount Ephraim. Its shape was at first globular, but it afterwards spread so as to involve the party of the traveller in a pale inoffensive blaze, then disappeared; again it re-assumed the globular form, and again expanded itself; at certain intervals, over more than two or three acres of the adjacent mountains. The atmosphere that evening had been very hazy, and the dew, as it fell on their bridles, felt unusually unctuous and clammy; a kind of weather, says Shaw, in which sailors observe the balls of fire that flit about the masts and yards of ships.

The cause of ignis fatuus has been disputed. It can scarcely be accounted for by the phosphorescence of the glow-worm, or any species of fire-fly. It differs also from electric flame, but has the greatest resemblance to the flame of phosphuretted hydrogen; a gas which spontaneously inflames, on coming into contact with air, and which is given out during the corruption of organic matter. This gas is absorbable by water and by fatty oils, to which last it imparts its phosphorescent qualities; and perhaps the luminousness of fish may depend upon the union of this substance with oily or mucous matter. There is some difficulty in accounting for the appearance so constant and considerable as that described near Bologna. The ground on which the largest meteor appeared is a hard clay, very retentive of water; whilst in the hilly district, where the Bolognian ignis fatuus was smaller, the soil was a loose sand. Becari however states, that they chiefly frequented the banks of streams. All accounts confirm the absence of sensible heat from these meteors.

SECT. VIII.—LIGHT EMANATING FROM LIVING ANIMALS.

A luminous appearance somewhat similar to that given out by decaying organic matter is occasionally observed to play round some classes of living animals; and regularly emanates from the bodies of others, at particular seasons; or as a constant concomitant of motion, by another class.

Of the first kind, probably, is that light sometimes observed playing around the ears and manes of horses, which, though by some attributed to electricity, is probably an emanation from the animal itself; and may perhaps consist of a phosphuretted gas, disengaged by some unknown process of the animal economy. A lambent flame, of a similar nature, has in a few instances been remarked around the heads of children; a circumstance which is happily seized by Virgil in his fine description of the glory that appeared on the temples of the young Ascanius.

*Ecce levis summo de vertice visus Iuli Fundere lumen apex, tructaque innoxia mollis Lambere flamma coma, et circa tempora pasch.*

En. ii. 663.

Living vegetables, in like manner, also occasionally give out light. This has been particularly noticed in the marigold, the orange, the Indian pink or lilium bulbosum, aconium napellus, tropaeolum majus, and other plants. But there are animals in whom luminousness forms a necessary part of their economy.

The most familiar instance of this is the common glowworm, *Lampyris noctiluca*, and its congeners. The male of this species is a coleopterous insect, and sports in the air; whilst the female is apterous, doomed for ever to crawl amongst herbaceous plants, or to nestle on the leaves of shrubs; but when the shades of evening are drawn around, during the summer months, a spot of lucid yellow light, generally tinted with a shade of green, emanates from the extreme rings of her abdomen, and sprinkles the hedges, in some parts of our island, and the warmer parts of Europe, with brilliant stars. The final object of this light is probably to attract the notice of the male insect, who otherwise could with difficulty distinguish his wingless mate. The male of the English glow-worm is generally believed to be destitute of the apparatus for light; though Mr Walser (*Phil. Trans.* for 1884) asserts, that the male of one species of English glow-worm has the luminous appendage. The winged species of glow-worm are common in Italy, Spain, the south of France, and still more so in equinoctial America, in which the flickering light of the numerous fireflies affords a pleasing and interesting spectacle. This light is found to belong to both sexes, though it is most striking in the females.

But the luminous property of the *Fulgora Lanternaria* of South America surpasses that of all animals, in the splendour of its light. It is a large insect, of the order Hemiptera, three and a half inches long. It has a sort of thick proboscis, about one inch in length, which is the luminous organ. The light emitted by this species is so splendid that two or three of them will illuminate a chamber.

In all these animals the light has so much the appearance of phosphorus dissolved in oil, that probably it may be a secretion of an analogous nature.

Many animals inhabiting the sea are highly luminous; and it is almost established that the luminousness so often exhibited by the ocean depends entirely upon myriads of minute phosphorescent animals.

This appearance is not constant, but is very frequent in most latitudes, and generally, whenever the night is dark, may be seen exceedingly brilliant around our own coasts. When the water is still, it is seen as numerous bright points of a bluish white phosphorescent light of considerable intensity; but when the water is agitated, as by the waves, the motion of a ship, or the dashing of oars, the light appears often in flashes so intense as to show the hours on a watch, or to render legible the pages of a large printed book. The number of the luminous points varies greatly at different times and in different places; and often in the course of a short sail this fluctuation is very conspicuous.

We have observed the coruscations, during a gentle breeze, like a line of fire several hundred yards in length from the bows, or in the wake of the vessel. This appearance is found in every sea, but with some difference in intensity and colour, as it seems to proceed from various genera of animals.

The luminousness of the sea was long ascribed to electricity; but about the beginning of this century it was proved, in many instances, to depend on the presence of animalcules in the ocean, particularly of a minute species of medusa, which abounds in our seas, and seems to be the same as *M. Hemisphaerica* of Müller. Several years before, Sir Joseph Banks discovered other two animals which rendered the sea luminous, viz. a sort of shrimp, *Cancer Fulgens*, and a large medusa, *M. Pellicula*, both of which abound on the coasts of Brazil.

From 1810 to 1814, the writer of this article made many observations on this subject, both on the coasts of Britain and in the Bay of Biscay. When the water was very luminous in that sea, on several evenings he drew water and carried it for examination into the cabin. Of course it ceased to appear luminous when viewed by candle-light, but he could distinguish, floating in a glass of sea-water, a number of pellicoid animalcules, which, when magnified by the globular form of the containing vessel, evidently belonged to several species of *Medusaria*, amongst which he could observe the genera *Noctiluca* of Lamarck, a *Cyanea*, and a *Beroe*. The general size of these animals was from one twentieth to one thirtieth of an inch in diameter. That the luminousness of the sea was derived from these minute animals, appears to be proved by the following simple experiments. Portions of this sea-water were put into separate beer glasses, and the number of animalcules in each was carefully ascertained. They were carried successively to the deck, and when the water was dashed on it, the number of lucid points was ascertained. In most instances the number of these points coincided with the number of animals previously observed in the glass; in no instance were the lucid points more; though occasionally they were not so numerous, probably owing to some of them adhering to the glass, or having escaped the shock that stimulated their light-making organs. The same animals he has often observed, even more numerous, in the luminous sea-water of the British coasts; and has obtained similar results by a repetition of the experiments. The luminous points often adhere to the fingers; and on introducing such a luminous speck into a glass of non-luminous sea-water, careful inspection will show a medusary animal Light floating in the water. The sea-water, on such occasions, if left at rest in glass vessels on deck, or even if suspended in gimbals, would occasionally exhibit luminous points; and the author conceives that the light either attends the voluntary movements of those minute creatures, or is emitted at their will. When sulphuric or other strong acid is poured into luminous water, it will produce a considerable flash of light, either by the effort of the animals to escape, or the unexpected stimulus it produces. In some climates the sea exhibits a fiery-red hue. This has often been observed in the Chinese seas, and in some parts of the Indian Ocean. It is produced, according to the best authorities, by myriads of minute animals that emit a reddish light. Captain Horsburgh and Mr Langstaff have described other kinds of light occasionally observed in the Indian Ocean. The latter mentions, that during a passage from China to New Holland, the sea at night had a faint milky appearance, as if snow had just fallen on the water. The sailors thought it was produced by a coral shoal, but seventy fathoms of line did not find bottom; and when the water drawn up was examined, it was observed to swarm with minute globular bodies, about the size of a pin's head, linked together, and of a milky hue, probably some minute species of medusa. It is well known that the larger fish are sometimes luminous. This has been remarked in the bonito and the shark; but whether they are naturally luminous, or only in consequence of the adhesion of luminous animalcules to them, is not determined.

Some have ascribed the luminousness of the sea to electricity, others to putrescent particles in the water. The former opinion is not probable, from the appearances observed; the latter is supported by the luminous matter of dead fish being diffusible through water, and imparting to it luminous qualities; but the known phosphorescence of many small sea animals, and the coincidence of the number of scintillations, in the experiments above detailed, with that of the animals observed, incline us to believe that the luminous appearance of the ocean depends on the presence of minute animals.

**Sect. IX.—Chemical Effects of Light.**

Light appears to be possessed of chemical properties and energies distinct from those of heat, which may be considered as further proofs of its materiality.

It is capable of decomposing various metallic salts. Thus, if a colourless solution of nitrate of silver be exposed to light, it gradually blackens; a powder is deposited which has the same colour, and the salt is found to have lost a portion of its oxygen. This change is more rapidly effected by the direct rays of the sun than by the diffused light of day. The neutral solutions of gold, also, when exposed to light, in contact with charcoal, with vegetable or animal matter, as cotton and silk, are decomposed. This is the principle of the beautiful process invented by Mrs Fulham, for ornamenting muslin and silk stuffs with flowers and sprigs of gold. The salt of gold parts with its acid and its oxygen to the vegetable or animal matter through the agency of light. The dry salt formed by dissolving gold in nitro-nitric acid is also slowly decomposed by light under similar circumstances.

Scheele examined the effects of light on metallic solutions, and discovered that the chemical effects of the different coloured rays of the prismatic spectrum were different. By enclosing solutions of silver in glasses of different colours, he found, that in red glass there was very little effect produced, whilst in violet-coloured glass the blackening was speedily produced. These interesting facts were confirmed by Senebier and by Thomas Wedgwood. The latter showed, that in the full sunshine, the blackening of muriate of silver was produced in two minutes, and in the shade that several hours were required to produce this effect. The sunbeams transmitted through red glass have very little blackening effect; yellow and green glass are somewhat more effectual; but blue and violet glass produce the most decisive effects. The discovery of Herschel, respecting the different refrangibility of light and heat, induced Ritter and Wollaston to try the effect upon the salts of silver beyond the violet ray; and they found that the blackening was most decided beyond the visible boundary of the spectrum at the violet end. In some experiments of Senebier, muriate of silver was darkened by the violet ray in 15°, by the blue in 29°, by the green in 37°, by the yellow in 5° 30', by the orange in 12°, and by the red in 20°. These effects are wholly due to light; for no effect is produced in the hottest point just beyond the red rays.

Berthollet proved, that during the action of light on many metallic oxides, as those of gold, silver, lead, and mercury, a portion of their oxygen was extricated; and this is supposed to be the change produced in the blackening of the salts of silver, viz. a partial reduction of the metal. Sir Humphry Davy found that tritoxide of lead, when moistened, and exposed to the red rays, became red, that is, it lost oxygen, and became a deutoxide. Oxide of mercury, obtained by mixing potash and calomel, was not changed by the most refrangible rays, but became red in the least refrangible rays, which must have resulted from the absorption of oxygen. The violet rays produced on the moistened red oxide of mercury the same effect as a stream of hydrogen gas, that is, a deoxidation. Wollaston found that these rays produced an oxidizing effect on one vegetable substance, resin of guaiacum. This resin becomes green by absorbing oxygen; and he found that it underwent this change in the violet rays, but again obtained its yellow hue on being exposed to the red rays.

Some of the acids suffer decomposition by light; thus, if we expose colourless nitric acid to the sun's rays, in a flask provided with a bent tube, and terminating in a pneumatic apparatus, the acid becomes coloured, from the formation of nitrous gas, and oxygen may be collected in the trough in the usual way, as Berthollet ascertained. Light also decomposes some vegetable acids, such as the hydrocyanic; and to preserve such compounds pure, it is necessary carefully to exclude light.

Light also in some cases favours chemical combination. If chlorine which has been collected over water, and therefore contains water, be exposed to light, the water is slowly decomposed; its hydrogen enters into combination with the chlorine to form hydro-chloric or muriatic acid, and its oxygen is liberated. The influence of light is still more striking on chlorine and on hydrogen. If we mix equal proportions of these gases, and the access of light be carefully excluded, no action takes place, or their union is very slowly produced; but if we expose them to the diffused light of day, combination will take place in a quarter of an hour. If exposed to the direct beams of the sun, the union is instantaneous, and with a violent explosion. Davy found that when such a mixture was exposed to the red rays only, the gases united without explosion, yet more rapidly than when exposed to the violet rays; but that the conversion of a solution of chlorine into muriatic acid took place most readily at the most refrangible end of the solar spectrum.

**Sect. X.—Effects of Light on Plants.**

The change produced on vegetable colouring matter by light is familiar in the process of bleaching by exposure to the sun. But the influence of light on living vegetables is much more remarkable.

If a plant grow in total darkness, its natural green hue is not acquired; but it will be white, though in other respects vigorous. This is familiar in the etiolation or blanching of certain garden-stuffs, as celery, sea-kale, endive, &c. The late Professor Robison, many years ago, remarked that plants growing in darkness were not only white, but that they did not attain the natural form of their leaves, nor their natural odour. In descending into a coal mine, he accidentally met with a plant growing luxuriantly, the form and qualities of which were entirely new to him. The sod on which it grew was removed, potted, and carefully attended to in his garden. The etiolated plant languished and died; but the roots speedily threw out vigorous shoots, which, from the form of the leaves, and peculiar odour, he readily recognised as common tansy. He repeated similar experiments upon plants of lovage, carvi, and mint, with analogous results.

The green colour of vegetables, and even the form of their leaves, are materially influenced by light.

Some experiments of Senebier would lead to the conclusion that it is the violet end of the spectrum which has the greatest influence in counteracting the effects of etiolation. This production of the dark colour of plants would appear to depend upon the decomposition of carbonic acid by the living vegetable, when acted on by light, and the fixation of its carbon. That light is necessary to this process, is proved by the experiments of Priestley, Senebier, Ingenhouz, De Saussure, Davy, and many vegetable physiologists. Growing plants in sunshine give out oxygen by the decomposition of carbonic acid; but in darkness carbonic acid is evolved; and Ellis has demonstrated, in his interesting essays, that oxygen, under such circumstances, always disappears from the air in which vegetation is going on. Light, then, acts an important part in the vegetable economy, by fixing the most dense and abundant of the three general elements of vegetable matter. The influence of light in maturing fruits is well known. The fruits which when ripe are saccharine, are previously acid; that is, their hydrogen and carbon are combined with an excess of oxygen. Light, by favouring the evolution of oxygen, and the fixation of carbon, converts the vegetable acid into sugar, and thus provides a suitable food for the embryo in the seed. On the other hand, the influence of light is not less conspicuous in another class of vegetables. In an early stage of the germination of seeds yielding farina when ripe, the tender rudiment of the seed is enveloped in a slightly saccharine juice, as may be seen in young wheat and maize. Now, what is wanted to convert them into farina is the evolution of oxygen, and the fixation of a larger quantity of carbon. These two indications are fulfilled by the influence of light on the growing vegetables; and thus we can explain why the sun's influence is so essential in the one case to the formation of sugar from an acid; and in the other, of farina from a saccharine juice. The same principles will explain why a farinaceous seed, when planted in the ground, becomes again saccharine. Light is now wanting; oxygen is absorbed, and partly aids in the evolution of the redundant carbon, partly is added to the two other general elements. This view affords an instance of the important agency of light in vegetation, and a beautiful example of the simplicity of the means employed to produce diversified effects in the works of creation.

SECT. XI.—EFFECTS OF LIGHT ON ANIMALS.

The facts noticed in the last section show the powerful influence of light on vegetable forms. But its effects on the exterior of animals, though less striking, are not unimportant.

