in a strict sense, signifies a large portion of water almost surrounded by land, as the Baltic and Mediterranean Seas; but it is frequently used for that vast body of water which encompasses the whole earth. See the articles Geology and Physical Geography.

Sea-Air is that part of the atmosphere which is above the sea. Sea-air has been found salubrious and beneficial in certain distempers. This may be owing to its containing a greater portion of oxygenous gas or vital air, and being less impregnated with noxious vapours, than the land. Dr Ingenhousz made several experiments to ascertain the salubrity of sea-air. By mixing equal measures of common air and nitrous air, he found that at Gravesend they occupied about 1:04, or one measure and 1/100ths of a measure; whereas on sea, about three miles from the mouth of the Thames, two measures of air, one of common and one of nitrous air, occupied from 0:91 to 0:94. He attempted a similar experiment on the middle of the channel between the English coast and Ostend; but the motion of the ship rendered it impracticable. He found that in rainy and windy weather the sea-air contained a smaller quantity of vital air than when the weather was calm. On the sea-shore at Ostend it occupied from 0:44 to 0:7; at Bruges he found it at 105, and at Antwerp 109. Dr Ingenhousz thus concludes his paper. "It appears, from these experiments, that the air at sea and close to it is in general purer and fitter for animal life than the air on the land, though it seems to be subject to the same inconstancy in its degree of purity with that of the land; so that we may now with more confidence send our patients labouring under consumptive disorders to the sea, or at least to places situated close to the sea, which have no marshes in their neighbourhood. It seems also probable that the air will in general be found much purer far from the land than near the shore, the former being never subject to be mixed with land-air." Sea-Lights. Sea-Light, and Lighthouse, 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 based upon 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 reconcileable 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 three hundred years before the Christian era, Chares, the disciple of Lysippus, constructed the celebrated brazen statue, called the Colossus of Rhodes, which was of such dimensions as to allow vessels to sail into the harbour between its legs, which spanned the entrance. 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 eighty 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 £36,000.

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 forty 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 "flamnis" of which he speaks, were not so produced. The word lampada may, however, be used metaphorically; and flamnis 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. 791.; et seq. Cesar, Comment. de Bell. Civit. l. iii. Pompon. Mela. l. ii. c. 7.; Ammian. Marcellin. l. xxii. c. 16.; Joseph. de Bell. Judaic. l. vi. Nicolas Lloyd's Lexicon Geographicum, and the Notitia Orbis Antiqui of Celarius, l. iv. c. 1., p. 13.

Mr. Moore, in his History of Ireland, (vol. i. p. 16.) Corros 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 Æthius, the cosmographer. This in all probability is the tower which Humboldt mentions in his Narrative under the name of the Iron

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1 Septima nox, Zephyro nonquam laxante rudentem, Ostendit Pharis' Egyptia littora flamnis. Sed prius orta dies nocturnum lampada tequit, Quam totas intraret aquas.

Pharal. ix. 1004. Sea-Lights Tower, which was built as a lighthouse by Caius Saevius Lupus, an architect of the city of Aqua Flavia, the modern Chaves.

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 Hydrographique. The building is 197 feet in height, and consists of a pile of masonry, forming successive galleries, enriched with pilasters and friezes, and rising above each other with gradually diminished diameters. These galleries are surmounted by a conical tower, which terminates in the lantern. 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. This wall is an outwork of defence, and receives the chief shock of the waves. The tower itself contains a chapel, and various apartments; and the ascent is by a spacious staircase. The first light exhibited in the Tour de Corduan, was obtained by burning billets of oak-wood, 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.

Plate I. fig. 7, shews the form of this celebrated lighthouse, and is made from drawings in the possession of the writer of this article, who, in 1834, spent several days there.

The history of the celebrated lighthouse on the Eddystone 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 sixty 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 shewn, and continued to be regularly exhibited till the year 1755, when the whole fabric was destroyed by accidental fire, after standing forty-seven 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 twelve feet of the tower form a solid mass of masonry, and the stones are united by means of stone joggles, dovetail 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. Plate I. fig. 3, shews 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 twelve 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, seventy-four, with all her crew, part of the wreck having Sea Lights have afterwards been 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 twelve 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 twelve miles from land, and covered to the depth of twelve 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 sixteen feet below the high-water of spring-tides, and at the end of the second season, the building was five feet six inches above the lowest part of the foundation. The third season's operations terminated by finishing the solid part of the structure, which is thirty 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 render 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 lighthouse, are six in number. The light is a revolving red and white light, and is produced by the revolution of a frame containing twenty 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 shewn successively. The machinery, which causes the revolution of the frame containing the lamps, is also applied to tolling two large bells, to give warning to the mariner of his approach to the rock in foggy weather. Plate I. fig. 6, shows a section of the Bell-Rock Lighthouse, and an elevation of the temporary barrack-house, which was removed on the completion of the work.

The most remarkable lighthouse on the coast of Ireland Carlingford is that of Carlingford, near Cranfield Point, at the entrance 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 Hawkhawling 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 Commissioners of the Northern Lighthouses have lately taken measures for erecting a lighthouse on the rock of Skerryvore, which lies in the channel between the Western Isles of Scotland and the north of Ireland. The solid rock is only about 40 yards square, and is about 13 miles from the nearest point of the Island of Tyree. It is exposed to the unbroken fury of the Atlantic, there being no land between it and the coast of America. The works were commenced last season on the rock, by the erection of part of a wooden barrack for the reception of the workmen, during the building of the tower of masonry. This barrack, which resembled that of the Bell-Rock, shown in Plate I. fig. 6, disappeared on the night of the 3rd November 1838; and when the writer of this article visited the rock, on the 16th of the same month, only a single beam remained. Several spars of a large vessel, and some of the beams of the barrack, afterwards came ashore on the Islands of Tyree and Coll; a circumstance which has led to the belief, that the fabric had sustained some injury by the collision of some heavy body in motion. From the tremendous height to which the sea was observed to rise on the rock, there is, on the other hand, some reason to suspect, that the fury of the waves alone may have demolished the unfinished structure of the barrack. The works were resumed in the spring of 1839.

There are various other lighthouses which, in themselves, are sufficiently deserving of a separate notice, were it not that they have, more or less, something in common with those already described, which are unquestionably the most remarkable edifices of the kind. We shall, therefore, now proceed to consider the methods of illumination, which have been adopted in lighthouses; a subject to which much attention has of late years been directed, and which is the most important consideration connected with those establishments, whose utility depends solely upon the distance at which the light can be seen, and the facility with which the mariner can recognise their individual appearance as indicative of a particular part of the coast.

There can be little doubt, that down to a very late period, Coal lights, 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 chimney 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 Catoptric system. Sea-Lights 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 Lenoir, 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 passage 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 fixed 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 connexion 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 proportionally 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 paraboloids 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, $c$ 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 $\sin \Delta = \frac{d}{c}$, 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 350 times the effect of paraboloid mirrors, 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 Sea-Lights 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 beating 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 bizzle, 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

$$\frac{b}{d} = \frac{r^2}{d}$$

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 shewn in Plate II. figs. 7, 8, and 9, is very important, as it insures the burner always being in the focus, and does not require that the reflector 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. $a a a$ represents one of the reflectors, $b$ is the lamp, $c$ is a cylindric 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. 8. An aperture of an elliptical form, measuring about two 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 Sea-Lights, which the copper tube $h$ is attached, serving for ventilation; $i$ shews 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, as seen at $k k$, is soldered on the brass band $l$, that clasps the exterior of the reflector.

Plate II. fig. 8, is a section of the reflector $o a$, shewing the position of the burner $b$, with the glass chimney $b'$, and oil-cup $l$, which receives any oil that may drop from the lamp.

Plate II. fig. 7, shews 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, $e$, the fountain, is moved partly down; $d d$ shews 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 Arrangement of reflectors on the frame.

shown in Plate I. figs. 2, 4, and 5. 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 of the gauge. The different arrangements of the reflectors will be more fully understood by referring to the Plates.

Plate I. figs. 2 and 5, shew an elevation and plan of a revolving apparatus on the catoptric principle. In these figures, $a a$ shews the reflector frame or chandelier; $o o$, the reflectors with their oil-fountains $p p$. The whole is attached to the revolving axis or shaft $q$. The copper tubes $r r$ convey the smoke from the lamps; $s s$ are cross bars which support the shaft at $t t$; $u u$ is a copper pan for receiving any moisture which may accidentally enter at the central ventilator in the roof of the light-room; $l$ is a cast-iron bracket, which supports the pivot on the shaft; $m m$ are bevelled wheels, which convey motion from the machine to the shaft.

Plate I. fig. 4, shews a plan of one tier of reflectors arranged in the manner employed in a fixed catoptric light; $n n$ shews the chandelier, $q$ the fixed shaft in the 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 by M. Bordier Marcelet, the pupil and successor of Ar-Marcelet's grand, who has laboured with much enthusiasm in perfecting catoptric instruments, more especially with a view to their application in 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 Marcelet has in his work-shop an instrument of this kind, eight feet in diameter, which he constructed on speculation. The apparatus used in harbour-lights, on the French coast, is of much smaller dimensions, and does Sea Lights not exceed fifteen inches in diameter. A perfect idea of the construction and effect of this apparatus, may be formed, by conceiving a parabola to revolve about its parameter as an axis, so that its upper and lower limbs would become the generating lines of two surfaces possessing the property of reflecting, in lines parallel to the axis of the parabola, all the rays incident upon them, from a light placed in the point where the parameter and axis of the generating parabola intersect each other. This point being the focus of each parabolic section of this apparatus, the light is equally dispersed in every point of the horizon, when the axes of the parabolic sections are in a plane perpendicular to a vertical line. But however perfectly this apparatus may attain this important object, it does so at the sacrifice of the most efficient part of the parabolic surface, which lies between the vertex and the parameter; and, therefore, produces a proportionally feeble effect. This beautiful little instrument is shown in Plate II., fig. 5; in which $p$ shows the burner $p$, $p'$ the upper reflecting surface, and $p''p''$ the lower reflecting surface, both generated in the manner above described by the revolution of a parabola about its parameter $a b$; $F$ is the focus of the generating parabola; and $l l$ are small pillars, which connect the two reflecting plates, and give strength to the apparatus.

M. Bordier Marcelet has also prepared a very ingenious modification of the paraboloidal mirror, which he has described under the name of fanal à double aspect; and the object of which is, to obtain a convenient degree of divergence from parabolic mirrors, by the use of two flames and two reflecting surfaces, each of which is acted upon by its own flame, and also by that of the other. This modification consists in the union of two portions of hollow paraboloidal mirrors, generated by the revolution of two parabolas about a common horizontal axis, and illuminated by two lamps placed in the focus of each. The first surface is generated by the revolution on its axis of a segment of a paraboloid intercepted between the parameter and some double ordinate greater than it, and may, from its form, be called the ribbon-shaped mirror. The second surface is that of a parabolic conoid, which is cut off by a vertical plane passing through a double ordinate, which is equal to the parameter of the parabolic ribbon, which is placed in front of it. The elements of the curve which forms the conoidal mirror, must be so chosen as to have its focus at a convenient distance in front of that of the ribbon-shaped mirror, so as to admit of placing the two lamps separate from each other, as well as to produce the necessary degree of divergence, which is to be obtained by the action of these mirrors respectively on the flame placed in the focus of the other. These two mirrors are thus joined together. Each mirror produces, by means of the lamp placed in its focus, an approach to parallelism of the reflected rays, which M. Bordier Marcelet has not inaptly termed the principal effect; whilst the action of each surface on the lamp which is placed in the focus of the other, causes what the inventor calls the secondary or lateral effect. Their secondary action may be described thus: The lamp, which is in the focus of the ribbon, is much nearer the vertex of the conoid than its own focus; so that its rays making, with normals to the surface of the conoid, angles greater than those which are formed by the rays proceeding from its focus, are necessarily reflected in lines diverging from the axis of the mirror. Those, on the contrary, which proceed from the focus of the conoid, meet the ribbon-shaped surface, so as to make angles with its normals more acute than those which the rays from its own focus could do, and which are, therefore, reflected in lines converging to the axis of the mirror. These reflected rays must therefore cut the axis, and diverge from it on the other side. This apparatus has been tried with success at La Hève and some other lights on the French coast. But it is impossible not to perceive the great loss of light which results from the use of two flames in one mirror; and it must not be forgotten, that the divergence which is obtained is not confined to the horizontal direction in which only it is wanted; but the light is scattered in every direction round the edge of the mirror.

Spherical mirrors have been employed in lighthouses only when they can be introduced as subsidiary parts in dioptric apparatus; and any observations regarding them will, therefore, be made in treating of the dioptric lights of Fresnel.

Floating lights are only resorted to in cases of absolute necessity, as their maintenance is extremely expensive, whilst they are less to be relied on, and, in all respects, less efficient than land lights. They are large vessels, built with great breadth of beam, and are generally moored off shoals, or serve as guides for taking channels. The lights are from lamps placed in front of small reflectors, ranged in lanterns, which are hoisted on the masts of the vessel. The number of lights varies from one to three as the only means of distinction, the feebleness of the light generally rendering it inexpedient to adopt the distinctions derived from the use of coloured media.

Catoptric lights are susceptible of nine separate distinctions, which are called fixed, revolving white, revolving reds of and white, revolving red with two whites, revolving white with two reds, flashing, intermittent, double fixed lights, and double revolving white lights. The first exhibits a steady and uniform appearance, which is not subject to any change; and the reflectors used for it, (as already noticed), 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 characterised 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. This 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 the present 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 Calf 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 re- The possibility 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. Léonard Fresnel, the present 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 Smolton 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 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. Above 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 Roche; 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 Encyclopedia; 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. Both Buffon and Condorcet, however, chiefly speak of reducing the thickness of the material, and do not seem to have thought of determining the radius and centre of the curvature of the generating arcs of each zone, having contented themselves with simply depressing the spherical surface in separate portions. Fresnel, on the other hand, determined these centres, which constantly recede 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 strike. 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. 6, Plate II., 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 Cylindric 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 simply refracting the rays at right angles to the line of the section, or in a direction parallel to the horizon, and cannot collect the rays from either side of the vertical line; and if this section, by its revolution about a vertical axis, becomes the generating line

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1 In all probability directly derived from the Greek δορπος, an optical instrument with holes for looking through, which is a compound of ὁδός, through, and ἀνάγω, I see.