1. The influence of the sun's rays in deepening the colour, or in giving a brown tint to the skin, seems to be more due to the light than to the heat of the sunbeams; for the parts of the skin covered by the clothes, though kept thus hotter than the parts exposed, do not undergo this change. The pale visage and enfeebled vitality of those who live much in obscure apartments, in prisons, and in mines, are well known; and though probably the most violent symptoms that characterize the anaemia of miners, in which the skin assumes a yellowish, waxy hue, and the lips become bloodless, be chiefly due to breathing a vitiated atmosphere, yet some influence is certainly attributable to want of light. The anaemia of persons long confined in dungeons has often been remarked, and was lately described as strikingly exemplified in the person of Caspar Hauser, the young man whose mysterious birth, confinement, and assassination, have hitherto baffled conjecture.

In climates where the heat renders a state almost approaching to nudity desirable, all travellers agree that the development of the human frame is early, and the form has fewer deviations from the symmetry natural to the race than amongst northern nations. Deformity is said to be comparatively rare in hot climates, where the surface is much exposed; and this has been attributed by Dr Edwards in some measure to the influence of light. Perhaps, after making due allowance for the less chance of rearing sickly or deformed children amongst barbarous nations, and for the prevalence of infanticide amongst them in such cases, there is considerable justice in his remarks on this subject; without which it would be difficult to account for the fine muscular and rounded forms so often observed amongst nations agreeing in nothing except in the prevalence of a very free exposure of their persons to the full influence of light; as amongst the Mexicans and Peruvians, the Chaymas and Muyscas of South America, the Caribbs of the Antilles, the inhabitants of the numerous groups of the South Seas, or the free inhabitants of Africa. Probably, too, a part of the effects attributed to country air, in restoring to health the sickly child of the city artizan, is due to insolation, or the exposure to light, which appears to have the property of invigorating the vital functions, and of elevating the spirits of those who have suffered a long deprivation of its cheering influence.

2. The effect of light on the lower animals is more marked, and is strikingly illustrated in the curious experiments of Dr W. F. Edwards upon the spawn of frogs. He enclosed portions of the spawn of the frog in different vessels permeable to water, some of which were perfectly opaque, while others freely admitted light; the temperature was the same, yet the eggs exposed to light came to maturity; but those deprived of light were not hatched.

He even found that the development of the perfect form of such animals depended on light. By enclosing tadpoles of both frogs and toads in different boxes, some of which freely admitted light, whilst others totally excluded it, and placing them in running water, he found that the tadpoles exposed to light underwent the change to the perfect form of the animal, as usual; whilst the tadpoles excluded from the light, though they seemed perfectly vigorous, did not undergo the transformation, even though they had increased to double or triple their primitive weight. He also conjectures that the Anguine Syren, Proteus anguinus, may only be the larva of some reptile retained in its imperfect form by the profound obscurity of the subterranean lakes of Carniola, in which it is found. In this conjecture, however, he is mistaken; for that singular animal has been kept alive without change a considerable time, under circumstances favourable to its transformation, if really an imperfect species, The facts, however, already detailed, show that the influence of light on the animal economy is by no means inconsiderable. But the most interesting property of light, in relation to animals, is its effect in producing vision.

3. The rays of light emanating from bodies, or reflected from their surfaces, are destined to impress the sensorium through the eye; an admirable and complex optical apparatus, in which the rays, after passing through the transparent cornea, \(a\), sustain various refractions, until they reach the sentient expansion of the optic nerve, \(f\), called retina. The cornea constitutes the front of the eyeball, the rest of its surface is formed of a dense white tunic, called the sclerotic coat, \(b\), which joins the cornea by its edges, forming with the latter an almost globular body, which contains the more delicate structures of the eye. Just within the cornea is observed a coloured ring, apparently formed of diverging fibres, termed the iris, \(h\). This ring is perforated by the pupil, a circular aperture in its centre, which is capable of contraction and expansion, by the action of the fibres of the iris, according to the increase or diminution of the light. This movement of the pupil is not under the will of the animal, but is regulated by the stimulus of light, so as to exclude any quantity that would be hurtful to the organ. The iris is attached to the outer coat of the eye, the sclerotic, by the ciliary processes, \(m\). Within the sclerotic lies a delicately thin vascular membrane named the choroid coat, having its interior surface lined with a black secretion termed the pigmentum nigrum, the probable use of which is to aid clear vision, by preventing a multiplicity of reflections in the bottom of the eye.

Still nearer to the centre of the eye lies the retina, which is continuous with the optic nerve, \(f\), and is the part of the eye sentient to the impressions of light. Entering the bottom of the eye, but not just in its axis, is seen a thick cord, the optic nerve, by which the impressions upon the retina are conveyed to the brain.

The bulk of the eyeball is made up of three substances, which have been improperly termed humours. The first lies just within the cornea, and fills the space between it and the iris, as well as the smaller space immediately behind the latter. From its fluidity, it is termed the aqueous humour, \(g\). The iris divides the chamber of the aqueous humour into an anterior and a posterior portion. The posterior wall of this chamber is formed by a delicate, transparent, capsular membrane, enclosing a lenticular body, convex on both sides, though more so posteriorly, which is named the crystalline lens, \(l\). This body consists of concentric layers, formed of fibres externally less consistent, but increasing in density towards its centre; a structure intended for the purpose of correcting its spherical aberration. The capsule of the crystalline lens is retained in its situation by the ciliary circle or ciliary processes, \(n\); and it lies imbedded in the most considerable of all the humours of the eye, the vitreous, \(m\), which consists of a clear gelatinous substance, that recent anatomical investigations prove to be lodged in very delicate membranous cells. In the healthy state, especially in young persons, these humours are perfectly colourless and transparent, but they become coloured by age; and the crystalline lens has been seen in very old persons of a bright amber colour, though still transparent. Opacity of the lens forms the disease termed cataract. The obvious use of the humours of the eye is to refract the rays of light entering the eye by the transparent cornea, and to concentrate them on the retina. The difference in their consistence seems intended to make that organ a true and nicely adjusted achromatic optical instrument.

The ray from any object falling directly in the centre of the eye, perpendicular to its surface, will not undergo any refraction, but pass straight through to the retina; and its direction may be termed the axis of the eye. All the rays which fall obliquely on the eye will, by the refraction of the humours, be bent from their course, and cut the axis somewhere between the lens and the retina; and the refractive power of the humours is such as to converge on the retina all rays proceeding from any one point of a visible object, whether these rays fall on the cornea in a parallel or a diverging condition. But as these rays have all crossed the axis of the eye by the refractive power of the humours, it is obvious that the rays from the upper part of the object will impinge on the lower part of the retina, and those from the lowest point, on the upper part of that membrane. Hence the image of the object will be inverted on the retina. That this actually takes place we can easily satisfy ourselves, by removing from the posterior part of the eye of a sheep a portion of the sclerotic coat with a sharp knife, until it becomes translucent, when the image of a candle placed before the cornea will be seen inverted on the back part of the eye thus prepared.

In the usual state of the eye, its refractive apparatus is so contrived that all the rays from the same points of objects at some distance from the eye will converge in the retina, or the principal focus of the eye will lie in that membrane; but divergent rays from very near objects would converge in a point behind the retina, were there not some power of adjustment in the eye for correcting the confused image which would thus be formed. This may be effected in two ways; either by increasing the convexity of the cornea, or by bringing the lens to a greater distance from the retina. Some have attributed this adjustment to an alteration in the form of the eyeball, and consequently of the cornea, by the action of the motor muscles of the organ; but there is much more probability in the opinion that this adjustment is produced by the contractions and expansions of the iris. This part Light of the eye is attached to the ciliary ligament, which is also connected with the capsule of the crystalline lens. Hence contraction of the pupil would tend to draw forward the lens, or remove it farther from the retina; and thus the rays which have their point of convergence behind the retina, would meet in that membrane, and a distinct image would be produced. That some such adjustment is necessary is obvious; for if, after contemplating a distant object, our attention be suddenly turned to a very near one, it is some time ere the second object is distinctly perceived; and the movement of the iris will be seen to take place before the second image is well defined.

The retina is capable of affording the perception of external objects in its whole extent, except at the insertion of the optic nerve, a spot about \( \frac{1}{40} \)th of an inch in diameter in man. Any image falling on this spot is invisible, as multiplied experiments have shown. This insensible point in either eye may be indicated by the familiar optical experiment of placing three coloured wafers horizontally on a wall, on a white ground, about two feet from each other. Let the operator place himself about two feet from the wall, opposite the middle wafer; shut one eye, and then gradually retire backwards, while he fixes his open eye on the wafer nearest the closed eye. When an ordinary eye is distant from the wall about five times the distance of the wafers from each other, the image of the middle wafer will have fallen on the insertion of the optic nerve, and will no longer be visible.

Two very interesting speculations connected with vision have long exercised the ingenuity of physiologists and metaphysicians, viz., how it happens that animals with two eyes see objects single, by means of double images on the retina; and how they appear erect by means of inverted images?

The first has been explained on the supposition of there being corresponding points in each eye that convey similar perceptions to the mind, and that such afford only the idea of unity. In the natural movements of the eyes, the images of external objects are supposed always to fall on such points; but should any cause throw the images on dissimilar points, then we have double vision. Hence, if with the finger we push one eyeball slightly out of its parallel position with regard to the other, two objects are perceived. In persons who squint, the parallel movement of the eyes is lost; and it has been supposed that in cases where the squinting has come on in adults from imperfection in one eye, that habit enables the individual to correct the illusion of double vision, and that the person ceases to notice the impressions made on the least perfect of the two eyes.

The second fact alluded to is that of erect vision by inverted images. That images of external objects are depicted inverted on the retina is perfectly well ascertained; and philosophers have been much puzzled to explain how the indications of vision and touch are reconciled. Some have imagined that at first we really see objects inverted, but learn by experience and by the sense of touch to correct the illusion. Other attempts have been made to explain it by what has been termed the law of visible direction; by which it is supposed that when any point of an object is viewed, the rays proceeding from that point must fall on the eye with different degrees of obliquity, yet that point will be only seen in the direction of the central ray of the cone of light proceeding from that point: And as the lines of visible direction must necessarily cross each other at the centre of visible direction, those of the lower part of the image on the retina must go to the upper part of the object, and those of the upper part of the image to the lower part of the object; and hence an erect object is considered as the necessary result of an inverted image.

This is very ingenious, but it is obviously hypothetical and obscure. The physiological explication of single vision by double images suggested by Newton, and since supported by Wollaston, is based on what has been termed the semi-decussation of the optic nerves at their commissure, whereby the right half of the retina of each eye is placed in direct nervous communication with the right optic lobe, or right half of that part of the brain termed corpus quadrigeminum, and vice versa. Hence impressions made on corresponding points in each retina may, in fact, be impressions on the same points of the common sensorium, and therefore co-operate in producing the same perception. The anatomical proof of this is still defective, but many facts give it probability. The subject has engaged the attention of Professor Alison of Edinburgh, who considers that this arrangement of the optic nerves is but a part of the provision by which nature has secured harmony between perceptions afforded by sight and touch. His explanation of one of the most perplexing questions in animal physiology is so ingenious, that we gladly avail ourselves of his permission to lay it before our readers.

According to him, the peculiar contorted or involuted course of the optic nerves (in all vertebrate animals) around the crura cerebri, until they are lost in the optic lobes, seems designed to secure that the position of the impression on the sensorium should be conformable to the true position of the object.

It is only in those animals intended by nature to contemplate objects with both eyes at once, as in mammalia and birds, that the semi-decussation of the optic nerve is found. The contrivance in fact implies that both eyes are to be at the same time directed to the same object, or that both optic lobes are to be constantly employed in vision at the same time; the right half of each retina being in connection with each optic lobe, and the left half of each retina in like manner connected with the left optic lobe. Now, the right half of each retina contains the image of the left half of the field of vision; and therefore the impressions made by the left half of the field of vision fall on the right optic lobe, and have, on their left, the impression resulting from the right side of the field of vision; on which, as both optic lobes are necessarily exercised together, the attention of the mind is equally directed.

Dr Alison considers that the grand contrivance adopted by nature to secure harmony between the indications of sight and touch is the decussation at the pyramids of the nervous fibres concerned in common sensation and in voluntary movements; the effect of which must be, that whilst the right side of the brain is that to which impressions from the left half of the field of vision are brought, it is also that on which all the other sensations of the left side of the body depend; or, in other words, we both see and feel what is on our left by the right side of the brain. Accordingly, it is in those animals only in which the semi-decussation of the optic nerves exists (namely, in Mammalia and birds) that the decussation at the pyramids exists, or that the sensations and voluntary motions of each side of the body appear to be in connection with the opposite side of the brain.

As the admirable mechanism of the eye, and its beautiful adaptation to the necessities and comfort of the animal creation, afford a striking instance of that wisdom and beneficence so conspicuous in the handiwork of the Deity; so the extinction of this exquisite and important organ must be considered as one of the severest of human calamities; a misfortune that perhaps cannot be fully appreciated by those who have not experienced the loss of sight, so feelingly deplored by our mighty bard in poetry that can only perish with the language in which it is expressed.

Thus with the year Seasons return, but not to me returns Lighthouses.

Lighthouse, and sea-light, are terms which, although not strictly synonymous, are indifferently employed to denote the same thing. A Sea-light may be defined as a light so modified and directed as to present to the mariner an appearance which shall at once enable him to judge of his position during the night, in the same manner as the sight of a landmark would do during the day.

The early history of lighthouses is very uncertain; and many ingenious antiquaries, finding the want of authentic records, have endeavoured to supply the deficiency by conjectures founded on casual and obscure allusions in ancient writers, and have invented many vague and unsatisfactory hypotheses on the subject, drawn from the heathen mythology. Some writers have gone so far as to imagine, that the Cyclopes were the keepers of lighthouses; whilst others have actually maintained that Cyclops was intended, by a bold prosopopoeia, to represent a lighthouse itself. A notion so fanciful deserves little consideration; and in order to show how ill it accords with that mythology of which it is intended to be an exposition, it seems enough to quote the lines from the ninth Odyssey, where Homer, after describing the darkness of the night, informs us that the fleet of Ulysses actually struck the shore of the Cyclopean island before it could be seen.

There does not appear any better reason for supposing, that under the history of Tithonus, Chiron, or any other personage of antiquity, the idea of a lighthouse was conveyed; for such suppositions, however reconcilable they may appear with some parts of the mythology, involve obvious inconsistencies with others. Nor does it seem at all probable, that in those early times, when navigation was so little practised, the advantages of beacon-lights were so generally known and acknowledged as to render them the objects of mythological allegory.

About 300 years before the Christian era, Chares, the disciple of Lysippus, constructed the celebrated brazen statue, called the Colossus of Rhodes, whose height was upwards of 100 feet, and which stood at the entrance to the harbour. There is considerable probability in the idea, that this figure served the purposes of a lighthouse; but we do not remember any passage in ancient writers, where this use of the Colossus is expressly mentioned. There is much inconsistency in the account of this fabric by early writers, who, in describing the distant objects which could be seen from it, appear to have forgotten the height which they assign to the figure. It was partly demolished by an earthquake, about 80 years after its completion; and so late as the year 672 of our era, the brass of which it was composed was sold by the Saracens to a Jewish merchant of Edessa, for a sum, it is said, equal to L36,000.