2 On pourrait même composer de plusieurs pièces ces loups à échelons; on y gagnerait plus de facilité dans la construction, une grande diminution de dépense, l'avantage de pouvoir leur donner plus d'étendue, et celui d'employer suivant le besoin un nombre des cercles plus ou moins grand, et d'obtenir ainsi du même instrument différents degrés de force. Eloge de Buffon, p. 35. Œuvres de Condorcet, tom. iv. Paris, 1804. Sea Lights 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 used in the British lighthouses.

An ingenious modification of the fixed apparatus is due to the inventive mind of Fresnel, who also 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.

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 this gentleman, with great candour, gives the whole credit to his deceased friend, in a notice regarding lighthouses, which appeared in the Annuaire 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 carbonisation 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. The writer of this article has seen the large lamp at the Tour de Corduan burn for seven 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. The first 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. The next shows a section of this burner. C, C', C'', C''' Sea Lights, 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. The last 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 by the failure of the lamp for a few minutes, 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 re- The lamp invented by Mr. Oldham of Dublin, appears, from the simplicity of its construction, to be very suitable for the purposes of a lighthouse; and the writer of this article is at present engaged in some experiments to ascertain the possibility of applying the pressure used in Mr. Oldham's lamp to produce the same regular supply of oil to the concentric burner, which is at present effected by means of the mechanism of the French 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 $5^\circ 9'$, 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 union of the several zones which compose an annular lens is effected by means of small slips of thin copper, which having one half passed into a groove in one zone, and the other half into a corresponding groove in the adjoining zone, prevent, in the same manner as a joggle in masonry, or a chock in carpentry, the one zone from slipping past the other. The pieces are also 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

$$d = \frac{\phi \phi'}{\phi - \phi'}$$

The curved mirrors, as already mentioned, 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 by screws, which are attached to the backs of the cases, and which afford the means of adjusting them to their true position in the lightroom, 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 Cataldiopic light, from the compound action by which it is characterised, was another of Fresnel's applications of Sea-Lights 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 (30 cm) and refract equally over the horizontal plane of the focus the light which they receive from it, and thus operate precisely in the same manner as the dioptric part of the fixed light apparatus. 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 reflecting 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 refracting 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 Thilorier, 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. 1. Plate I. which shews its section and plan. F is the focal point in which the flame is placed, r, r cylindric 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 around them. These exterior refractors in front of the inner 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 shew 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 M, M, shewn in Plate II. figs. 3 and 4; 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.

This consideration, together with some other views regarding the greater convenience of the catadioptric apparatus, led Mr. Alan Stevenson to propose to the Commissioners of the Northern Lighthouses, in a report, dated 8th October 1835, that an inquiry should be instituted regarding the practicability of substituting in lights of the first order, a series of catadioptric prisms in the room of the curved mirrors which are at present used in France. Having received authority from the Lighthouse Board, he corresponded with M. Fresnel at Paris, who, in the most liberal manner, furnished him with all the information regarding the steps which he had pursued in reference to the smaller apparatus, and at the same time suggested many important views regarding the larger one.

In the small apparatus of the fourth order, the nearness of the prisms to the flame makes the angle subtended by the first refracting side considerable; and as the curvature of the reflecting side of the prisms depends upon this angle, the excess of the secant, above the radius of the arc, is a notable quantity, and the radius of curvature is proportionally small. But in the prisms of the first order, Mr. A. Stevenson found, that the radius of curvature of the reflecting side is upwards of 24 feet, and that, even when the generating triangles are larger than those of the small apparatus, the excess of the secant over the radius is only about 0.125 inch. Where a flame of great size, like that which illuminates the dioptric apparatus, is used, this curvature may be safely disregarded; and is, indeed, such, that it could not be accurately ground, on account of the great length and unwieldiness of the radius. It is true, that an approximation to the true figure might be made, by giving to the reflecting side the form which would be traced by two tangents to the curve, and thus forming a trapezoidal prism; but the difficulty of such an operation, where so little glass is to be removed, and the increased source of error from the necessity of an additional shifting of the glass on the chucks, seem to be sufficient reasons for rejecting this expedient. It ought also to be recollected, that in this case the tendency of the deviation from the theoretical form, is towards the side of safety, as the error in the path of any ray which it could cause, would throw the light at its emergence from the prism below, and not above the horizon. The prisms may therefore be considered as rings generated by the revolution of isosceles triangles round a vertical axis passing through the focus of the system.

Mr. Edward Sang, F.R.S.E., in an interesting communication on the grinding of glass, which was lately read before the Society of Arts for Scotland, pointed out the advantage of adopting the highest point of the flame as the focus of the system of reflecting rings. By this arrangement, the light from the upper part of the flame, which would otherwise escape upwards, will be directed to the horizon, whilst the rays from the lower parts of the flame will be usefully directed to illuminate the space between the horizon and the lighthouse.

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 refractorators 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 are generally simply catoptric, and consist of curved mirrors 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. At Corduan, however, and at Planier, near Marseilles, the apparatus above the principal lenses is dia-catoptric, being composed of an union of eight lenses of 19-68 inches (50 cm) 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° inclination. These upper lenses are 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, as already noticed in speaking of the appearance of Corduan light. The use of the accessory apparatus is to collect the rays, which would otherwise pass above and below the main lenses, without contributing to the brilliancy of the light. The nature Sea-Lights of the whole apparatus will be more fully understood by referring to the Plates.

Plate II. fig. 1, is a section of a revolving dioptric apparatus of the first order; F is the focal point in which the flame is placed; L, L, 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; L', L' upper lenses, forming by their union a frustum of an octagonal pyramid of 50° inclination, and having their foci coinciding in the point F. They parallelise the rays of light which pass over the lenses. M, M, plane mirrors, placed above the pyramidal lenses, and so inclined, as to project the beams reflected from them in planes parallel to the horizon; X, X show the rollers and wheel work which give motion to the lenses; W is the weight which moves the clock-work of the mechanical lamp H.

Plate II. fig. 2, is the plan of the apparatus shown in fig. 1.

Plate II. fig. 3, shows a section of a fixed dioptric light of the first order. F is the focal point in which the flame is placed; R, R cylindric refractors, forming by their union a prism of thirty-two sides, or a true cylinder, with the lamp in its axis, and producing a zone of light of equal intensity in every point of the horizon; M, M curved mirrors, ranged in tiers above and below the cylindric refractors, and having their foci coinciding in the point F; the effect of the mirrors increases the power of the light, by collecting and transmitting the rays which would otherwise pass above and below them, without increasing the effect of the light; W is the weight which moves the mechanical lamp H.

Plate II. fig. 4, is a plan of the fixed apparatus of the first order.

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 multilinear section of an annular lens, revolving about its principal focus as a vertical axis. 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 has been realized. An improvement of some importance might also be made upon the arrangement of this apparatus, 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, will not in any azimuth intercept the light throughout the whole height of the refracting belt, but the interception will be confined to a small rhomboidal space, whose area will be inversely proportional to the sine of the angle of inclination; and if the helical joints be formed between the opposite angles of the present rectangular frames, the amount of intercepted light will be absolutely equal in every azimuth."

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 thirteen 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 thirteen 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 thirteen 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 thirty miles."

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 36-22 inches, (92 cm.) 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, (70 cm.) 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, (50 cm.) and the other of 9-84 inches (25 cm.)

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

The four orders, as already hinted above, and explained by M. Fresnel in his observations prefixed to the list of the French lights for 1833, are not intended as distinctions; but are characteristic of the power and range of lights, which render them suitable for different localities on the coast, according to the distance at which they can be seen. This division, therefore, is analogous to that which separates our lights into sea-lights, secondary lights, and harbour lights, terms which are used to designate the power and position, and not the appearance of the lights to which they are applied.

Each of the above orders is susceptible of certain combinations, which produce various appearances, and constitute the distinctions used for dioptric lights; but the following are those which have been actually employed as the most useful in practice:

The first order contains, 1st. Lights producing great flashes, preceded by smaller ones, once in every minute, by the revolution of eight great annular, and eight smaller lenses, as at Corduan; 2d. Lights flashing once in every half minute, and composed of sixteen half lenses. These lights may have the subsidiary parts simply catoptric, as at Biarritz, or dia-catoptric, as at Planier; and, 3d. Fixed lights, composed of a combination of cylindrical pieces, with mirrors ranged in tiers above and below them as at the Isle d'Yeu.

The second order comprises revolving lights with sixteen or twelve lenses, which make flashes every half minute; and fixed lights varied by flashes once in every four minutes, as at Pillar, an effect which is produced by the revolution of exterior cylindrical pieces.

The third order (larger diameter) contains common fixed lights, and fixed lights varied by flashes once in every four minutes, as at Aiguesmortes.

The third order, (smaller diameter,) contains fixed lights, varied by flashes once in three minutes, as at Commerce on the Loire, and common fixed lights, as at Aiguillon, Grave, and Dunkerque.

The fourth order has fixed lights varied by flashes once in every three minutes, and fixed lights of the common kind, as at Pertuis-Breton, and La Coubre. It has been thought necessary to change the term "fixed lights varied by flashes," for "fixed light with short eclipses," because it has been found that, at certain distances, a momentary eclipse precedes the flash.

These distinctions depend upon the periods of revolution, rather than upon the characteristic appearance of the light; and therefore seem less calculated to strike the eye of a seaman, than those employed on the coasts of Great Britain and Ireland. In conformity with this system, and in consideration of the great loss of light which results from the application of coloured media, all distinctions based upon colour have been discarded in the French lights.

Having thus fully described the nature of the catoptric and dioptric modes of illuminating light-houses, we shall next proceed to compare the merits of both systems, with a view to determine their eligibility in revolving or in fixed lights.

Repeated experiments were made at Galan-hill, which is distant from Edinburgh about fifteen miles, during the winters of 1832 and 1833, under the inspection of the Commissioners of Northern Lights, the result of which was, that the light of one of the great annular lenses used in the revolving lights of the first order, was equal to the united effect of about eight of the large reflectors employed in the revolving lights on the Scotch coast. It may be said, however, that the diacatoptric combination of pyramidal lenses and plane mirrors of Corduan, adds the power of more than two reflectors to the effect of the great lens; but it ought to be remembered that in the French lights, this additional power is used only to lengthen the duration of the flash, and therefore in no degree contributes to render the light visible to the mariner at a greater distance. M. Fresnel found from the smaller divergence of the lens, that the eclipses were too long, and the bright periods of the revolution too short; and he therefore determined to adopt the horizontal inclination of 7° for the upper lenses, with a view to remedy this defect. Assuming, therefore, that it were required to increase the number of reflectors in a revolving light of three sides, so as to render it equal in power to a dioptric revolving light of the first order, it would be necessary to place eight reflectors on each face, so that the greatest number of reflectors required for this purpose may be taken at twenty-four. M. Fresnel has stated the expenditure of oil in the great lamp of four concentric wicks at 750 grammes of oil of colza per hour; and it is found by experience at the Isle of May and Inchkeith, that the quantity of spermaceti oil consumed by the great lamp, is equal to that burned by fourteen of the Argand lamps used in the Scotch lights. It therefore follows that, by dioptric means, the consumption of oil necessary for fourteen reflectors, will produce a light as powerful as that which would require the oil of twenty-four reflectors in the catoptric system followed on the coast of Scotland; and consequently, that there is an excess of oil equal to that consumed by ten reflectors, or 400 gallons in the year, against the Scotch system. But in order fully to compare the economy of producing two revolving lights of equal power by these two methods, it will be necessary to take into the calculation the interest of the first outlay in establishing them.

The expense of fitting up a revolving light with twenty-four reflectors, ranged on three faces, may be estimated at L.1298, and the annual maintenance at L.418, 8s. 4d. The fitting up a revolving light with eight lenses, and the diacatoptric accessory apparatus, may be estimated at L.1263, and the annual maintenance at L.280, 10s. 4d. It therefore follows, that to establish, and afterwards maintain a catoptric light, of the kind called revolving white, (as that of Start Point in Orkney, where the frame has only three faces,) so as to be equal in power to the dioptric light of Corduan, an annual outlay of L.137, 18s. more would be required for the reflecting light than for the lens light; whilst for a light of the kind called revolving red and white, (as the Bell Rock or Cape Wrath, where the frame has four faces,) thirty-six reflectors would be required to make the light equal in power to that of Corduan; and the catoptric light would in this case cost L.333 more than the dioptric light.

We shall now speak of fixed lights, to which the dioptric Fixed method is peculiarly adapted. The effect produced by the lights, consumption of a gallon of oil in a fixed light, with twenty-six reflectors, like that formerly exhibited at the Isle of May, may be estimated as follows:—The mean intensity of the light spread over the horizontal sector subtended by one reflector, as measured at each degree by the method of shadows, is equal to that of 174 unassisted Argand burners. If, then, this quantity be multiplied by 360 degrees, we shall obtain an aggregate effect of 6240, which, divided by 1040, the number of gallons burned during a year, by twenty-six reflectors, would give sixty Argand flames for the intensity of light maintained throughout the year by the combustion of a gallon of oil. On the other hand, the effect of a dioptric light like that lately established at the Isle of May, may be estimated thus. The mean intensity of the light produced by the joint effect of both the dioptric and catoptric parts of a fixed light apparatus, may be valued at 376 Argand flames, which, multiplied by 360 degrees, gives an aggregate of 135860; and if this quantity be divided by 570, the number of gallons burned by the great lamp in a year, we shall have nearly 237 for the intensity of light produced by the combustion of a gallon of oil. It would Sea-Lights thus appear that in fixed lights, the French apparatus produces nearly four times more light, by the combustion of the same quantity of oil, than can be obtained by the catoptric mode.