Little is known with certainty regarding the Pharos of Alexandria, which was regarded by the ancients as one of the seven wonders of the world. It was built by Ptolemy Philadelphus, about 300 years before Christ; and it is recorded by Strabo, that the architect Sostratus, the son of Dexiphanes, having first secretly cut his own name on the solid walls of the building, covered the words with plaster, and, in obedience to Ptolemy's command, made the following inscription on the plaster—"King Ptolemy to the gods, the saviours, for the benefit of sailors." What truth there may be in this account of the fraud of Sostratus there is now no means of determining; and the story is only now interesting, in so far as it shows the object of the royal founder and the use of the tower. The accounts which have reached us of the dimensions of this remarkable edifice are exceedingly various; and many of the statements regarding the distance at which it could be seen are clearly fabulous. Josephus approaches nearest to probability, and informs us, that the fire which was kept constantly burning in the top was visible by seamen at a distance equal to about 40 miles. If the reports of some writers are to be believed, this tower must have far exceeded in size the great pyramid itself; but the fact that a building of comparatively so late a date should have so completely disappeared, whilst the pyramid remains almost unchanged, is a sufficient reason for rejecting, as erroneous, the dimensions which have been assigned by most writers to the Pharos of Alexandria. Some have pretended that large mirrors were employed to direct the rays of the beacon-light on its top, in the most advantageous direction; but there is nothing like respectable evidence in favour of this supposition. Others, with greater probability, have imagined that this celebrated beacon was known to mariners, simply by the uncertain and rude light afforded by a common fire. In speaking of the Pharos, the poet Lucan, on most occasions sufficiently fond of the marvellous, takes no notice of the gigantic mirrors which it is said to have contained. It is true that, by using the word "lampada," which can only with propriety be applied to a more perfect mode of illumination than an open fire, he appears to indicate that the "flammar" of which he speaks were not so produced. The word lampada may, however, be used metaphorically; and flammar would, in this case, not improperly describe the irregular appearance of a common fire. Those who are desirous of knowing all that occurs in ancient authors on the subject of the Pharos of Alexandria may consult Pliny, l. xxxvi., c. 12.; l. v., c. 13., and l. xiii., c. 11. Strabo, l. xvii., p. 794, et seq. Caesar, Comment. de Bell. Civili, l. iii. Pompon. Mela, l. ii., c. 7. Ammian. Marcellin, l. xxii., c. 16. Joseph. de Bell. Judaica, l. vi. Nicolas Lloyd's Lexicon Geographicum, and the Notitia Orbis Antiqui of Cælius, l. iv., c. 1., p. 13.

Mr Moore, in his History of Ireland (vol. i., p. 16), speaks of the Tower of Coruña, which, he says, is mentioned in the traditionary history of that country as a lighthouse erected for the use of the Irish in their frequent early intercourse with Spain. In confirmation of this opinion, he cites a somewhat obscure passage from Æthicus, the cosmographer. This in all probability is the tower which Humboldt mentions in his Narrative under the name of the Iron Tower, which was built as a lighthouse by Cains Sevius Lupus, an architect of the city of Aqua Flavia, the modern Chaves. A lighthouse has lately been established on this headland, for which dioptric apparatus was supplied from the workshop of M. Léonard de Paris. (See also a curious account of the traditions about this tower in Southey's Letters from Spain and Portugal, p. 17.)

There is also a record in Strabo of a magnificent lighthouse of stone at Capio, or Apio, near the Harbour of Menestheus (the modern Mess Asta, or Puerto de Sta. Maria), built on a rock nearly surrounded by the sea, as a guide for the shallows at the mouth of the Guadalquivir, which he describes in terms almost identical with those used by him in speaking of the Pharos of Alexandria. I am not aware of any other notice of this great work, for such it seems to have been, to have deserved the praises of Strabo.

In Camden's Britannia, a passing notice is taken of the ruins called Caesar's Altar, at Dover, and of the Tour d'Or dre, at Boulogne, on the opposite coast; both of which are conjectured, on somewhat doubtful grounds, to have been ancient lighthouses. Pennant describes the remains of a Roman Pharos near Holywell, but cites no authorities for his opinion as to its use. There were likewise remains of a similar structure at Flamborough-head. A very meagre and unintelligible account is also given of a lighthouse at St Edmund's Chapel, on the coast of Norfolk, in Gough's additions to Camden, by which it might seem that the lighthouse was erected in 1272.

Such seems to be the sum of our knowledge of the ancient history of lighthouses, which, it must be admitted, is neither accurate nor extensive. Our information regarding modern lighthouses is of course more minute in its details, and more worthy of credit, as the greater part of it is drawn from authentic sources, or is the result of the actual observation of the writer of this article, who has visited the most important lighthouses of Europe. It seems sufficient here to notice briefly the most remarkable establishments of the kind now in existence; reserving, for the latter part of the article, the more appropriate and important topics of the methods of illumination, and the systems of management.

The first lighthouse of modern days which merits attention is the Tour de Corduan, which, in point of architectural grandeur, is unquestionably the noblest edifice of the kind in the world. It is situated on an extensive reef at the mouth of the River Garonne, and serves as a guide to the shipping of Bordeaux and the Languedoc Canal, and, indeed, of all that part of the Bay of Biscay. It was founded in the year 1584, and was not completed till 1610, under Henri IV. It is minutely described in Belidor's Architecture Hydraulique. The building is 197 feet in height, and is shown in the accompanying woodcut, fig. 1. Round the base is a wall of circumvallation, 134 feet in diameter, in which the light-keepers' apartments are formed, somewhat in the style of casemates. The first light exhibited in the Tour de Corduan was obtained by burning billets of oakwood, in a chafier at the top of the tower; and the use of coal instead of wood was the first improvement which the light received. A rude reflector, in the form of an inverted cone, was afterwards added, to prevent the loss of light which escaped upwards. About the year 1780, M. Lenoir was employed to substitute reflectors and lamps; and in 1822 the light received its last improvement, by the introduction of the dioptric instruments of M. Fresnel.

The history of the celebrated lighthouse on the Eddy-stone rocks is well known to the general reader, from the narrative of Mr Smeaton the engineer. These rocks are 9½ miles from the Ram-Head, on the coast of Cornwall; and from the small extent of the surface of the chief rock, and its exposed situation, the construction of the lighthouse was a work of very great difficulty. The first erection was of timber, designed by Mr Winstanley, and was commenced... in 1696. The light was exhibited in November 1698. It was soon found, however, that the sea rose upon this tower to a much greater height than had been anticipated, so much so, it is said, as to "bury under the water" the lantern, which was 60 feet above the rock; and Mr Winstanley was therefore afterwards under the necessity of enlarging the tower, and carrying it to the height of 120 feet. In November 1703, some considerable repairs were required, and Mr Winstanley, accompanied by his workmen, went to the lighthouse to attend to their execution; but the storm of the 26th of that month carried away the whole erection, when the engineer and all his assistants unhappily perished.

The want of a light on the Eddystone soon led to a fatal accident; for, not long after the destruction of Mr Winstanley's lighthouse, the Winchilsea man-of-war was wrecked on the Eddystone rocks, and most of her crew were lost. Three years, however, elapsed after this melancholy proof of the necessity of a light before the Trinity House of London could obtain a new act to extend their powers; and it was not till the month of July 1706 that the construction of a new lighthouse was begun, under the direction of Mr John Rudyerd of London. On the 28th of July 1708 the new light was first shown, and continued to be regularly exhibited till the year 1755, when the whole fabric was destroyed by accidental fire, after standing 47 years. But for this circumstance, it is impossible to tell how long the lighthouse might, with occasional repair, have lasted, as Mr Rudyerd seems to have executed his task with much judgment, carefully rejecting all architectural decoration, as unsuitable for such a situation, and directing his attention to the formation of a tower which should offer the least resistance to the waves. The height of the tower, which was of a circular form, and constructed of timber, was, including the lantern, 92 feet, and the diameter at the base, which was a little above the level of high water, was 23.

The advantages of a light on the Eddystone having been so long known and acknowledged by seamen, no time was permitted to elapse before active measures were taken for its restoration; and Mr Smeaton, to whom application was made for advice on the subject, recommended the exclusive use of stone as the material, which, both from its weight and other qualities, he considered most suitable for the situation. On the 5th of April 1756, Mr Smeaton first landed on the rock, and made arrangements for erecting a lighthouse of stone, and preparing the foundations, by cutting the surface of the rock into regular horizontal benches, into which the stones were carefully dovetailed or notched. The first stone was laid on 12th June 1757, and the last on the 24th of August 1759. The tower measures 68 feet in height, and 26 feet in diameter at the level of the first entire course, and the diameter under the cornice is 15 feet. The first 12 feet of the tower form a solid mass of masonry, and the stones are united by means of stone joggles, dovetailed joints, and oak treenails. It is remarkable that Mr Smeaton should have adopted an arched form for the floors of his building, instead of employing these floors as tie-walls formed of dovetailed stones. To counteract the injurious tendency of the outward thrust of these arched floors, Mr Smeaton had recourse to the ingenious expedient of laying, in circular trenches or beds in the stones which form the outside casing, sets of chains, which were heated by means of an application of hot lead, and became tight in cooling. The light was exhibited on the 16th October 1759; but such was the state of the lightroom apparatus in Britain at this period, that a feeble light from tallow candles was all that decorated this noble structure. In 1807, when the property of this lighthouse again came into the hands of the Trinity House, on the expiry of a long lease, Argand burners, and parabolic reflectors of silvered copper, were substituted for the chandelier of candles. Fig. 2 shows a section of the Eddystone lighthouse, as executed according to Mr Smeaton's design.

The dangerous reef called the Inch Cape, or Bell-Rock, so long a terror to mariners, was well known to the earliest navigators of Scotland. Its dangers were so generally acknowledged, that the Abbots of Aberbrothick, from which the rock is distant about 12 miles, caused a float to be fixed upon the rock, with a bell attached to it, which, being swung by the motion of the waves, served by its tolling to warn the mariner of his approach to the reef. Amongst the many losses which occurred on the Bell-Rock in modern times, one of the most remarkable is that of the York, 74, with all her crew, part of the wreck having been afterwards found on the rock, and part having come ashore on the neighbouring coast. During the survey of the rock also, many instances were discovered of the extent of loss which this reef had occasioned, and many articles of ships' furnishings were picked up on it, as well as various coins, a bayonet, a silver shoe-buckle, and many other small objects. Impressed with the great importance of some guide for the Bell-Rock, Captain Brodie, R.N., set a small subscription on foot, and erected a beacon of spars on the rock, which, however, was soon destroyed by the sea. He afterwards constructed a second beacon, which soon shared the same fate. It was not, however, until 1802, when the Commissioners of Northern Lights brought a bill into parliament for power to erect a lighthouse on it, that any efficient measures were contemplated for the protection of seamen from this rock, which, being covered at every spring-tide to the depth of 12 feet, and lying right in the fairway to the Firths of Forth and Tay, had been the occasion of much loss both of property and life. In 1806 the bill passed into a law, and various ingenious plans were suggested for overcoming the difficulties which were apprehended, in erecting a lighthouse on a rock 12 miles from land, and covered to the depth of 12 feet by the tide. But the suggestion of Mr Robert Stevenson, the engineer to the Lighthouse Board, after being submitted to the late Mr Rennie, was at length adopted; and it was determined to construct a tower of masonry, on the principle of the Eddystone. On the 17th of August 1807, Mr Stevenson accordingly landed with his workmen, and commenced the work by preparing the rock to receive the supports of a temporary wooden pyramid, on which a barrack-house, for the reception of the workmen, was to be placed; and during this operation much hazard was often incurred in transporting the men from the rock, which was only dry for a few hours at spring-tides, to the vessel which lay moored off it. The lowest floor of this temporary erection, in which the mortar for the building was prepared, was often broken up and removed by the force of the sea. The foundation having been excavated, the first stone was laid on the 10th July 1808, at the depth of 16 feet below the high-water of spring-tides, and at the end of the second season, the building was 5 feet 6 inches above the lowest part of the foundation. The third season's operations terminated by finishing the solid part of the structure, which is 30 feet in height; and the whole of the masonry was completed in October 1810. The light was first exhibited to the public on the night of the 1st of February 1811. The difficulties and hazards of this work were chiefly caused by the short time during which the rock was accessible between the ebbing and flowing tides; and amongst the many eventful incidents which rendered the history of this work interesting, was the narrow escape which the engineer and thirty-one persons made from being drowned, by the rising of the tide upon the rock, before a boat came to their assistance, the attending vessel having broken adrift. This circumstance occurred before the barrack-house was erected, and is narrated by Mr Stevenson in his account of the work, published at the expense of the Lighthouse Board in 1824, to which we may refer for more minute information on the subject of this work, and the other lights of the coast of Scotland.

The Bell-Rock Tower is 100 feet in height, 42 feet in diameter at the base, and 15 at the top. The door is 30 feet from the base, and the ascent is by a massive copper ladder. The apartments, including the light-room, are six in number. The light is a revolving red and white light, and is produced by the revolution of a frame containing 20 Argand lamps, placed in the foci of parabolic mirrors, arranged on a quadrangular frame, whose alternate faces have shades of red glass placed before the reflectors, so that a red and white light is shown successively. The machinery, which causes the revolution of the frame containing the lamps, is also applied to telling two large bells, to give warning to the mariner of his approach to the rock in foggy weather. Fig. 3 shows a section of the Bell-Rock Lighthouse, and of the temporary barrack-house, which was removed on the completion of the work. The entire cost of the lighthouse was £61,331, 9s. 2d.

The great merit of Mr Stevenson, as architect of the Bell-Rock Lighthouse, lies in his bold conception and unshaken belief in the possibility of erecting a tower of masonry on a reef 12 miles from the nearest land, and covered by every tide,—a situation, undoubtedly, much more difficult than that of the Eddystone. But his mechanical skill in carrying on the work is also deserving of high praise. Not only did he conceive the plan of the moveable jib and balance crane, which he afterwards used with much advantage in building the tower; but his zeal, ever alive to the possibility of improving on the conceptions of his great master Smeaton, led him to introduce several beneficial changes into the arrangements of the masonry. In particular, he converted the stone floors of the apartments, which in the Eddystone exert an outward, and in its tendency disruptive, thrust, into bonds of union and efficient sources of stability. This thrust was by Smeaton himself considered so disadvantageous, that he thought fit to counteract it, as already noticed, by means of metallic girders, concealed in the stone-work, and most ingeniously applied. The Lighthouse Board placed in the upper apartment of the tower a bust of Mr Stevenson, "in testimony," as the minutes record, "of the sense entertained by the Commissioners of his distinguished talent and indefatigable zeal in the erection of the Lighthouse."

The most remarkable lighthouse on the coast of Ireland Carlingford is that of Carlingford, near Cranfield Point, at the entrance ford, of Carlingford Lough. It was built according to the design of Mr George Halpin, the Inspector of the Irish Lights; and was a work of an arduous nature, being founded twelve feet below the level of high-water on the Hawkbowl Rock, which lies about two miles off Cranfield Point. The figure is that of a frustum of a cone, 111 feet in height, and 48 feet in diameter at the base. The light, which is fixed, is from oil burned in Argand lamps placed in the foci of parabolic mirrors. It was first exhibited on the night of the 20th December 1830.

The Skerryvore Rocks, which lie about 12 miles W.S.W. of the seaward point of the Isle of Tyree, in Argyllshire, were long known as a terror to mariners, owing to the numerous shipwrecks, fatal alike to the vessels and the crews, which had occurred in their neighbourhood. A list, confessedly incomplete, enumerates thirty vessels lost in the forty years preceding 1844; but how many others, which during that period had been reported as "foundered at sea," or as to whose fate not even an opinion has been hazarded, may have been wrecked on this dangerous reef, which lies so much in the track of the shipping of Liverpool and the Clyde, it would be vain to conjecture. The Commissioners of the Northern Lighthouses had for many years entertained the project of erecting a lighthouse on the Skerryvore. vore; and with this object had visited it, more especially in the year 1814, in company with Sir Walter Scott, who, in

through which the sea plays almost incessantly. The cutting of the foundation for the tower in this irregular flinty mass occupied nearly two summers; and the blasting of the rock, in so narrow a space, without any shelter from the risk of flying splinters, was attended with much hazard.

In such a situation as that of Skerryvore everything was to be provided beforehand and transported from a distance; and the omission in the list of wants of even a little clay for the tamping of the mine-holes might for a time have entirely stopped the works. Barracks were to be built at the workyard in the neighbouring Island of Tyree, and also in the Isle of Mull, where the granite for the tower was quarried. Piers were also built in Mull and Tyree for the shipment and landing of materials; and at the latter place a harbour or basin, with a reservoir and sluices for scouring the entrance, were formed for the accommodation of the small vessel which attends the lighthouse. It was, besides, found necessary, in order to expedite the transport of the building materials from Tyree and Mull to Skerryvore Rock, to build a steam-tug, which also served, in the early stages of the work, as a floating barrack for the workmen. In that branch of the service she ran many risks, while she lay moored off the rock in a perilous anchorage, with two-thirds of the horizon of foul ground, and a rocky and deceitful bottom, on which the anchor often tripped.

The operations at Skerryvore were commenced in the summer of 1838, by placing on the rock a wooden barrack, similar to that first used by Mr Robert Stevenson at the Bell-Rock. (See Fig. 3.) The framework was erected in the course of the season on a part of the rock as far removed as possible from the proposed foundation of the lighthouse tower; but in the great gale which occurred on the night of the 3rd of November following, it was entirely destroyed and swept from the rock, nothing remaining to point out its site but a few broken and twisted iron stanchions, and attached to one of them a piece of a beam, so shaken and rent by dashing against the rock as literally to resemble a bunch of laths. Thus did one night obliterate the traces of a season's toil, and blast the hopes which the workmen fondly cherished of a stable dwelling on the rock, and of refuge from the miseries of sea-sickness, which the experience of the season had taught many of them to dread more than death itself. After the removal of the roughest part of the foundation of the tower had been nearly completed, during almost two entire seasons, by the party of men who lived on board the vessel while she lay moored off the rock, a second and successful attempt was made to place another barrack of the same description, but strengthened by a few additional iron ties, and a centre post, in a part of the rock less exposed to the breach of the heaviest waves than the site of the first barrack had been. This second house braved the storm for several years after the works were finished, when it was taken down and removed from the rock, to prevent any injury from its sudden destruction by the waves.

"Perched 40 feet above the wave-beaten rock, in this singular abode," says Mr Alan Stevenson, the engineer, "with a goodly company of thirty men, I have spent many a weary day and night at those times when the sea prevented anyone going down to the rock, anxiously looking for supplies from the shore, and earnestly longing for a change of weather favourable to the recommencement of the works. For miles around nothing could be seen but white foaming breakers, and nothing heard but howling winds and lashing waves. At such seasons much of our time was spent in bed; for there alone we had effectual shelter from the winds and the spray, which searched every creanny in the walls of the barrack. Our slumbers, too, were at times fearfully interrupted by the sudden pouring of the sea over the roof; the rocking of the house on its pillars, and the spurting of water..." through the seams of the doors and windows—symptoms which, to one suddenly aroused from sound sleep, recalled the appalling fate of the former barrack, which had been engulfed in the foam not twenty yards from our dwelling, and for a moment seemed to summon us to a similar fate. On two occasions, in particular," says the engineer, "those sensations were so vivid as to cause almost every one to spring out of bed; and some of the men fled from the barrack, by a temporary gangway, to the more stable but less comfortable shelter afforded by the bare wall of the lighthouse tower, then unfinished, where they spent the remainder of the night in the darkness and the cold."

The design for the Skerryvore Lighthouse was given by Mr Alan Stevenson, and is an adaptation of Smeaton's Eddystone Tower to the peculiar situation and the circumstances of the case at the Skerryvore, with such modifications in the general arrangements and dimensions of the building as the enlarged views of the importance of lighthouses which prevail in the present day seemed to call for.

The tower is 188 feet 6 inches high, and 42 feet in diameter at the base, and 16 feet at the top. It contains a mass of stone-work of about 58,580 cubic feet, or more than double that of the Bell-Rock; and not much less than five times that of the Eddystone. The lower part of the tower was built by means of jib-cranes, and the upper part with shear-poles, needles, and a balance-crane. The shear-poles were similar to those used by Smeaton at the Eddystone; and the jib-cranes and balance-crane were the same as those which were designed and first employed by Mr Robert Stevenson in the erection of the Bell-Rock Lighthouse. The mortar used was compounded of equal parts of lime-stone (from the Halkin Mountain, near Holywell, in North Wales), burnt and ground at the works, and of Pozzolano earth. The light of Skerryvore is revolving, and reaches its brightest state once every minute. It is produced by the revolution of eight great annular lenses around a central lamp with four wicks, and belongs to the first order of dioptric lights in the system of Fresnel. The light may be seen from a vessel's deck at the distance of 18 miles. The entire cost of the lighthouse, including the purchase of the steam-vessel, and the building of the harbour at Hynish for the reception of the small vessel, which now attends the lighthouse, was £86,977, 17s. 7d.

"In such a situation as the Skerryvore," says the engineer, "innumerable delays and disappointments were to be expected by those engaged in the work; and the entire loss of the fruit of the first season's labour in the course of a few hours was a good lesson in the school of patience, and of trust in something better than an arm of flesh. During our progress, also, cranes and other materials were swept away by the waves; vessels were driven by sudden gales to seek shelter at a distance from the rocky shores of Mull and Tyree; and the workmen were left on the rock desponding and idle, and destitute of many of the comforts with which a more roomy and sheltered dwelling, and the neighbourhood of friends, are generally connected. Daily risks were run in landing on the rock in a heavy surf, in blasting the splintery gneiss, or by the falling of heavy bodies from the tower on the narrow space below, to which so many persons were necessarily confined. Yet had we not any loss of either life or limb; and although our labours were prolonged from dawn to night, and our provisions were chiefly salt, the health of the people, with the exception of a few slight cases of dysentery, was generally good throughout the six successive summers of our sojourn on the rock. The close of the work was welcomed with thankfulness by all engaged in it; and our remarkable preservation was viewed, even by many of the most thoughtless, as, in a peculiar manner, the gracious work of Him by whom 'the very hairs of our heads are all numbered.'"

There can be little doubt that, down to a very late period, the only mode of illumination adopted in the lighthouses, even of the most civilized nations of Europe, was the combustion of wood or coal in a chaffeur on the top of a high tower. It is needless to enlarge upon the evils of such a method; they need only be named to be understood; for it is difficult to conceive how an efficient system of lighting a coast could be managed under such disadvantages. The uncertainty caused by the effects of wind and rain, and the impossibility of rendering one light distinguishable from another, must have at all times rendered the early lighthouses in a great measure useless to the mariner.

M. Teulère, a member of the Royal Corps of Engineers Cateoptre of Bridges and Roads in France, is, by some, considered the first who hinted at the advantages of parabolic reflectors; and he is said, in a memoir dated the 26th June 1783, to have proposed their combination with Argand lamps, ranged on a revolving frame, for the Corduan lighthouse. Whatever foundation there may be for the claim of M. Teulère, certain it is, that this plan was actually carried into effect at Corduan under the directions of the Chevalier Borda, and to him is generally awarded the merit of having conceived the idea of applying parabolic mirrors to lighthouses. These were prodigious steps in the improvement of lighthouses, as not only the power of the lights was thus greatly increased, but the introduction of a revolving frame proved a valuable source of distinction amongst lights, and has since been the means of greatly extending their utility. The exact date of the change on the light of the Corduan is not known; but as it was made by Lenour, the same young artist to whom Borda, about the year 1780, intrusted the construction of his reflecting circle, it has been conjectured by some that the improvement was made about the same time. If this conjecture be correct, the claim of M. Teulère must of course fall to the ground. The reflectors were formed of sheet copper, plated with silver, and had a double ordinate of 31 French inches. It was not long before these improvements were adopted in England by the Trinity House of London, who sent a deputation to France to inquire into their nature. In Scotland, one of the first acts of the Northern Lights Board, in 1786, was to substitute reflectors in the room of coal lights, then in use at the Isle of May in the Firth of Forth, and the Cumbrae Isle in the Firth of Clyde, which had, till that period, been the only beacons on the Scotch coast. The reflectors employed were formed of facets of mirror glass, placed in hollow parabolical moulds of plaster, according to the designs of the late Mr Thomas Smith, the engineer of the Board, who, as appears from the article Reflector in the Supplement to the third edition of the Encyclopaedia Britannica, was not aware of what had been done in France, and had, himself, conceived the idea of this combination. The system of Borda was also adopted in Ireland; and, in time, variously modified, it became general wherever lighthouses were known.

The property of the parabola, by which all lines incident on its surface from the focus make with normals to the curve at the points of incidence, angles equal to the inclination of these same normals respectively to lines drawn parallel to the axis of the curve, is that which fits it for the purposes of a lighthouse. A hollow mirror, formed by the revolution of a portion of a parabola about its axis, has, in consequence of this property, the power of projecting the repeated images of a luminous point placed in its focus, in directions parallel to the axis of the generating curve; so that, when the mirror is placed with its axis parallel to the horizon, a cylindric beam of light is thereby sent forward in a horizontal direction. When such mirrors are placed side by side, with their axes parallel on the faces of a quadrangular frame which revolves about a vertical axis, a distant observer receives the successive impressions which result from the pas- sage of each face of the frame, over a line drawn between the observer's eye and the centre of the revolving frame. This arrangement constitutes what is called a revolving light. A fixed light is produced by placing, side by side, round a circular frame, a number of reflectors, with their axes inclined to each other, so as to be radii containing equal arcs of the frame on which they are placed. It is obvious that a perfect parabolic figure, and a luminous point mathematically true, would render the illumination of the whole horizon by means of a fixed light impossible; and it is only from the aberration caused by the size of the flame which is substituted for the point, that we are enabled to render even revolving lights practically useful. But for this aberration, even the slowest revolution in a revolving light, which would be consistent with a continued observable series, such as the practical seaman could follow, would render the flashes of a revolving light greatly too transient for any useful purpose; whilst fixed lights, being visible in the azimuths only in which the mirrors are placed, would, over the greater part of the distant horizon, be altogether invisible. The size of the flame, therefore, which is placed in the focus of a parabolic mirror, when taken in connection with the form of the mirror itself, leads to those important modifications in the paths of the rays, and the form of the resultant beam of light, which have rendered the catoptric system of lights so great a benefit to the benighted seaman.

It is obvious, from a consideration of the nature of the action which takes place in this combination of the paraboloidal mirrors with Argand lamps, that the revolving light is not only more perfect in its nature than the fixed light, but that it possesses the advantage of being susceptible of an increase of its power, by increasing the number of reflectors, which have their axes parallel to each other, so as to concentrate the effect of several mirrors in one direction. The perfect parallelism of the axes of separate mirrors, it is true, is unattainable, but approaches may be made sufficiently near for practical results; and in order to prolong the duration of the flash, the reflectors are sometimes placed on a frame, having each of its sides slightly convex, by which arrangement the outer reflectors of each face of the frame have their axes less inclined inwards from the radius of the revolving frame which pass through their foci.

The best proportions for the paraboloidal mirrors depend upon the object to which they are to be applied; as mirrors which are intended to produce great divergence in the form of the resultant beam should have one form; whilst those which are designed to cause a near approach to parallelism of the rays will have another form. These objects may also be attained by variations of the size of the flame applied in the same mirror; but it is much more advantageous to produce the effect by a change in the form of the mirror, as any increase of the flame beyond the size which is found to be most advantageous in other respects cannot be regarded otherwise than as a wasteful expenditure of light. The details into which a full investigation of this matter would lead us are quite beyond the scope of this article, and it therefore seems sufficient to give the formulae which express the relations which exist between the size of the flame, the reflecting surface, and the corresponding divergence of the reflected ray. If $\Delta$ represent the inclination of any reflected ray to the axis of a paraboloidal mirror, $e$ the distance of the focus from the point of reflection, and $d$ the distance from the edge of the flame to the focus in the plane of reflection, we shall have $\sin \Delta = \frac{d}{e}$; and when the flame in the given plane of reflection is circular, or has its opposite sides equidistant from the focus of the mirror, we shall, by putting $\Delta'$ for the effective divergence of the mirror in the given plane, have $\Delta' = 2\Delta$. When, therefore, great divergence, as in the case of the fixed lights, is required, the prolate form of the curve is to be preferred; and the oblate is conversely more suited to revolving lights.

The power of the reflectors ordinarily employed in lighthouses is generally equal to about 360 times the effect of the unassisted flame which is placed in the focus. This value, however, is strictly applicable only at the distances at which the observations have been made, as the proportional value of the reflected beam must necessarily vary with the distance of the observer, agreeably to some law dependent upon the unequal distribution of the light in the luminous cone which proceeds from it. The ordinary burners used in lighthouses are one inch in diameter, and the focal distance generally adopted is four inches, so that the effective divergence of the mirror in the horizontal plane may be estimated at about $14^\circ 22'$. In arranging reflectors on the frame of a fixed light, however, it would be advisable to calculate upon less effective divergence, for beyond $11^\circ$ the light is feeble; but the difficulty of placing many mirrors on one frame, and the great expense of oil required for so many lamps, have generally led to the adoption of the first valuation of the divergence.

The reflectors used in the best lighthouses are made of sheet copper plated, in the proportion of 6 oz. of silver to 16 oz. of copper. They are moulded to the paraboloidal form by a delicate and laborious process of heating with mallets and hammers, of various forms and materials, and are frequently tested during the operation, by the application of a carefully-formed mould. After being brought to the curve, they are stiffened by means of a strong bezale, and a strap of brass, which is attached to it for the purpose of preventing any accidental alteration of its figure. Polishing powders are then applied, and the instrument receives its last finish.

Two gauges of brass are applied to test the form of the reflector. One is for the back, and is used by the workmen during the process of hammering, and the other is applied to the concave face as a test, while the mirror is receiving its final polish. It is then tested, by trying a burner in the focus, and measuring the intensity of the light at various points of the reflected conical beam. Another test may also be applied successively to various points in the surface, by masking the rest of the mirror. Having placed a screen in the line of the axis of the mirror at some given distance from it, it is easy to find whether the image of a very small object placed in the conjugate focus, which is due to the distance of the screen, be reflected at any distance from that point on the centre of the screen through which the prolongation of the axis of the mirror would pass, and thus to obtain a measure of the error of the instrument. For this purpose it is necessary to find the position of the conjugate focus, which corresponds to the distance of the screen. If $b$ be the distance which the object should be removed outwards from the principal focus of the mirror, $d$ the distance from the focus to the screen, and $r$ the distance from the focus to the point of the mirror which is to be tested, we shall have $b = \frac{r^3}{d}$ as the distance which the object must be removed outwards from the true focus on the line of the axis.