But the great superiority of the dioptric method chiefly rests upon its fulfilling perfectly the condition required in a fixed light, by distributing a more intense light equally in every point of the horizon. In the event of the whole horizon not requiring to be illuminated, the dioptric light would lose a part of its superiority in economy, and when half the horizon only is lighted, it would be more expensive than the reflected light; but the greater power and more equal distribution of the light, may be considered of so great importance, as far to outweigh any difference of expense. In the latter case, too, an additional power might be given to the light, by placing at the landward side of the lighthouse, a spherical mirror with its centre in the focus of the dioptric apparatus. The luminous cone of which such a reflector forms the base, instead of passing off useless to the land, would thus be thrown back through the focal point, and finally refracted, so as to contribute to the effect of the light seaward.

The expense of establishing a fixed light composed of twenty-six reflectors, may be estimated at L950, and the annual maintenance at L425, 10s.; and the expense of fitting up a fixed light on the dioptric principle is L1038; and the annual maintenance may be taken at L267, 6s. 4d. It thus appears that the annual expenditure of the dioptric fixed light is L158, 3s. 8d. less than that of a fixed light composed of twenty-six reflectors; and the light given out is four times more powerful, and it is at the same time more equally diffused over the horizon.

The comparative views already given of the catoptric and dioptric modes of illuminating lighthouses, demonstrate that the latter produces more powerful lights by the combustion of the same quantity of oil; while it is obvious that the catoptric system insures a more certain exhibition of the light, from the fountain lamps being less liable to derangement than the mechanical lamps used in dioptric lights. The balance, therefore, of real advantages or disadvantages, and consequently the propriety of finally adopting one or other, involves a mixed question, not susceptible of very absolute solution, and leaving room for different decisions, according to the value which may be set upon obtaining a cheaper and better light, on the one hand, as contrasted with less certainty in its exhibition, on the other. A few general considerations, which may serve briefly to recapitulate the arguments for and against the two systems, may not be out of place. And, first, regarding the fitness of dioptric instruments for revolving lights, it may be observed, that, from the details above given, it appears,

1st. That by placing eight reflectors on each face of a revolving frame, a light may be obtained as brilliant as that derived from the great annular lens; and that in the case of a frame of three sides, the excess of expense by the reflecting mode, would be L137, 18s.; and in the case of a frame of four sides, the excess would amount to L333.

2nd. The diverging property of the lens being less than that of the reflector, it becomes difficult to produce, by lenses, the appearance which characterises catoptric revolving lights, which are already so well known to British mariners; and any change which might affect their appearance, would involve many grave practical objections.

3rd. The uncertainty of the management of the lamp renders it more difficult to maintain the revolving dioptric lights without fear of extinction, an accident which has several times occurred at Corduan and other French lighthouses.

4th. The extinction of one lamp in a revolving catoptric light is not only less probable, but leads to much less serious consequences than the extinction of the single lamp in a dioptric light; because, in the first case, the evil is limited to diminishing the power of one face by an eighth part; whilst, in the second, the whole horizon is totally deprived of light. The extinction of a lamp, therefore, in a dioptric light, leads to evils which may be considered infinitely great in comparison with the consequences which attend the same accident in a catoptric light.

In comparing the fixed dioptric, and the fixed catoptric apparatus, the results may be ranged under the following heads:

1st. It is impossible, by means of any practical combination of parabolic reflectors, to distribute round the horizon a zone of light of exactly equal intensity; while this may be easily effected, by dioptric means, in the manner already described. In other words, the qualities required in fixed lights cannot be so perfectly obtained by reflectors as by refractors.

2nd. The light produced by burning one gallon of oil in Argand lamps, with reflectors, has only about one-fourth of the intensity of that produced by burning the same quantity in the dioptric apparatus; and the annual expenditure is L158, 3s. 8d. less for the dioptric than for the catoptric light.

3rd. The characteristic appearance of the fixed reflecting light would not be changed by the adoption of the dioptric method, although its increased intensity would render it visible at a greater distance.

4th. From the equal distribution of the rays, the dioptric light would be observed at equal distances in every point of the horizon; an effect which cannot be fully attained by any practicable combination of parabolic reflectors.

5th. The inconveniences arising from the uncertainty which attends the use of the mechanical lamp, are not so much felt in a fixed as in a revolving light, because the less complex nature of the apparatus admits of easier access to it, in case of accident. In those situations, too, where the horizon is not illuminated all round, the mechanical lamp may perhaps be supplanted by a large fountain lamp of four concentric wicks, the use of which would, in a great measure, remove the objection of uncertainty in the exhibition of the light.

6th. But the extinction of a lamp in a catoptric light, leaves only one-26th part of the horizon without the benefit of the light, and the chance of accident arising to vessels from it may, therefore, be considered as incalculably less than the danger resulting from the extinction of the single lamp of the dioptric light, which deprives the whole horizon of light.

7th. There may also, in certain situations, be some danger arising from the irregularity in the distances at which the same fixed catoptric light may be seen in the different points of the horizon. This defect, of course, does not exist in the dioptric light.

There can be little doubt, that the more fully the system of Fresnel is understood, the more certainly will it take the place of all other systems of illumination for lighthouses, in those countries where this important branch of administration is conducted with the care and solicitude which Britain deserves. To the Dutch belongs the honour of having first 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. Leonor Fresnel, brother of the late illustrious inventor of the system, and his successor as the Secretary of the Lighthouse Commission of France, afforded Mr. Stevenson the means of making such a report on his return, as induced the Commissioners to authorise him to remove the reflecting apparatus of the revolving light at Inchkeith, and substitute dioptric instruments in its place. This change was complet- Sea-Lights.

Sea-Lights ed., 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 begin to show symptoms of interest in this important change; and America, it is believed, is likely soon to adopt active measures for the improvement of her lighthouses. 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.

The fuel commonly adopted in the best lighthouses of Great Britain, is spermaceti oil, which is obtained from the South Sea whale (Physeter macrocephalus); and in France the oil generally burned is expressed from the seed of a species of wild cabbage, (Brassica oleracea colza), and is called huile de colza. It appears from some experiments made by M. Léon Fresnel at Paris, in which he compared the intensity of the light produced by the combustion of equal parts of this oil and the spermaceti oil used in England, of which specimens had been sent to him by the Trinity House of London, that there is but little difference. This conclusion differs somewhat from the result of the trials at the Isle of May and Inchkeith, where flames of similar dimensions to those produced from the colza oil are obtained by the combustion of nearly one-fourth less spermaceti oil. In the lights on the shores of the Mediterranean, olive oil is chiefly used; but the light obtained from it is feeble compared with that of spermaceti or colza oil.

In a few lighthouses which are near towns, the gas of 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, whose sudden failure might involve consequences of the most fatal description, and whose situation is often such, that their re-establishment must be a work of great expense and time. Gas is, besides, far from being suitable in catoptric lights, to which, where the frame is moveable, as in revolving lights, it could not be applied.