The flame generally used in reflectors is from an Argand fountain-lamp, whose wick is an inch in diameter. Much care is bestowed upon the manufacture of these lamps for the Northern Lighthouses, which have their burners tipped with silver, to prevent wasting by the great heat which is evolved. These burners are also fitted with a slide apparatus, accurately formed, by which the burner may be removed from the interior of the mirror at the time of cleaning it, and returned exactly to the same place, and locked. by means of a key. This arrangement, which is shown in figs. 5, 6, and 7, is very important, as it ensures the burner always being in the focus, and does not require the reflector to be lifted out of its place every time it is cleaned; so that, when once carefully set and screwed down to the frame, it is never altered. In these figs. aaa represents one of the reflectors, b is the lamp, c is a cylindrical fountain, which contains 24 oz. of oil. The oil-pipe and fountain of the former is connected with the rectangular frame d, and is moveable in a vertical direction upon the guide-rods e and f, by which it can be let down and taken out of the reflector by simply turning the handle g, as will be more fully understood by examining fig. 6. An aperture of an elliptical form, measuring about 2 inches by three, is cut in the upper and lower part of the reflector, the lower serving for the free egress and ingress of the lamp, and the upper, to which the copper tube h is attached, serving for ventilation; i shows a cross section of the main bar of the chandelier or frame, on which the reflectors are ranged, each being made to rest on knobs of brass, one of which is seen at kk, and which are soldered on the brass band l, that clasps the exterior of the reflector. Fig. 5 is a section of the reflector aa, showing the position of the burner b, with the glass chimney k, and oil-cup l, which receives any oil that may drop from the lamp.

Fig. 6 shows the apparatus for moving the lamp up and down, so as to remove it from the reflector at the time of cleaning it. In the diagram, o, the fountain, is moved partly down; dd shows the rectangular frame on which the burner is mounted, e, e the elongated socket-guides, f, the rectangular guide-rod, connected with the perforated sockets on which the checking-handle g slides.

The modes of arranging the reflectors in the frames are shown in figs. 8, 9, and 10. It seems quite unnecessary, after what is said on the subject of divergence, to do more than remark, that in revolving lights the reflectors are placed with their axes parallel to each other, so as to concentrate their power in one direction; whilst in fixed lights it is necessary, in order to effect as equal a distribution of the light over the horizon as possible, to place the reflectors, with their axes inclined to each other at an angle somewhat less than that of the divergence of the reflected cone. For this purpose a brass gauge, composed of two long arms, somewhat in the form of a pair of common dividers, connected by means of a graduated limb, is employed. The arms having been first placed at the angle, which is supplemental to that of the inclination of the axes of the two adjacent mirrors, are made to span the faces of the reflectors, one of which is moved about till its edges are in close contact with the flat surface of one of the arms centre, which supports the whole, o, o the reflectors, and p, p the fountains of their lamps.

A variety of the parabolic reflectors has been invented Bordier by M. Bordier Marce, the pupil and successor of Argand, Marce's who has laboured with much enthusiasm in perfecting catoptric instruments, more especially with a view to their application to the illumination of lighthouses and the streets of towns. Amongst many other ingenious combinations of parabolic mirrors, he has invented and constructed an apparatus, which is much used in harbour-lights on the French coast. The object of this apparatus is to fulfil, as economically as possible, the conditions required in a fixed light, by illuminating, with perfect equality, every part of the horizon, by means of a single burner; and M. Bordier Catoptric lights are susceptible of nine separate distinctions, which are called fixed, revolving white, revolving red and white, revolving red with two whites, revolving white with two reds, flashing, intermittent, double fixed lights, Distinct and double revolving lights. The first exhibits a steady tone of and uniform appearance, which is not subject to any change; and the reflectors used for it are of smaller dimensions than those employed in revolving lights. This is necessary, in order to permit them to be ranged round the circular frame, with their axes inclined at such an angle as shall enable them to illuminate every point of the horizon. The revolving light is produced by the revolution of a frame with three or four sides, having reflectors of a large size grouped on each side, with their axes parallel; and as the revolution exhibits a light gradually increasing to full strength, and in the same gradual manner decreasing to total darkness, its appearance is extremely well marked. The succession of red and white lights is caused by the revolution of a frame whose different sides present red and white lights; and these, as already mentioned, afford three separate distinctions, namely, alternate red and white; the succession of two white lights after one red; and the succession of two red lights after one white light. The flashing light is produced in the same manner as the revolving light; but owing to a different construction of the frame, and the greater quickness of the revolution, a totally different and very striking effect is produced. The brightest and darkest periods being but momentary, this light is characterized by a rapid succession of bright flashes, from which it gets its name. The intermittent light is distinguished by bursting suddenly into view, and continuing steady for a short time, after which it is suddenly eclipsed for half a minute. Its striking appearance is produced by the perpendicular motion of circular shades in front of the reflectors, by which the light is alternately hid and displayed. This distinction, as well as that called the flashing light, are peculiar to the Scotch coast, having been first introduced by Mr R. Stevenson, the late engineer of the Northern Lights Board. The double lights, which are generally used only where there is a necessity for a leading line, as a guide for taking some channel, or avoiding some danger, are exhibited from two towers, one of which is higher than the other; and when seen in one line, form a direction for the course of shipping. At the Cala of Man a striking variety has been introduced into the character of leading lights, by substituting for two fixed lights, two lights which revolve in the same periods, and exhibit their flashes at the same instant; and these lights are, of course, susceptible of the other variety enumerated above, that of two revolving red and white lights revolving in equal periods. The utility of all these distinctions is chiefly to be imputed to their at once striking the eye of an observer, and being instantaneously obvious to strangers.

Before entering upon the subject of the dioptric lights, the writer of this article embraces with pleasure the opportunity afforded to him of acknowledging the liberality of M. Léonor Fresnel, the late Secretary of the Lighthouse Commission of France. It was entirely owing to the readiness with which M. Fresnel afforded him access to every avenue of information on the subject of lighthouses that he was enabled to effect the object of a mission to France, on which he was sent in the year 1834, by the Commissioners of Northern Lights.

The first proposal of applying lenses to lighthouses is recorded by Smeaton in his account of the Eddystone Lighthouse, where he mentions that, in 1759, an optician in London proposed grinding the glass of the lantern to a

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1 In all probability directly derived from the Greek ἑρεστικόν, an optical instrument with holes for looking through, which is a compound of ἑρεστικόν, through, and ἑρεστικόν, I see. radius of seven feet six inches; but the description is too vague to admit of even a conjecture regarding the proposed arrangement of the apparatus. About forty years ago, however, lenses were actually tried in several lighthouses in the south of England; but their imperfect figure, and the quantity of light absorbed by the glass, which was of inferior quality and of considerable thickness, rendered their effect so much inferior to that of the parabolic reflectors then in use, that, after trying some strange combinations of lenses and reflectors, the former were finally abandoned.

The object to be attained by the use of lenses in a lighthouse is, of course, identical with that which is answered by employing reflectors; and both instruments effect the same end by different means, collecting the rays which diverge from a point called the focus, and projecting them forward in a beam, whose axis coincides with the produced axis of the instrument. The actions by which these similar results are effected have been termed reflection and refraction. In the one case the light, as has been already said, merely impinges on the reflecting surface, and is thrown back; whilst in the other, the rays pass through the refracting medium, and are bent or refracted from their natural course.

The celebrated Buffon, to prevent the great absorption of light by the thickness of the material, which would necessarily result from giving to a lens of great dimensions a figure continuously spherical, proposed to grind out of a solid piece of glass a lens in steps or concentric zones. This suggestion of Buffon, regarding the construction of large burning glasses, was first executed, with tolerable success, about the year 1780, by the Abbé Rochon; but such are the difficulties attending the process of working a solid piece of glass into the necessary form, that it is believed the only other instrument ever constructed in this manner is that which was made by Messrs Cookson of Newcastle-upon-Tyne, for the Commissioners of Northern Lighthouses.

The merit of having first suggested the building of these lenses in separate pieces seems to be due to Condorcet, who in his *Eloge de Buffon*, published so far back as 1773, enumerates the advantages to be derived from this method. Sir David Brewster also described this mode of building lenses in 1811, in the *Edinburgh Encyclopaedia*; and in 1822, the late eminent Fresnel, alike unacquainted with the suggestions of Condorcet, or the description by Sir David Brewster, explained, with many ingenious and interesting details, the same mode of constructing these instruments. To Fresnel belongs the additional merit of having first followed up his invention by the construction of a lens, and, in conjunction with MM. Arago and Mathieu, of placing a powerful lamp in its focus, and, indeed, of finally applying it to the practical purposes of a lighthouse. The fertile genius of the French Academician has produced many ingenious combinations of dioptric instruments for lighthouses, which we shall have occasion to notice in the sequel.

The great advantages which attend the mode of construction proposed by Condorcet are, the ease of execution, by which a more perfect figure may be given to each zone, and spherical aberration almost entirely corrected, and the power of forming a lens of larger dimensions than could easily be made from a solid piece. Buffon appears only to have had in view the reduction of the thickness of the lens, but Condorcet distinctly suggests the possibility of correcting the spherical aberration by properly selected centres for the various zones.

To Fresnel, however, is due the credit of having determined these centres, which constantly receive from the axis of the lens in proportion as the zones to which they refer are removed from its centre; and the surfaces of the zones of the annular lens, consequently, are not parts of concentric spheres, as in Buffon's lens. It deserves notice, that the first lenses constructed for Fresnel by M. Soleil had their zones polygonal, so that the surfaces were not annular, a form which Fresnel considered less accommodated to the ordinary resources of the optician. He also, with his habitual penetration, preferred the plano-convex to the double-convex form, as more easily executed. After mature consideration, he finally adopted crown glass, which, notwithstanding its greenish colour, he considered more suitable than flint glass, as being less liable to strie. All his calculations were made in reference to an index of refraction of 1.51, which he had verified by repeated experiments, conducted with that patience and accuracy for which, amidst his higher qualities, he was so remarkably distinguished. These instruments have received the name of annular lenses, from the figure of the surface of the zones.

Fig. 12 exhibits a plan and section of an annular lens of the largest size, whose focal distance is 92 centimetres, or about 36'22 inches, and which subtends a luminous pyramid of 46° of inclination, having its apex in the flame.

Having once contemplated the possibility of illuminating lighthouses by dioptric means, Fresnel quickly perceived the advantage of employing for fixed lights a lamp placed in the centre of a polygonal hoop, consisting of a series of cylindric refractors, infinitely small in their length, and having their axes in planes parallel to the horizon.

Such a continuation of vertical cylindric sections of various curvatures, by refracting the rays proceeding from the focus only in a direction perpendicular to the vertical sections of the cylindric parts, must distribute a zone of light equally brilliant in every point of the horizon. This effect will be easily understood, by considering the middle vertical section of one of the great annular lenses or burning glasses, already described, abstractly from its relation to the rest of the instrument. It will readily be perceived that this section possesses the property of refracting the rays in the vertical plane only, without interfering with azimuthal divergence; and if this section, by its revolution about a vertical axis, becomes the generating line of the enveloping hoop above noticed, such a hoop would of course possess the property of refracting an equally diffused zone of light round the horizon. The difficulty, however, of forming this apparatus, appeared so great, that Fresnel determined to substitute for it a vertical polygon, composed of what have been improperly called cylindric lenses, but which in reality are mixtilinear and horizontal prisms, distributing the light which they receive from the focus equally over the horizontal sector which they subtend. This polygon has a sufficient number of sides to enable it to give, at the angle formed by the junction of two of them, a light not very much inferior to what is produced by one of the sides; and upper and lower courses of curved mirrors are so placed as to make up for the deficiency of the light at the angles. The effect sought for in a fixed light is thus obtained in a much more perfect manner than by any combination of the parabolic mirrors formerly used in the British lighthouses.

An ingenious modification of the fixed apparatus is due to the inventive mind of Fresnel, who conceived the happy idea of placing one apparatus of this kind in front of another, with the axes of the cylindric pieces crossing each other at right angles. As these cylindric pieces have the property of refracting all the rays which they receive from the focus into a direction perpendicular to the mixtilinear section which generates them, it is obvious that, if two refracting media of this sort be arranged as proposed by Fresnel, their joint action will unite the rays which come from their common focus into a beam, whose sectional area is equal to the overlapped surface of the two instruments, and thus produce the effect of an annular lens. It was by availing himself of this property of crossed prisms that Fresnel invented the distinction for lights, which he calls a fixed light varied by flashes; in which the flashes are caused by the revolution of cylindric media, with vertical axes round the fixed-light apparatus already described.

The modification just described is shown at fig. 16. This instrument is, however, now supplanted by a revolving apparatus, consisting of alternate sections of a fixed light apparatus, and a Holophotal apparatus to be afterwards described. By the revolution of this composite apparatus the same effect is obtained, while the flash is produced by the action of a single optical agent, instead of by two, as in Fresnel's arrangement.

Fresnel immediately perceived the necessity of combining, with the dioptric instruments which he had invented, a burner capable of producing a large volume of flame; and the rapidity with which he matured his notions on this subject, and at once produced an instrument admirably adapted for the end he had in view, affords one of the many proofs of that happy union of practical with theoretical talent for which he was so distinguished. Fresnel himself has modestly attributed much of the merit of the invention of this lamp to M. Arago; but that gentleman, with great candour, gives the whole credit to his deceased friend, in a notice regarding lighthouses, which appeared in the Annaire du Bureau des Longitudes of 1831. The lamp has four concentric burners, which are defended from the action of the excessive heat produced by their united flames by means of a superabundant supply of oil which is thrown up from the cistern below by a clock-work movement, and constantly overflows the wicks, as in the mechanical lamp of Carcel. A very tall chimney is found to be necessary, in order to supply fresh currents of air to each wick with sufficient rapidity to support the combustion. The carbonization of the wicks, however, is by no means so rapid as might be expected, and it is even found that, after they have suffered a good deal, the flame is not sensibly diminished, as the great heat evolved from the mass of flame promotes the rising of the oil in the cotton. So perfect, indeed, is the action of this great lamp, that it has been known to burn for upwards of twelve hours without being snuffed, or even having the wicks raised.

The annexed diagrams will give a more perfect idea of the nature of the concentric burner than can easily be conveyed by words alone.

Fig. 13 shows a plan of a burner of four concentric wicks. The intervals which separate the wicks from each other, and allow the currents of air to pass, diminish in width a little as they recede from the centre. Fig. 14 shows a section of this burner. C, C', C", C" are the rack handles for raising or depressing each wick. AB is the horizontal duct which leads the oil to the four wicks; L, L, L, are small plates of tin by which the burners are soldered to each other, and which are so placed as not to hinder the free passage of the air; P is a clamping screw which keeps at the proper height the gallery R, R which carries the chimney. Fig. 15 shows the burner with its glass chimney and damper. E is the glass chimney, F is a sheet-iron cylinder, which serves to give it a greater length, and has a small damper D, capable of being turned by a handle for regulating the supply of air; and B is the pipe which supplies the oil to the wicks. The great risk in using this lamp arises from the leather valves, that force the oil by a clock-work movement, being occasionally liable to derangement; and several of the lights on the French coast, and more especially the Corduan, have been extinguished for a few minutes by the failure of the lamp, an accident which has never, and scarcely can happen with the fountain lamps which illuminate the reflectors. To prevent the occurrence of such accidents, and to render their consequences less serious, various precautions have been resorted to. Amongst others, an alarm is attached to the lamp, consisting of a small cup pierced in the bottom, which receives part of the overflowing oil from the wicks, and is capable, when full, of balancing a weight placed at the opposite end of a lever. The moment the machinery stops, the cup ceases to receive the supply of oil, and the remainder running out at the bottom, the equilibrium of the lever is destroyed, and, in falling, it disengages a spring which rings a bell sufficiently loud to waken the keeper, should he chance to be asleep. There is another precaution of more importance, which consists in having always at hand in the light-room a spare lamp trimmed and adjusted to the height for the focus, which may be substituted for the other in case of accident. It ought to be noticed, however, that it takes about twenty minutes from the time of applying the light to the wicks to bring the flame to its full strength, which, in order to produce its best effect, should stand at the height of nearly four inches (10 cm.). The inconveniences attending this lamp have led to several attempts to improve it; and amongst others M. Delaveleye has proposed to substitute a pump having a metallic piston, in place of the leather valves, which require constant care, and must be frequently renewed. A lamp was constructed in this manner by M. Lepaute, and tried at Corduan; but it was afterwards discontinued until some further improvements could be made upon it. It has lately been much improved by M. Wagner, an ingenious artist whom M. Fresnel employed to carry some of his improvements into effect. In the dioptric lights on the Scotch coast, a common lamp with a large wick is kept constantly ready for lighting; and in the event of the sudden extinction of the mechanical lamp by the failure of the valves, it is only necessary to unscrew and remove its burner, and put the reserve-lamp in its place. The height of this lamp is so arranged, that its flame is in the focus of the lenses, when the lamp is placed on the ring which supports the burner of the mechanical lamp; and as its flame, though not very brilliant, has a considerable volume, it will answer the purpose of maintaining the light for an hour or two until the light-keepers have time to repair the valves of the mechanical lamp.

The divergence of the annular lens is greatly less than that of the parabolic mirror. It may be estimated in the following manner. Let $\Delta$ be the angle of divergence of any ray emerging from the lens, $l$ the distance of the point of incidence from the principal focus of the lens, and $r$ the radius of the flame, and we have $\sin \Delta = \frac{r}{l}$, and when $\Delta'$ is made the angle of the effective divergence of the lens, we have $\Delta' = 2\Delta$.

Adopting this rule, we find the effective divergence of the lens to be about 5° 9', which does not differ much from the observed divergence.

The manufacture of the dioptric instruments is not distinguished by any peculiarity which requires special notice, the grinding and polishing being performed by means of powders gradually increasing in fineness, successively applied as in the ordinary process of grinding glass. The zones are united by a glue which possesses the important property of being able to resist the action of considerable heat, whilst it is by no means brittle. M. Fresnel intrusted the work of building the first lens to the late M. Soleil, optician to the King of France, to whose zeal and intelligence he bears ample testimony in the Memoire, in which he describes the invention.

In order to test the figure of the lenses, moulds carefully made may be applied; or the lens being mounted on a stand which permits its being set at any angle, the accuracy of the whole instrument, and of each portion of it, may be separately tested by the form and size of the spectrum which is formed in the principal focus, by permitting the solar rays to fall upon the lens at right angles. When any particular portion is to be tried, the rest of the surface is covered with discs of strong grey paper or pasteboard.

Another method may be employed similar to that already described as applicable to reflectors. This method consists in finding whether a small object placed in any point of the axis farther from the lens than the principal focus, has its image refracted accurately to a point on a screen placed in the conjugate focus which is due to that distance. The same principle of testing the instrument is also applied when a person stationed at a given short distance in front of the lens observes whether its whole surface be completely illuminated by a small flame placed in the conjugate focus corresponding to that distance. All that is necessary, therefore, is to determine these distances by means of formulae which express the relations of the distances of the object and its image. If $\delta$ represent the distance of the eye from the lens, $\phi$ the principal focus, and $\phi'$ the distance of the conjugate focus corresponding to the observer's distance $\delta$, then we have $\phi' = \frac{\delta \phi}{\delta - \phi}$.

If, again, adopting the same notation, we wish to find the distance at which the image of an object placed at a given distance from the lens greater than that of the principal focus, should be accurately impressed on a screen, we have $\delta = \frac{\phi \phi'}{\phi' - \phi}$.

The curved mirrors, placed above and below the lenses as a supplement to them, are, strictly speaking, generated by portions of parabolas having their foci coincident with the common flame of the system. In practice, however, they are made portions of a curve surface, ground by the radius of the circle which osculates the given parabola, and passes tangentially through the middle of the chord which subtends the arc of the mirror. These mirrors are plates of glass, silvered on the back, and set in flat cases of sheet brass. They are suspended on a circular frame of screws, which are attached to the backs of the cases, and which afford the means of adjusting them to their true position in the light-room, so that they may reflect the horizon of the lighthouse to an observer's eye placed in the focus of the system. In order to test the accuracy of the mirrors, recourse may again be had to the formulae of conjugate foci; thus, if we put $r$ equal to the radius of curvature of the mirror, $d$ equal to the given distance of any object from the mirror, and $d'$ equal to the distance of a moveable screen, which shall receive the true image of the object if the mirror be accurately formed, we shall have for this latter distance $d' = \frac{rd}{2d-r}$.

The effect of an annular lens may be estimated at moderate distances to be nearly equal to that of 3000 Argand flames of about an inch diameter; that of a cylindric refractor at about 250; and that of a curved mirror may, perhaps, on an average, be assumed at about 10 Argand flames.

A beautiful apparatus, which has received the name of Cataldioptric light, from the compound action by which it is characterized, was another of Fresnel's applications of dioptric instruments to the purposes of a lighthouse. This elegant apparatus consists of thirteen rings of glass of various diameters, arranged one above another, in an oval form. The five middle rings have an interior diameter of 11-81 inches (30cm), and form a cylindric lens, similar to that already described under the head "cylindric refractors." The other rings or prisms, five of which are upper and three lower, are ground and set in such a manner that they project all the light derived from the focus in a direction parallel to the other rays by total reflection. This effect is produced by arranging the prisms, so that the incident rays, after being refracted at the first surface, shall strike the second side of the prism at such an angle that, instead of passing through the prism at that point, they shall be totally reflected from it; and, after a second refraction, emerge from the third side in a direction parallel to those transmitted by the middle or simply reflecting rings. When this apparatus is employed to light only a part of the horizon, the rings are discontinued on the side next to the land, and room is thus obtained for using a common fountain lamp; but when the whole horizon is to be illuminated, the apparatus must inclose the flame on every side, so that it has in this case been found necessary to employ the hydrostatic lamp of Thillotier, in which the balance is sulphate of zinc in solution. Fresnel was prevented, by an early death, the consequence of severe application to scientific pursuits, from ever constructing this beautiful instrument; and it was reserved for the present enlightened secretary of the Commission des Phares to complete his brother's invention.

The nature of this apparatus will be fully understood by a reference to fig. 16, which shows its section and plan. F is the focal point in which the flame is placed, r, r cylindrical refractors, forming by their union a cylinder with a lamp in its axis, and producing a zone of light of equal intensity all round the horizon, and $r'$, $r''$ are cylindric refractors having their axes at right angles to those of the refractors $r$, $r'$, and revolving round them. These exterior refractors in front of the inner refractors, which have been already described under the head of Cylindric Refractors, produce, by compound refraction, a beam similar to that resulting from an annular lens. $x$, $x'$ are catadioptric prismatic rings acting by total reflection, and giving out zones of light of equal intensity at every point of the horizon. The dotted lines show the course traversed by the rays of light which proceed from the lamp and are acted upon by the rings of glass. The catadioptric rings supply the places of the curved mirrors, which had at one time to be employed in the larger class of lights for a similar purpose, and as the reflection from the inner surface of a prism is, theoretically speaking, total, and the whole loss of light is merely that which is due to absorption in passing through the glass, and that which takes place at the two surfaces, there must of necessity be a much greater proportion of the incident light transmitted by the catadioptric action than can ever be obtained from the most perfect reflecting surface, the loss from reflection being held to be in no case less than one-half of the incident light.

The loss of light by reflection at the surface of the most perfect mirrors, and the perishable nature of the material composing their polish, induced Mr Alan Stevenson, so far back as 1835, in a report on the Light of Inchkeith, which had just been altered to the dioptric system, to propose the substitution of totally reflecting prisms, even in lights of the first order or largest dimensions. In this attempt he was much encouraged by the singular liberality of M. Léon Fresnel, who not only freely communicated the method pursued by his distinguished brother Augustin Fresnel, in determining the forms of the zones of the small apparatus, introduced by him into the Harbour Lights of France, and his own mode of rigorously solving some of the preliminary questions involved in the computations; but also made various important suggestions, which substantially embrace the whole subject. The result was the preparation of a table containing Mr Stevenson's calculations of the forms of the zones of the first order, which are verifications of those of M. Fresnel; and the first catadioptric apparatus ever constructed, through the ardour and perseverance of M. François Soleil, on so magnificent a scale, was fitted up in the Skerryvore Lighthouse. In December 1843, a trial of the apparatus, attended with complete success, was made at the Royal Observatory of Paris, whereby it appeared that the illuminating effect of the cupola of zones was to that of the seven upper tiers of mirrors of the first order, as 140 to 87.

Mr Alan Stevenson having been directed by the Commissioners of the Northern Lighthouses to convert the fixed catoptric light of the Isle of May into a dioptric light of the first order, proposed that an attempt should be made to form a true cylindric, instead of a polygonal belt, for the refracting part of the apparatus; and this task was successfully completed by Messrs Cookson of Newcastle. The defect of the polygon lies in the excess of the radius of the circumscribing circle over that of the inscribed circle, which occasions an unequal distribution of light between its angles and the centre of each of its sides; and this fault can only be fully remedied by constructing a cylindric belt, whose generating line is the middle mixtilinear section of an annular lens, revolving about a vertical axis passing through the principal focus. This is, in fact, the only form which can possibly produce an equal diffusion of the incident light over every part of the horizon.

In a report to the Commissioners of the Northern Lights there is the following description of the refractors constructed for the Isle of May light:—"I at first imagined," says Mr Alan Stevenson, "that the whole hoop of refractors might be built between two metallic rings, connecting them to each other solely by the means employed in cementing the pieces of the annular lenses; but a little consideration convinced me that this construction would make it necessary to build the zone at the lighthouse itself, and would thus greatly increase the risk of fracture. I was therefore reluctantly induced to divide the whole cylinder into ten arcs, each of which being set in a metallic frame, might be capable of being moved separately. The chance of any error in the figure of the instrument has thus a probability of being confined within narrower limits; whilst the rectification of any defective part becomes at the same time more easy. One other variation from the mode of construction at first contemplated has been forced upon me by the repeated failures which occurred in attempting to form the middle zone in one piece; and it was at length found necessary to divide this belt by a line passing through the horizontal plane of the focus. This division of the central zone, however, is attended with no appreciable loss of light, as the entire coincidence of the junction of the two pieces with the horizontal plane of the focus, confines the interception of the light to the fine joint at which they are cemented. With the exception of these trifling changes, the idea at first entertained of the construction of this instrument was fully realized, at the very first attempt, in the manufactory of Messrs Cookson." At a subsequent period the central zones were formed in one piece, and the arrangement of the apparatus greatly improved, by giving to the metallic frames which contain the prisms a rhomboidal instead of a rectangular form. The junction of the frames being thus inclined from the perpendicular, do not in any azimuth intercept the light throughout the whole height of the refracting belt, but the interception is confined to a small rhomboidal space, whose area is inversely proportional to the sine of the angle of inclination; and when the helical joints are formed between the opposite angles of the rectangular frames, the amount of intercepted light becomes absolutely equal in every azimuth. Time and perseverance, and the patience and skill of M. François Soleil, who was urged to undertake the task, was at length crowned with success, and Mr Stevenson had the satisfaction of at last seeing a fixed light apparatus, of a truly cylindric form, with its central belt in one piece and the joints of each panel inclined to the horizon at such an angle as to render the light perfectly equal in every azimuth. Lanterns with diagonal framings are now also constructed in conformity with this arrangement of the zones.

The change of the light at the Isle of May, from the catoptric to the dioptric system, was generally acknowledged to be an improvement. A committee of the Royal Society of Edinburgh made some observations on the two lights which were exhibited in contrast on the night of the 26th of October 1836, from the town of Dunbar, which is distant about 13 miles from the lighthouse. Their report, which was drawn up by Professor Forbes, concludes in these words:—

"The following conclusions seem to be warranted:— 1. That at a distance of 13 miles, the mean effect of the new light is very much superior to the mean effect of the old light (perhaps in the ratio of two to one).

2. That at all distances, the new light has a prodigious superiority to the old, from the equality of its effects in all azimuths.

3. That the new light fulfils rigorously the conditions required for the distribution of light to the greatest advantage.

4. That at distances much exceeding 13 miles, the new light must still be a very effective one, though to what extent the committee have not observed. The light is understood to be still a good one, when seen from Edinburgh at a distance of about 30 miles.

There are few finer specimens of art than an entire apparatus for a fixed light of the first order, as shown in fig. 17. It consists of a central belt of refractors, forming a hollow cylinder 6 feet in diameter, and 30 inches high; below it are six triangular rings of glass, ranged in a cylindrical form, and above a crown of thirteen rings of glass, forming by their union a hollow cage, composed of polished glass, 10 feet high and 6 feet in diameter! There is no work of art more beautiful or more creditable to the boldness, intelligence, and zeal of the artist.

All the lights on the dioptric principle are illuminated by a flame placed in the centre of the apparatus or common focus of the principal lenses and cylindric refractors which are ranged round it. The burner of the lamp varies in its dimensions and its consumption of oil, according to the size of the instruments employed, which also determines what is called the order of the light, a name expressive of its power and range. Above and below the strictly dioptric part of the apparatus of each order there are also accessory parts, which, as just described, and shown in fig. 17, consist in fixed lights of catadioptric prisms arranged in tiers, one above another, like the leaves of a Venetian blind, and placed so as to reflect to the horizon the rays received from the lamp, which is in their common focus.

In all revolving lights, up till 1850, the apparatus above the principal lenses either consisted of prisms similar to those described for fixed lights, or was diacatoptric, being composed of an union of eight lenses of 19'68 inches (50cm) of focal distance, inclined inwards to the flame, which is in their common focus, and thus forming a frustum of an octagonal pyramid of 50° of inclination. These upper lenses were surmounted by plane mirrors, placed so as to reflect horizontally the beams transmitted by the lenses. In placing these upper lenses, it has been thought advisable to give their axes a horizontal inclination of 7° from that of the great lenses. By this arrangement, the flash of the upper lenses always precedes that of the principal lenses.

The use of the necessary apparatus is to collect the rays, which would otherwise pass above and below the main lenses, without contributing to the brilliancy of the light. These inclined mirrors and lenses have, since 1850, been entirely laid aside on the introduction of the Holophotal system of revolving lights, to be afterwards explained, in which totally reflecting prisms form part of the revolving apparatus, and supersede the inclined mirrors and lenses. The nature of the whole revolving apparatus of Fresnel will be more fully understood by referring to fig. 18, which is a section of a revolving dioptric apparatus of the first order; L, L, are the great annular lenses, forming by their union an octagonal prism, with the lamp in its axis, and projecting in horizontal beams the light which they receive from the focus; a, a, upper lenses, forming by their union a frustum of an octagonal pyramid of 50° of inclination, and having their feet coinciding in the focal point. They parallelize the rays of light which pass over the lenses. b, b, plane mirrors, placed above the pyramidal lenses, and so inclined as to project the beams reflected from them in planes parallel to the horizon.

The dioptric lights used in France are divided into four orders, in relation to their power and range; but in regard to their characteristic appearances, this division does not apply, as, in each of the orders, lights of identically the same character may be found, differing only in the distance at which they can be seen, and in the expense of their maintenance. The four orders may be briefly described as follows:

1st. Lights of the first order, having an interior radius or focal distance of 35'22 inches (92cm), and lighted by a lamp of four concentric wicks, consuming 570 gallons of oil per annum.