The application of the Drummond and Voltaic lights to lighthouse purposes is, owing to their prodigious intensity, a very desirable consummation; but it is surrounded by so many practical difficulties, that it may safely, 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.

In 1835, Mr. Gurney proposed the combination of streams of oxygen with the flame of oil or wax, in order to obtain a powerful light of sufficient size to produce the divergence required for the illumination of lighthouses. The Trinity House of London entertained the proposal, and have since been engaged in making experiments on this important subject; and their efforts, it is believed, have been attended with a measure of success which holds out a reasonable prospect of this lamp being finally used in lighthouses. In applying this light to reflectors, it is intended to use three small flames, each about three-eighths of an inch in diameter, which produce, it is said, an effect equal to that of ten common Argand lamps. The burner intended for lenses has seventeen films of flame, and is said to possess six times the power of the Fresnel lamp. The light is considered cheaper than that which is obtained by the combustion of oil in atmospheric air.

We shall conclude by a brief account of the lights of various countries, and the mode of management that is adopted, so far as we have the means of speaking with certainty on this subject.

The lights on the coast of England are under the management of the Corporation of Trinity House of Deptford House of Stroud. Before the reign of Henry VIII., the Trinity House appears to have been merely a fraternity of seamen, and it was not till the sixth year of Henry's reign that it was incorporated by royal charter. Elizabeth afterwards granted the Trinity House certain privileges of ballastage, beaconage, and buoyage, and empowered them to erect and preserve "beacons and signs for the sea." It does not appear, however, that this body specially undertook the erection of lighthouses, till about the year 1676. Before that time it was common to grant letters patent in favour of the proprietors of the lands adjoining the site of a lighthouse, empowering them to erect a lighthouse, and to levy certain duties on shipping for its maintenance. In some cases the patentee was bound to pay an annual sum to the Trinity House towards the support of their charities; but in other cases, he was quite independent. The Trinity House have, in numerous instances, entered into new arrangements with the lessees at the expiry of their leases; but it is now the practice of this board to erect and maintain lighthouses on the coast, without the intervention of any Sea-Lights arrangement of the nature of a lease; and the result, as might be expected, has been very favourable to the interests of the public. The lights, which were formerly maintained by lessees, whose interests, in some cases, led them to adopt every saving, without regard to the efficiency of the light, are now maintained in a manner worthy of the country to which they belong. This condemnation of the manner in which the lessees of some of the lights discharged their obligations, by no means applies indiscriminately to all of them; for there were several honourable exceptions. In 1834, the court of the corporation of Trinity House consisted of one master, four wardens, eight assistants, and eighteen elder brothers. Of these, eleven are in the honorary line of the brotherhood, and twenty-one are chosen from the merchant service. There are also younger brothers, whose number is unlimited, and who are elected by the elder brothers. The elder brothers are self-elected from the list of younger brothers. The business is managed by seven committees, who separately superintend the treasury and accounts, the examination of and granting certificates to masters in the navy and pilots, the supervision of ballastage in the Thames, the lighthouses, the collection of the dues, the pensioners and the management of the house affairs. In each of these committees a majority constitutes a quorum.

The rate of dues chargeable by the Trinity House before the passing of the last act in 1836, varied from 1/6th of a penny to one penny per ton on each light passed; and it appears from the Parliamentary Report of 1834, that in 1832, the nett amount of revenue was £77,371, and the expense of maintaining the lights was £36,904, leaving a surplus of £40,467. This surplus is partly expended in the extensive charities which are distributed by the Corporation, to the annual amount of £35,000, and partly in the erection of new lighthouses, and the maintenance of the general establishment.

The public lights in England, including Heligoland, are 71 in number, and may be arranged in the following classes:

1st, Those belonging to, and under the management of, the corporation of the Trinity House of Deptford Strond, in number.............. 55 lights. 2d, Those in the charge of individuals, under lease from the Trinity House, and having different periods to run, viz. the Longships, Smalls, and Mumbles, in number.................................. 3 3d, Those let by the crown to individuals for a period of years, on leases renewed since 1822, viz. Harwich, 2; Dungeness, 1; Wintertonness and Oxfordness, 3; and Hunstanton Cliff, 1; in number.................................................. 7 4th, Lights held originally under patents, subsequently sanctioned by acts of Parliament, and now in the hands of proprietors, viz. the Spurn, Tynemouth Castle and Skerries, in number..... 4 5th, One light at Heligoland.......................... 1 6th, One floating light at Benbridge Lodge... 1

Total number of public general lights in England......................................................... 71

The lights in Scotland may be divided into public and local or harbour lights. The public lights are under the management of a board denominated "Commissioners of Northern Lights." The Board was incorporated in 1786, by the 38th Geo. III. c. 58, and the Commissioners hold their office at the board by virtue of the public situations they fill. The Act of the 26th Geo. III. c. 101, gives authority to erect lighthouses and to collect duties.

The commissioners, twenty-five in number, are the Lord Advocate and the Solicitor-General for Scotland; the provosts of Edinburgh, Glasgow, Aberdeen, Inverness, and Campbeltown; the first bailies of Edinburgh and Glasgow; the sheriffs of the counties of Edinburgh, Lanark, Renfrew, Bute, Argyll, Inverness, Ross, Orkney, Caithness, Aberdeen, Ayr, Fife, Forfar, Wigton, Sutherland, and Kin- cardine, which are maritime counties or shires. The services of the commissioners are entirely gratuitous; there are two or three general meetings annually, and the general business is conducted by committees, whose meetings take place as occasion requires. A committee of the board was appointed at the time the Bell-Rock lighthouse was erected, which has been continued ever since, and to which all general business is referred; special committees have also been appointed for particular objects, as for accounts, stores, experiments on lenses, light-duties, visitation of lighthouses, special repairs and new works; and there is besides a sub-committee on each lighthouse. No public lights on the Scotch coast are in the hands of private individuals; and all the light-dues collected from the general shipping in Scotland, are received by the Commissioners of Northern Lighthouses, for public use. In the year 1641, a patent for the erection of a private light in the island of May, was ratified in the Scottish Parliament; and this is supposed to have been the earliest sea-light on the shores of Scotland. In 1814, the Commissioners purchased the Island from the Duke of Portland for £60,000, and erected a new lighthouse there. There are now 25 land-lights under the charge of the Commissioners, for which light-dues are levied from the shipping generally; and there are 28 local or harbour lights under the management of trustees and corporations, maintained by dues levied on the trade of the respective ports where the lights are situated, and on vessels resorting to them. Some of these lights are established by act of Parliament, as those of Cumbrae, Clough, and Toward, under the 29th of Geo. III. c. 20. Others, as those of Leith and Dundee, are secured, by ancient charters, to the fraternities of the ports; and others, as those of Montrose and Arbroath, were erected and are maintained, by the ship-owners and merchants of the ports.