2nd. Lights of the second order, having an interior radius of 27'55 inches (70cm), lighted by a lamp of three concentric wicks, consuming 384 gallons of oil per annum.

3rd. Lights of the third order, lighted by a lamp of two concentric wicks, consuming 183 gallons of oil per annum.

The instruments used in these lights are of two kinds, one having a focal distance of 19'68 inches (50cm), and the other of 9'84 inches (25cm).

4th. Lights of the fourth order, or harbour lights, having an internal radius of 5'9 inches (15cm), and lighted by a lamp of one wick, or Argand burner, consuming 48 gallons of oil per annum.

The great loss of light by natural divergence in the parabolic reflector, and the separation of the rays into as many systems... Lighthouses.

The first instrument constructed upon this principle was for the North Harbour of Peterhead, where it has been in use since August 1849. Mr A. Stevenson has also adopted an instrument of this kind, on a large scale, in Hoy Sound Lighthouse, one of the Northern Lights' stations.

Experiments were lately made at Gullan Hill to test the comparative power of an instrument on this principle (the reflecting part being of brass, and but roughly finished), and a highly-finished silver reflector of the usual construction; both instruments being 25 inches in diameter at the lips. The lights were viewed at distances of from 7 to 12 miles every night during a week, and in every instance the brass reflector on the holophotal principle had the advantage of the silver reflector; and on one occasion, when the atmosphere was thick, the light from the holophotal brass reflector was alone visible.

In so far as concerns the arrangement of the different catadioptric parts, irrespective of the nature of the materials of which they are composed, the light emitted from any given flame by the instruments just described, should be the light of maximum intensity. But the most accurate experiments which have been undertaken by scientific observers have shown that reflection from the best silvered mirrors, and even from metallic specula made with the utmost care for experimental purposes, involves a loss of light by absorption of not less than about one-half of the whole incident rays.

The advantage of employing as largely as possible the principle of total reflection from glass, in place of ordinary reflection from metallic specula, induced Mr T. Stevenson to attempt further improvements in the holophotal system of illumination. If we retain the lens and the spherical mirror of the holophotal apparatus just described, and, in place of the paraboloid, conceive the arc between the lens and the spherical mirror to be filled up with glass rings, which are the solids of revolution generated by the rotation of the cross section of the totally reflecting prisms used in fixed lights, round a horizontal axis passing through the flame, we shall then have extended the action of the lens, so as to parallelize one-half of the whole sphere of incident rays. Such an arrangement is shown in figs. 21 and 22, where L is the lens, p the totally reflecting prisms, and b is the spherical mirror. The distinguishing peculiarity of this arrangement is, that the prisms, instead of transmitting the light in parallel vertical plates, diverging all round as in the fixed light apparatus of Fresnel, produce an extension of the lenticular or quadruplex action of the common annular lens, by assembling the light around its axis in the form of concentric hollow cylinders. In order to distinguish this system of prisms from those introduced by M. Fresnel, which have no lenticular action, they may therefore be termed catadioptric lenses.

The writer of this article has lately heard, that, although it had never been proposed to use such prisms for lighthouse purposes, a small lamp, for lighting the quays of a canal in Paris, was made by M. A. Fresnel, in 1824, in which some prisms, similar to those above described, formed an accessory part; but no drawing or description of such an apparatus was ever published, and all knowledge of it seems to have been Lighthouses.

lost till Mr T. Stevenson described his plan in 1849. Since that time, thirty-six instruments have been made at Edinburgh, on the small scale, both for home and foreign use. The first instance in which those prisms were ever applied to lighthouses was at the Horburgh Light, near Singapore; the optical part of which was designed by Mr T. Stevenson. This was the first time that the principle of total reflection was applied to the moving apparatus of revolving lights. The first light of the large order, on this principle, was constructed under the direction of Mr Alan Stevenson, for North Ronaldsay, in the Orkneys; and seven others have recently been made in Paris for French and American lighthouses. There can be no doubt that this arrangement will be generally adopted, instead of the combination of inclined lenses and plane mirrors, first employed at Corduan, and subsequently in other revolving lights in France and Britain.

By an elegant adaptation of totally reflecting zones, Mr Thomas Stevenson has further succeeded in substituting, for a reflector of metal or silvered glass, a polygonal totally reflecting hemisphere of glass (vide fig. 5). By this arrangement, reflection from metallic specula is abolished from every part of the system, and the principles of total reflection and simple refraction alone are employed.

The action of these glass zones will be best understood by referring to fig. 23, which gives the cross section of one of them; \( f \) shows the flame or centre of the system, and the diverging rays are represented by dotted lines, the arrows indicating the direction of a ray before and after being altered by the prism. The side \( bc \) is concave, the centre of curvature being in \( f \), the centre of the flame. The surfaces of the other sides, \( ab \) and \( ac \), are portions of parabolas, whose common focus is \( f \); or of circles osculating the parabolic curves. Those parabolic surfaces face each other, and their tangents form an angle of 90° with each other at the vertex of the prism. Any ray proceeding from the centre \( f \) will be received as a normal to the surface \( bc \), and will consequently pass on without suffering any deviation at \( e \), where it meets the prism, to its incidence on the surface \( ab \), where total reflection, and finally emerges in a radial direction without deviation at the point \( e \). An exactly similar action will take place simultaneously with another ray in the same path from the flame, though passing in an opposite direction. The concentric zones, \( a \), which compose the dome (vide fig. 23), are solids of revolution, generated by the rotation round the horizontal axis of the instrument of triangles similar to \( a, b, c \) (fig. 23), with a radius equal to \( af \). The angle formed by the radius with the horizontal axis of the instrument varies from nearly 90° down to zero, as shown in fig. 24. Where those angles vanish at \( a \), a conoid will result, having the radius of its base equal to the semichord of its inner surface. It will be seen that the prisms \( b, a, c \), fig. 23, resemble in their action that of the drops of rain which give rise to the phenomenon of the secondary rainbow. In fig. 24, which shows the whole instrument complete, \( L \) represents the common lens acting on the rays by refraction only; \( p \), the cataleptic portion of the lens acting by refraction and total reflection; and \( a, a, a \), the prisms acting by total reflection only. Part of a hemispherical dome on this principle has now been successfully executed.

Fig. 25 shows the adaptation of the holophtal system to Fresnel's revolving light of the first order. \( L \) are the lenses, and \( p \), the cataleptic lenticular rings. The advantage of this system will clearly appear by comparing it with Fresnel's apparatus (fig. 18). At each of figs. 25 and 18, diagrams marked \( xy \) show proportionate sections of the beam of light given out by the two arrangements respectively. The objectionable arrangement of the lenses and mirrors \( a \) and \( b \) (fig. 18), in which so much of the light is lost, by the introduction of so many surfaces, and is also superceded.

For pointing out sunken reefs, on which no lighthouse Ding can be placed, two plans have been proposed by Mr Thomas and Stevenson. The first, or dipping light, consists in throwing a pencil of rays from a lantern on shore down upon the reef, by inclining the vertical axis of the apparatus at the requisite angle; and by this arrangement the visibility of the light is confined within certain limits, the passing within which is, to the seaman, a sign of danger, and a warning to haul off seaward. The second, or apparent light, applies to reefs on which a small beacon, capable of bearing a cage on its top, can be built, but which affords no room for a lighthouse tower. In this cage is placed some reflecting apparatus, either wholly cataleptic, or combining a plane mirror with a vertical system of straight lenticular prisms; and upon this there is projected, from a lantern on shore, a beam of light, which is diverted (according to the angle at which the cage apparatus on the beacon is set) into any given direction that may be required. This latter plan has been most successfully applied to the entrance of Stormoway Loch, a much-used harbour of refuge in the Lewis, and has drawn from many shipmasters frequenting that port written expressions of their satisfaction with the light, which is well seen a mile and a half in the offing; a distance amply sufficient for the limited purposes for which such lights should be used. The name apparent is given to this light, because, while it is really produced by a flame on shore, it seems to proceed from the beacon on the rock.

In all fixed lights of the present construction, which are not required to show all round the horizon, the light of the darkened part is either allowed to be lost, or is returned through the focus by means of a portion of a spherical holophotal mirror. There are, however, cases frequently occurring to the lighthouse engineer where it would be very desirable to employ this spare light, not in the direction diametrically opposite to the darkened arc, but in some other direction more suited to the configuration of the coast. The apparatus for Isle Oronsay, of which fig. 26 is a horizontal section through

the focus, is the first which has been designed with a view to the most advantageous employment of this spare light.

The lighthouse of Isle Oronsay is situated in the narrow Sound of Skye, and, throughout nearly the whole of the illuminated arc, does not require to be seen to a greater distance than 3 or 4 miles, while in one direction (down the sound) it can be seen for 15 miles, and in another (up the sound) it can be seen for about 7 miles. If, therefore, a light were erected sufficiently powerful to be seen at the greatest distance required, it must be very greatly too powerful in every other direction, and consequently there would be a great waste of oil; while, if an apparatus only powerful enough for the short range were employed, it would become necessary to make use of subsidiary apparatus, implying additional lamps, and an increased expenditure of oil, to show up and down the sound. The apparatus shown was designed by Mr T. Stevenson, with the view of reinforcing the ordinary fixed-light apparatus in the directions of greatest range. This is effected by distributing in these directions the spare light on the dark side of the lantern, which, if returned through the focus in the ordinary manner by means of a spherical mirror, would only tend to strengthen that portion of the light which is already sufficiently powerful.

A is 167° of a small fixed-light apparatus subtending the entire arc to be illuminated; so that all the rest of the light, or 193°, is spare light. Of this, 129° are parallelized by the holophotal apparatus B; and the rays, falling on a series of equal and similar prisms, α, are again refracted, but in the horizontal plane only; and after passing through a focal point (independent for each prism), emerge in a series of twelve equal and parallel beams, having a divergence of about 10°, which are also equal and parallel to the diverging beam, e, and, consequently, according to well-known optical laws, have the effect of strengthening it as much as if they were actually superimposed upon it. As the light of 139° is, in this manner, condensed into about 10°, the effect must be from 12 to 14 times that of the unassisted apparatus, and should, therefore, be amply sufficient for a range of 15 miles. In like manner, the light parallelized by the other holophote C, are refracted by the prisms b, so as to strengthen the arc β, which will thus be rendered amply powerful for a range of 7 miles. The greater number of rays in the arcs α and β are intended to indicate the additional density due to the action of the subsidiary prisms. We shall only add, that when arcs of coloured light are to be employed, this method of strengthening any particular portion of the light becomes very important, as offering a ready method of equalizing the general range, otherwise interfered with by the enormous absorption of coloured media; and we might suggest, that, even where the light requires to be shown all round, it is possible to supplement any arc at the expense of any other, by cutting off an ordinary apparatus by a horizontal plane at any suitable height, and mounting a portion of a holophote and a series of prisms above it. By a somewhat similar arrangement to what has been described for Oronsay, the whole of the rays proceeding from a plane may be spread over any desired arc of the horizon.

Having thus fully described the catoptric and dioptric General systems of illumination, it might be expected that we should remarks institute a comparison between them. This, however, may now be considered unnecessary, as the universal adoption of the dioptric system speaks for itself. Its advantages are indeed too numerous and too palpable to be overlooked, combining, as they do, efficiency of action and fitness to meet every requirement, with economy in the consumption of oil; and we may safely say, that in all those countries where this important branch of administration is conducted with the care and solicitude which it deserves, the dioptric system has been adopted to the complete exclusion of the catoptric, except, indeed, in certain cases where economy of first construction and simplicity of detail are objects of primary importance.

To the Dutch belongs the honour of having first, after the French, embraced the system of Fresnel in their lights. The Commissioners of the Northern Lights followed in the train of improvement, and, in 1834, sent Mr Alan Stevenson on a mission to Paris, with full power to take such steps for acquiring a perfect knowledge of the dioptric system, and forming an opinion on its merits, as he should find necessary. The singular liberality with which he was received by M. Léon Fresnel, brother of the late illustrious inventor of the system, and his successor as the Secretary of the Lighthouse Commission of France, afforded Mr A. Stevenson the means of making such a report on his return as induced the Commissioners to authorize him to remove the reflecting apparatus of the revolving light at Inchkeith, and substitute dioptric instruments in its place. This change was completed, and the light exhibited on the evening of 1st October 1835; and so great was the satisfaction which the change produced, that the Commissioners immediately instructed Mr Stevenson to make a similar change at the fixed light of the Isle of May, where the new light was exhibited on the 22nd September 1836. The Trinity House of London followed next in adopting the improved system, and employed Mr A. Stevenson to superintend the construction of a revolving dioptric light of the first order, which was afterwards erected at the Start Point in Devonshire. Other countries followed, and the Report of the Lighthouse Board of America, published in 1852, which recommends (p. 13) the adoption of Fresnel's dioptric system, and the holophotal improvements, is a very full body of information on lighthouse subjects, extending over about 750 pages. Even Turkey has followed in the train of improvement; and we believe that a light on the dioptric principle will shortly be exhibited (if it be not already completed) from the Isle of Serpents. Fresnel, who is already classed with the greatest of those inventive minds which extend the boundaries of human knowledge, will thus, at the same time, receive a place amongst those benefactors of the species who have consecrated their genius to the common good of mankind; and, wherever maritime intercourse prevails, the solid advantages which his labours have procured will be felt and acknowledged.

When, however, this system was in its infancy, there were several objections raised to its adoption, which appeared to be of very considerable importance, though the experience of years has proved that they are not insurmountable. The first, and probably the most important, was the liability of the lamp to be extinguished from the failure of the leather work of the oil-pumps—a most serious objection, insomuch as, from there being only one lamp, its failure implies the extinction of the light. The means adopted to remedy this have been already described (vide "mechanical lamp"), and an experience of twenty-one years in the Northern Lighthouses has proved them to be sufficient for the purpose; for during the whole of that time (although it has on several occasions been necessary to light the spare lamp) the light has only on one occasion been totally extinguished, a casualty which was caused by the keeper sleeping on his watch.

The only other objection worthy of mention is the short duration of the flash in revolving lights, owing to the small divergence (5° 9') of the annular lens. This has been corrected by setting the inclined mirrors, or holophotal prisms, a little in advance of the great lenses, so that they precede, and consequently prolong, the principal flash. M. Degrand has also proposed to cut the whole apparatus by a horizontal plane passing through the focus, and to set one portion a few degrees in advance of the other, a plan which has considerable advantages, as all the portions of the beam are more nearly of equal intensity.

Mr T. Stevenson, moreover, suggests an ingenious method of remedying this evil, by constructing lenses whose aberration in the vertical plane is corrected, while that in the horizontal plane may be adjusted to any determinate amount. In the application of this method of construction to the annular lenses they would be ground on the external surface as before; but the internal surface would be a portion of a vertical cylinder of suitable radius. Thus each vertical section would be similar to that of a plano-convex lens as at present, and would refract the rays accordingly, while the horizontal sections would be of a meniscal form, and would act only by the excess of their convexity over their concavity. Thus, by varying the radius of the cylinder, any amount of horizontal divergence may be obtained, and this without much increasing the thickness of the glass, at least in the case of revolving lights, in which a curve of long radius might be applied.

The oil, until lately, most generally employed in the lighthouses of the United Kingdom was the sperm oil of commerce, which is obtained from the South Sea whale (*Physeter macrocephalus*). In France, the colza oil, which is expressed from the seed of a species of wild cabbage (*Brassica oleracea colza*), and the olive oil, are chiefly used; and a species of the former has now been successfully introduced into the British lighthouses.