Before the passing of the Act 6th and 7th of William IV., there was no separate charge for each of the lights under the management of the Commissioners of the Northern Lights; but each vessel paid a fixed sum according to the limits within which she came. The rates were as follows:—For every British ship or decked vessel sailing within the limits of the light of the island of May, viz. between the Castle of Dunnottar on the north, and St. Abb's Head on the south, 2½d. per ton; and for foreign vessels sailing within these limits, 3d. per ton. For every British ship or decked vessel liable to dues without these limits, 2d. per ton; and for foreign vessels, 4d. per ton. Vessels sailing to or from any place between Holyhead and Howth Head, both inclusive, on the north, paid ½d. per ton for each of the three lights in the Isle and Calf of Man; and foreign ships 4d. per ton; but if they had paid the other northern duties, they were exempted from the charge for these lights. The rates were levied on the registered tonnage of all vessels passing any of the lighthouses, whether loaded or in ballast, outwards or homewards bound, on a foreign voyage, or sailing coastways; and it appears from the Parliamentary Report of 1834, that the revenue of the Commissioners was £36,283. Since the passing of the act 6th and 7th of William IV. cap. 79, in 1836, however, the rates of the duties, and the mode of levying them, have been completely altered, and in some degree assimilated to the system pursued by the Trinity House, so that a certain duty is now paid for every light that is passed.

The lights of Ireland have had frequent changes in regard to superintendence, and they were finally placed under "The Corporation for Preserving and Improving the Port of Dublin," by the act 50th Geo. III. c. 35. The powers of the corporation, which is generally named the Ballast Board, have received various alterations by subse- It consists of 23 members, viz., the lord mayor and sheriffs of Dublin, three aldermen chosen by the board of aldermen from their own body, and 17 members appointed in the first instance by the act of incorporation, and who are on all future vacancies empowered to elect new members. The board devotes one day a week to the business of the lighthouses.

There are thirty-six land lights, and three floating lights, supported by the Ballast board in Ireland. Of these land lights, twenty-six are public lights, and ten are local or harbour lights. There are besides five other harbour lights on the coast, supported by the trustees of the respective harbours.

It appears from the Parliamentary Report of 1834, that the revenue derived from collection of lighthouse dues in Ireland during the year 1833, was £42,060. The mode of charging the light dues was formerly as follows:—Any British or Irish vessel and foreign privileged vessels on overseas voyages, paid one farthing per ton for every light they passed in the track of their voyage; foreign ships not privileged paid one halfpenny per ton. Coasters, loaded, paid one farthing per ton for every lighthouse or floating light they passed; if in ballast, one eighth of a penny only per ton; and rules were laid down as to the number of lights to be paid for by vessels navigating St. George's Channel, to or from the Atlantic Ocean; as well as for passing through the northern channel bound to the northward, and returning; for going down St. George's Channel, to the eastward; and for sailing from a western port of Ireland to the eastward, without entering St. George's Channel.

The last act of Parliament regarding lighthouses, was the result of a report of a select committee on lighthouses, which was moved for by Mr. Joseph Hume, M.P., and which sat in 1834. The act was passed in 1836. The chief alterations upon the former state of the Boards in which the management of the lights is vested, are the following. The duties levied under all former acts were repealed; and it was enacted, that every British vessel, and every privileged foreign vessel, should pay the toll of one halfpenny, for every time of passing or deriving advantage from any light, with exception of the Bell-rock, for which one penny per ton is the toll. Every foreign vessel not privileged, must pay double toll. The only exemptions from the payment of duties, are in favour of the King's vessels, those of the Trinity House, and all vessels going in ballast, or engaged in the herring fisheries. Power was also given to the Commissioners of the Northern Lighthouses, to erect beacons, and moor buoys; and the harbour lights on the Scotch coast were placed under their control. The Act confers upon the Trinity House, the power of entering any lighthouse under the charge of the other boards, to inspect their condition, and gives them a control as to the erection of new lighthouses, or the alteration of those already existing, both in Scotland and Ireland. But in the event of a difference of opinion arising between the Trinity House and either of the two other Boards, appeal may be made to the Privy Council. The Act further provides that an account of the receipt of all money, and a report of all alterations made during the preceding year, should be annually laid before each House of Parliament, one month after its meeting.

The average expense of maintaining a land light in Great Britain, is found to be about £500, and that of a floating light about £1,200.

The French lights, of which there are upwards of 100, including about 60 harbour lights, are managed by a particular section of the Direction Générale des Ponts et Chaussées, which is called the Commission des Phares; and more than half of the lights are on the dioptric principle of Fresnel.

By an imperial decree of 7th March 1806, the lighthouses and beacons were placed under the charge of the Minister of the Interior; before which time they had been under the immediate direction of the Administration of Roads and Bridges. This decree required that the establishment of every new beacon or light, should proceed upon the joint recommendation of the Ministers of the Marine and the Interior, and gave rise, in 1811, to the institution of the Commission des Phares, the members of which, with the exception of the secretary, who is also engineer-in-chief, act without any special remuneration, and in consequence of their holding other official situations. At the time of its first constitution, it appears by the report of Admiral de Rossel, that this Commission consisted of the following gentlemen, viz. Baron Becquey, councillor of state, director-general of roads and bridges, President of the Commission; M. Halgan, rear-admiral, and councillor of state; M. De Prony, inspector-general of roads and bridges; M. Arago, astronomer-royal, member of the Institute; M. Sganzin, inspector-general of roads and bridges; M. Rolland, inspector-general of naval works; M. Tarbe De Vauxclairis, inspector-general of roads and bridges; M. Mathieu, member of the Institute; and M. Augustin Fresnel, member of the Institute, secretary and engineer-in-chief to the Commission.

All the more important plans for the improvement and establishment of lights, are submitted to this Commission; but the plans for new lighthouse towers are only discussed in reference to their fitness for the lights; and every question regarding the buildings or estimates, is submitted to the General Council of the Administration of Roads and Bridges, for their final approbation. The Engineer of the Commission prepares all the plans, and directs the fitting up of the optical apparatus and the lanterns, and sends to the engineers of the departments, the schemes for new lights, that they may make the plans for the necessary buildings connected with them. He also inspects the lights on the coast, and is responsible for their efficient condition. In the discharge of these duties, he is assisted by three conducteurs of works, who generally see the apparatus fitted up, and attend to the due performance of the light-room duty.

As soon as a new light is ready, the Administration causes advertisements to be made in the most extensively circulated journals of Paris, containing a notice to mariners regarding its position, appearance, and time of exhibition. This notice is also circulated in every French port by means of placards, which are affixed by the maritime authorities, and generally appear about three months before the exhibition of the light. By a late decree, too, of the Director-general of roads and bridges, the Engineer publishes every year a summary description of all the lights on the coast of France.

The other lights of Europe are, with the exceptions already mentioned, on the catoptric principle. The writer of this article has seen the greater number of the lights along the coast, between St. Petersburgh and the Spanish frontier; and so far as he has been able to learn, their management is generally vested in some department of the executive government.