The advantages of the colza oil are thus stated by the engineer of the Scottish Lighthouse Board:

"It appears from pretty careful photometrical measurements of various kinds, that the light derived from the colza oil is, in point of intensity, a little superior to that derived from the spermaceti oil, being in the ratio of 1:056 to 1. The colza oil burns both in the Fresnel lamp and the single Argand burner with a thick wick during seventeen hours, without requiring any coaling of the wick or any adjustment of the damper; and the flame seems to be more steady and freer from flickering than that derived from spermaceti oil. There seems (most probably owing to the greater steadiness of the flame) to be less breakage of glass chimneys with the colza than with the spermaceti oil.

The consumption of oil seems in the Fresnel lamp to be 121 for colza, and 114 for spermaceti; while in the common Argand, the consumption appears to be 910 for colza, and 902 for spermaceti; and if we assume the means of these numbers, 515 for colza, and 508 for spermaceti, as representing the relative expenditure of these oils; and if the price of colza be 3s. 9d., while that of spermaceti is 6s. 9d. per imperial gallon; we shall have a saving in the ratio of 1 to 1:755, which, at the present rate of supply for the Northern Lights, would give a saving of about £3266 per annum."

In a few lighthouses which are near towns, the gas of gas pit coal has been used; and there are certain advantages, more especially in dioptric lights, where there is only one large central flame, which would render the use of gas desirable. The form of the flame, which is an object of considerable importance, would thus be rendered less variable, and could be more easily regulated, and the inconvenience of the clock-work of the lamp would be wholly avoided. But it is obvious that gas is by no means suitable for the majority of lighthouses, their distant situation, and generally difficult access, rendering the transport of large quantities of coal expensive and uncertain; whilst, in many of them there is no means of erecting the apparatus necessary for manufacturing gas. There are other considerations which must induce us to pause before adopting gas as the fuel of lighthouses; for, however much the risk of accident may be diminished in the present day, it still forms a question, which ought not to be hastily decided, how far we should be justified in running even the most remote risk of explosion in establishments such as lighthouses, the sudden failure of which might involve consequences of the most fatal description, and the situation of which is often such, that their re-establishment must be a work of great expense and time.

The application of the Drummond and Voltaic lights to Drummond lighthouse purposes is, owing to their prodigious intensity, and Volta's a very desirable consummation; but it is surrounded by so lights many practical difficulties, that it may, in the present state of our knowledge, be pronounced unattainable. The uncertainty which attends the exhibition of both these lights is of itself a sufficient reason for coming to this conclusion. But other reasons unhappily are not wanting. The smallness of the flame renders those lights wholly inapplicable to dioptric instruments, which require a great body of flame, in order to produce a degree of divergence sufficient to render the duration of the flash in revolving lights long enough to answer the purpose of the mariner. M. Fresnel made some experiments on the application of the Drummond light to dioptric instruments, which completely demonstrate their unfitness for this combination. He found that the light obtained by placing it in the focus of a great annular lens was much more intense than that produced by the great lamp and lens of Corduan; but the divergence did not exceed 30°; so that, in a revolution like that of Corduan, the flashes would last only 1½ second, and would not, therefore, be seen in such a manner as to suit the practical purposes of a revolving light. The great cylindric refractor, used in fixed lights of the first order, was also tried with the Drummond light in its focus; but it gave coloured spectra at the top and bottom, and only a small bar of white light was transmitted from the centre of the instrument. The same deficiency of divergence completely unfitting the combination of the Drummond light with the reflector for the purposes of a fixed light, and even if this cause did not operate against its application in revolving lights on the catoptric plan, the supply of the gases, which is attended with almost insurmountable difficulties, would, in any case, render the maintenance of the light precarious and uncertain in the last degree.

There are many questions of much interest regarding Lighthouses, which appear to open an extensive field of inquiry; and it may be doubted whether some of them have received that degree of consideration to which their importance entitles them. Amongst these we may rank the numerous questions which may be raised regarding the most effective kind of distinctions for lights. Those distinctions may be naturally expected to be the most effective which strike an observer by their appearance alone. Thus a red and white light, a revolving and a fixed light, offer appearances which are calculated to produce upon the observer a stronger sense of their difference than the same observer would receive from lights the sole difference of which lies in their revolutions being performed in greater or less intervals of time. On the other hand, the distinctions derived from time, if the intervals on which they depend do not approach too closely to each other, appear to afford very suitable means for characterizing lights; and the number of distinctions which may be founded upon time alone are pretty numerous. Coloured media have the great disadvantage of absorbing light, and the only colour which has hitherto been found useful in practice is red, all others, at even moderate distances, serving merely to enfeeble, without characterizing, lights. In the system of Fresnel, as already explained, all the distinctions are based upon time alone. Mr Robert Stevenson, the engineer of the Northern Lighthouses, has invented two distinctions, which, although they are produced by variations of the time, possess characteristic appearances, sufficiently marked to enable an observer to distinguish a light without counting time. The one is called a flashing light, in which the flashes and eclipses succeed each other so rapidly as to give the appearance of a succession of brilliant scintillations; and the other has been called intermittent, from its consisting of a fixed light, which is suddenly and totally eclipsed, and again as suddenly revealed to view. The effect of this light is entirely different from that of any revolving light, both from the great inequality of the intervals of light and darkness, and also from the contrast which is produced by its sudden disappearance and reappearance, which is completely different from the gradual diminution and increase of the light in revolving lights, more especially in those on the catoptric principle. The great and still increasing number of lights renders the means of distinguishing them one of the most important considerations connected with lighthouses.

Not less important, and very nearly allied to the subject of distinction, is that of the arrangement of lights on a line of coast. The choice of the most suitable places, and the assigning to each the characteristic appearances which are most likely to distinguish it from all the neighbouring lights, are points requiring much consideration; and it ought never to be forgotten, that the indiscriminate erection of lighthouses soon leads to confusion, and that the needless exhibition of a light, by involving the loss of a distinction, may afterwards prove inconvenient in the case of some future light, which time and the growing wants of trade may call for on the same line of coast. To enter at length upon this topic, or even to lay down the general principles which ought to regulate the distribution of lights, would exceed the limits of this article; but in connection with this it may be observed, that the superintendence of lighthouses should be committed to one general body, and ought not to be left to local trusts, whose operations are too often conducted on narrow principles, without reference to general interests. The inconveniences arising from interference between the distinctions of the lights under one trust, and those of the lights under another, are thereby avoided; and the full advantage is obtained of the means of distinction at the disposal of both.

The considerations which enter into the choice of the position and character of the lights on a line of coast are either, on the one hand, so simple and self-evident as scarcely to admit of being stated in a general form, without becoming mere truisms; or are, on the other hand, so very numerous, and often so complicated, as scarcely to be susceptible of compression into any general laws. We shall not, therefore, do more than very briefly notice, in the form of distinct propositions, a few of the chief considerations which should guide us in the selection of the sites and characteristic appearance of the lighthouses to be placed on a line of coast.

1. The most prominent points of a line of coast, or those first made on over-sea voyages, should be first lighted; and the most powerful lights should be adapted to them, so that they may be discovered by the mariner as long as possible before his reaching land. 2. So far as is consistent with a due attention to distinction, revolving lights of some description, which are necessarily more powerful than fixed lights, should be employed at the outposts on a line of coast. 3. Lights of precisely identical character and appearance should not, if possible, occur within a less distance than 100 miles of each other on the same line of coast, which is made by over-sea vessels. 4. In all cases, the distinction of colour should never be adopted except from absolute necessity. 5. Fixed lights, and others of less power, may be more readily adopted in narrow seas, because the range of the lights in such situations is generally less than that of open sea-lights. 6. In narrow seas, also, the distance between lights of the same appearance may often be safely reduced within much lower limits than is desirable for the greater sea-lights. Thus there are many instances in which the distance separating lights of the same character need not exceed 50 miles; and peculiar cases occur in which even a much less separation between similar lights may be sufficient. 7. Lights intended to guard vessels from reefs, shoals, or other dangers, should, in every case where it is practicable, be placed seaward of the danger itself, as it is desirable that seamen be enabled to make the lights with confidence. 8. Views of economy in the first cost of a lighthouse should never be permitted to interfere with placing it in the best possible position; and, when funds are deficient, it will generally be found that the wise course is to delay the work until a sum shall have been obtained sufficient for the erection of the lighthouse on the best site. 9. The elevation of the lantern above the sea should not, if possible, for sea-lights, exceed 200 feet; and about 150 feet is sufficient, under almost any circumstances, to give the range which is required. Lights placed on high headlands are subject to be frequently wrapped in fog, and are often thereby rendered useless at times when lights on a lower level might be perfectly efficient. But this rule must not, and indeed cannot, be strictly followed, especially on the British coast, where there are so many projecting cliffs, which, while they subject the lights placed on them to occasional obscuration by fog, would also entirely and permanently hide from view lights placed on the lower land adjoining them. In such cases, all that can be done is carefully to weigh all the circumstances of the locality, and choose that site for the lighthouse which seems to afford the greatest balance of advantage to navigation. As might be expected, in questions of this kind, the opinions of the most experienced persons are often very conflicting, according to the value which is set on the various elements which enter into the inquiry. 10. The best position for a sea-light ought rarely to be neglected for the sake of the more immediate benefit of some neighbouring port, however important or influential; and the interests of navigation, as well as the true welfare of the port itself, will generally be much better served by placing the sea-light where it ought to be, and adding, on a smaller scale, such subsidiary lights as the channel leading to the entrance of the port may require. 11. It may be held as a general maxim, that the fewer lights that can be employed in the illumination of a coast the Lightfoot better, not only on the score of economy, but also of real efficiency. Every light needlessly erected may, in certain circumstances, become a source of confusion to the mariner; and, in the event of another light being required in the neighbourhood, it becomes a deduction from the means of distinguishing it from the lights which existed previous to its establishment. By the needless erection of a new lighthouse, therefore, we not only expend public treasure, but waste the means of distinction among the neighbouring lights. Distinctions of lights, founded upon the minute estimation of intervals of time between flashes, and especially on the measurement of the duration of light and dark periods, are less satisfactory to the great majority of coasting seamen, and are more liable to derangement by atmospheric changes, than those distinctions which are founded on what may more properly be called the characteristic appearance of the lights, in which the times for the recurrence of certain appearances differ so widely from each other as not to require for their detection any very minute observation in a stormy night. Thus, for example, flashing lights of five seconds' interval, and revolving lights of half a minute, one minute, and two minutes, are much more characteristic than those which are distinguished from each other by intervals varying according to a slower series of 5', 10', 20', 40', &c. Harbour and local lights, which have a circumscribed range, should generally be fixed instead of revolving; and may often, for the same reason, be safely distinguished by coloured media. In many cases, also, where they are to serve as guides into a narrow channel, the leading lights which are used should, at the same time, be so arranged as to serve for a distinction from any neighbouring lights.

Floating lights, which are very expensive, and more or less uncertain, from their liability to drift from their moorings, as well as defective in power, should never be employed to indicate a turning-point in a navigation in any situation where the conjunction of lights on the shore can be applied at a reasonable expense.

British and Irish Lights.

In concluding, it may be necessary to state that the English lights are placed under the Corporation of Trinity House of Deptford, Stroud; the Scottish lights are under the management of the Commissioners of Northern Lights; and the Irish lights are under the care of the Corporation for preserving and improving the port of Dublin, commonly called the Ballast Board.

The last act of parliament on the subject of lighthouses forms part of one the general title of which is, "An act to amend various laws relating to merchant shipping." It passed 20th August 1853. The chief provisions which affect lighthouses are the following:—1. The light dues of the United Kingdom are to form one imperial fund, under the control of the Board of Trade. 2. From this fund all expenses of erecting and maintaining the lights of the United Kingdom are to be defrayed. 3. The three boards which manage the lighthouses in England, Scotland, and Ireland are to render account of their expenditure to the Board of Trade. 4. The Trinity House, or English board, is to exercise a certain control over the boards in Scotland and Ireland, and is to judge of all their proposals to erect new lights, or to change existing ones; but in every case the sanction of the Board of Trade must precede the acts of each of the three boards.

The following works may be consulted on the subject of lighthouses:—Smeaton's Narrative of the Eddystone Lighthouse, London, 1793; Stevenson's Account of the Bell Rock Lighthouse, Edinburgh, 1824; Beldorn, Architectura Hydraulica, vol. iv., p. 151; Pelet, Traité de l'éclairage des Phares, Paris, 1827; Fresnel's Memoire sur un Nouveau Système d'éclairage des Phares, Paris, 1822; Admiral de Monts's Report, containing l'Expérience du Système adopté par le Comité des travaux de la marine, ed. by F. Fremin, Paris, 1829; Treatise on Burning Instruments, containing the method of building large polygonal lenses, by D. Brewster, LL.D., F.R.S., Edin., 1819; Faciale di Salvore, nell' Istria, Illuminato a Gas, Vienna, 1821; On Construction of Polygonal Lenses and Mirrors of Great Magnitude, for Lighthouses, &c., by D. Brewster, LL.D., F.R.S. (Edin., Phil. Journ., 1823, vol. viii., p. 160); Account of a New System of Illumination for Lighthouses, by D. Brewster, LL.D., F.R.S., Edin., 1827; Saggio di Osservazioni, &c., or Observations on the Means of Improving the Construction of Lighthouses; with an Appendix, on the Application of Gas to Lighthouses, by the Chevalier G. Aldini, Milan, 1823; Bordier Marceau's Notice descriptive d'un Feuill à double organe, &c., Paris, 1820; Bordier Marceau's Plan de Sécurité de l'État, on Lampe de Sécurité, Paris, 1819; Fresnel's Description Sommaire des Phares et Feux fixes allumés sur les Côtes de France, au 1er d'Août, 1837; The Lighthouses of the British Islands, from the Hydrographical Office of the Admiralty, London, 1836; Instructions pour le service des Phares Lenticulaires, par L. Fresnel, Paris, 1836; Stevenson's Sketch of Civil Engineering in America, London, 1838, p. 296; Report of Select Committee of the House of Commons on Lighthouses, 1834; Report by a Committee of the Board to the Commissioners of the Northern Lighthouses, on the "Report of the Select Committee," 1836; Report to the Commissioners of the Northern Lighthouses on the Illumination of Lighthouses, by Alan Stevenson, M.A., Edin., 1834; Report to the same, on the Inchkeith Lighthouse, by Alan Stevenson, Edin., 1835; Report on the Isle of May Lighthouse, by Alan Stevenson, Edin., 1836; Report on the Isle of May Lighthouse, by a Committee of the Royal Society (Professor Forbes, reporter), Edin., 1836; Account of Skerryvore Lighthouse, with Notes on Lighthouse Illumination, by Alan Stevenson, LL.B., Edin., A. & C. Black, 1847; Stevenson's Treatise on the History, Construction, and Illumination of Lighthouses, London, 1850; Account of the Holophotal System of Illuminating Lighthouses, by Thomas Stevenson, F.R.S.E., C.E., in the Transactions of the Royal Scottish Society of Arts for 1849; Formula for Constructing Totally Reflecting Hemispherical Mirrors, by William Swan, F.R.S.E., Trans. Roy. Soc. Edin., 1850; Description of Spheroidal Cylinder Lenses, &c., by T. Stevenson, F.R.S.E., Edinburgh New Philosophical Journal, 1855; Report of the Lighthouse Board of America, Washington, 1852.