The Americans have been very active in the establishment of lighthouses. There are upwards of two hundred land lights along the coast of the United States, and twenty-eight floating lights. Their management is entrusted to a board called the General Lighthouse Establishment, which appears to have been regularly organised so early as the year 1791. The total expenditure connected with the Lighthouse Establishment in the year 1837, was about £71,352; a sum which includes about £7,000 for the maintenance of upwards of 600 buoys and beacons, which are also under the care of the Lighthouse Establishment. The Fifth Auditor of the Treasury is the chief officer of the lighthouse board. Sea-Lights. The apparatus is on the catoptric principle; but the reflectors, which are illuminated by means of Argand lamps, are of polished tin-plate, and of small dimensions. The light is from spermaceti oil, the produce of the American South Sea fishery; but experiments have lately been made upon oil produced from cotton-seed; and there is some probability that this oil will be universally employed in the lighthouses of America.

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 whose sole difference 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 characterising 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 ensnare without characterising 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.

Another important general inquiry, is that regarding the height of most advantageous height for lighthouses; but this subject tower is so extensive, and embraces the consideration of so many circumstances, that we can only glance at the chief elements of the question. The distance at which lights should be seen, depends very much upon their position in relation to the dangers of the coast; those which are outposts beyond the danger, require a less extensive range than those which, from unavoidable causes, are situated landward of the dangers which they are intended to point out. Upon this circumstance chiefly depends the height to which a lighthouse tower should be carried; but in many climates, the fogs by which the upper and lower regions of the atmosphere are obscured, introduce elements into the question, which, it is to be feared, must baffle all general rules.

The following works may be consulted on the subject of lighthouses: Smeaton's Narrative of the Eddystone Lighthouse. Lond. 1793. Stevenson's Account of the Bell-Rock Lighthouse. Edinburgh, 1824. Belidor, Architecture Hydraulique, vol. iv. p. 151. Peclet, Traité de l'éclairage. Paris, 1827. Fresnel's Mémoire sur un Nouveau Système d'éclairage des Phares. Paris, 1822. Admiral de Rossel's Rapport, contenant l'exposition du système adopté par la Commission des Phares pour éclairer les côtes de France. Paris, 1825. Treatise on Burning Instruments, containing the method of building large polygonal lenses. By David Brewster, LL.D. F.R.S. Edin. 1812. Fanale di Salvore, nell'Istria, illuminato a gaz. Vienna, 1821. On Construction of Polygonal Lenses and Mirrors of Great Magnitude, for Lighthouses and for Burning Instruments, and on the Formation of a Great National Burning Apparatus. By David Brewster, LL.D. F.R.S. (Edin. Phil. Jour. 1823. vol. viii. p. 160.) Account of a New System of Illumination for Lighthouses. By David Brewster, LL.D. F.R.S. Edin. 1827. Saggio di Osservazione, 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 Marce's Notice descriptif d'un fanal à double aspect, &c. Paris, 1823. Bordier Marce's Parabole Soumise à l'art, ou Essai sur la catoptrique de l'éclairage. Paris, 1819. L. Fresnel's Description Sommaire des Phares et Fanaux allumés sur les côtes de France, au 1er. d'Août. 1837. Stevenson's British Pharos. Leith, 1831. The Lighthouses of the British Islands, corrected to July 1836, from the Hydrographical Office of the Admiralty. Lond. 1836. Instructions pour le service des Phares Lenticulaires, par L. Fresnel. Paris, 1836. The Lighthouses, Floating Lights, and Beacons of the United States in 1838; prepared by order of Stephen Pleasonton, fifth auditor of the Treasury, and acting commissioner of the revenue. Washington, 1838. Captain Leontey Spafarielli's New Guide for the Navigation of the Gulf of Finland. St. Petersburg, 1813. Coulier's Guide des Marins. Paris, 1825. 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 Board on the Inchkeith Dioptric Light, by Alan Stevenson. Edin. 1835. Report on the Isle of May dioptric Light, by Alan Stevenson. 1835. Report on the Isle of May Light, by a Committee of the Royal Society, (Professor Forbes, reporter.) Edin. 1836. Sea-Plants. Sea-Plants are those vegetables that grow in salt-water, within the shores of the sea. The old botanists divided these into three classes. The first class, according to their arrangement, contained the algae, the fuci, the sea-mosses or confervae, and the different species of sponges. The second contained substances of a hard texture, like stone or horn, which seem to have been of the same nature with what we call zoophyta, with this difference, that we refer sponges to this class, and not to the first. The third class is the same with our lithophyta, comprehending corals, madreporites, &c. It is now well known that the genera belonging to the second and third of these classes, and even some referred to the first, are not vegetables, but animals, or the productions of animals. Sea-plants, then, properly speaking, belong to the class of cryptogamia, and the order of algae; and, according to Bonare, are all comprehended under the genus fucus. We may also add several species of the ulva and conferva, and the sargazo. The fuci and marine ulvae are immersed in the sea, are sessile, and without root. The marine conferva are either sessile or floating. The sargazo grows beyond soundings.

Sea-Serpent, a monstrous creature, said to inhabit the northern seas, about Greenland and the coasts of Norway. "In 1756," says Guthrie, "one of them was shot by a master of a ship. Its head resembled that of a horse; the mouth was large and black, as were the eyes, a white mane hanging from its neck. It floated on the surface of the water, and held its head at least two feet out of the sea. Between the head and neck were seven or eight folds, which were very thick; and the length of this snake was more than a hundred yards, some say fathoms. They have a remarkable aversion to the smell of castor; for which reason, ship, boat, and bark masters provide themselves with quantities of that drug, to prevent being overset, the serpent's olfactory nerves being remarkably exquisite. The particularities related of this animal would be incredible, were they not attested upon oath. Egede, a very reputable author, says, that on the 6th of July 1734, a large and frightful sea-monster raised itself so high out of the water, that its head reached above the main-topmast of the ship; that it had a long sharp snout, broad paws, and spouted water like a whale; that the body seemed to be covered with scales; the skin was uneven and wrinkled, and the lower part was formed like a snake. The body of this monster is said to be as thick as a hogshead; his skin is variegated like a tortoise-shell; and his excrement, which floats upon the surface of the water, is corrosive." No man of sound judgment, however, would think these recitals sufficient to establish the existence of such monsters.

Sea-Sickness, a disorder incident to most persons on their first going to sea, and occasioned by the agitation of the vessel. This disorder has not been much treated of, although it is very irksome and distressing to the patient during its continuance. It appears to be a spasmodic affection of the stomach, occasioned by the alternate pressure and recession of its contents against its lower internal surface, according as the rise and fall of the ship oppose the action of gravity.

Many methods of preventing, or at least mitigating, this disorder, have been recommended, of which the most efficacious appear to be the following: Not to go on board immediately after eating, and not to eat, when on board, any large quantity at a time; to lie down the moment the symptoms are felt, and to remain in the horizontal position till they are removed; to keep much upon deck, even when the weather is stormy, as the sea-breeze is not so apt to affect the stomach as the impure air of the cabin, rendered so for want of proper circulation; and not to watch the motion of the waves, particularly when strongly agitated with tempest.

Sea-Weed, or Alga marina, is commonly used as a manure on the sea-coast, where it can be procured in abundance. The best sort grows on rocks, and is that from which kelp is made. The next to this is called the peasy seaweed; and the worst is that with a long stalk.