There is not one of the appearances of nature which has so much engaged the attention of mankind as thunder. The savage, the citizen, and the philosopher, have observed it with dread, with anxiety, and with curiosity; and the philosopher of our times treats the others with a smile of condescension, while he here enjoys the fullest triumph of his superiority.
Felix qui potuit rerum cognoscere causas, Aique mutus omnes et inevitabile fulmen Subjecit pedibus.
But though this grand phenomenon has long engaged the curious attention of philosophers, it is but very lately that they have been able to explain it; that is, to point out the more general law of nature of which it is a particular instance. Inflammable vapours had long furnished them with a sort of explanation. The discovery of gunpowder, and still more that of inflammable air, gave some probability to the existence of extensive strata of inflammable vapours in the upper regions of the atmosphere, which, being set on fire at one end, might burn away in rapid succession, like a train of gunpowder. But the smallest investigation would show such a dissimilarity in the phenomena, and in the general effects, that this explanation can have no value in the eyes of a true naturalist. Horrid explosion; and a blast which would sweep everything from the surface of the earth, must be the effects of such inflammation. The very limited and capricious nature of the ravages made by thunder, render them altogether unlike explosions of elastic fluids.
No sooner were the wonderful effects of the charged electrical phial observed, than naturalists began to think of this as exhibiting some resemblance to a thunder-stroke (see ELECTRICITY, Encycl. n° 12); but it was not till toward the year 1750 that this resemblance was viewed in a proper light by the celebrated Franklin. In a dissertation written that year, he delivers his opinion at large, and notices particularly the following circumstances of similarity:
1. The colour and crooked form of lightning, perfectly similar to that of a vivid electrical spark between distant bodies, and unlike every other appearance of light. This angular, delirious, capricious form of an particular electrical spark, and of forked lightning, is very singular. No two successive sparks have the same form. Their sharp angles are unlike every appearance of motion through unrefilling air. Such motions are always curvilinear. The spark is like the simultaneous existence of the light in all its parts; and the fact is, that no person can positively say in which direction it moves.
2. Lightning, like electricity, always strikes the most advanced objects—hills, trees, steeples.
3. Lightning affects to take the best conductors of electricity. Bell wires are very frequently destroyed by it. At Leven house in Fifeshire, in 1733, it ran along a gilded moulding from one end of the house to the other, exploding it all the way, as also the tinfoil on the backs of several mirrors, and the gilding of screens and leather hangings.
4. It burns, explodes, and destroys these conductors precisely as electricity does. It dissolves metals; melts wires; it explodes and tears to pieces bodies which contain moisture. When a person is killed by lightning, his shoes are commonly burnt. When it falls on a wet surface, it spreads along it. The Royal William, in Louisburgh harbour, in 1738, received a thunder-stroke, which dissipated the maintop; gallant mast in dust, and came down on the wet decks in one spark, which spread over the whole deck as a spout of water would have done. This is quite according to electrical laws.
5. It has sometimes struck a person blind. Electricity has done the same to a chicken which it did not kill.
6. It affects the nervous system in a way resembling some of the known effects of electricity. The following is a most remarkable instance: Mr Campbell, Esq; of Succoth, in Dumbartonshire, had been blind for several years. The disorder was a gutta serena. He was led one evening along the streets of Glasgow by his servant Alexander Dick, during a terrible thunder storm. The lightning sometimes fluttered along the streets for a quarter of a minute without ceasing. While this fluttering lasted, Mr Campbell saw the street distinctly, and the changes which had been made in that part by taking down one of the city gates. When the storm was over, his entire blindness returned.—We have from a friend another instance, no less remarkable. One evening in autumn he was fitting with a gentleman who had the same disorder, and he observed several lambeat flashes of lightning. Their faces were turned to the parlour window; and immediately after a flash, the gentleman said to his wife "Go, my dear, make them shut the white gate; it is open, you see." The lady did so, and returned; and, after a little, said, "But how did you know that the gate was open?" He exclaimed, "My God! I saw it open, and two men look in, and go away again," (which our friend also had observed). The gentleman, on being close questioned, could not recollect having had another glance, nor why it had not surprized him; but of the glimpse itself he was certain, and described the appearance very exactly.
7. Lightning kills; and the appearances perfectly resemble resemble those of a mortal stroke of electricity. The muscles are all in a state of perfect relaxation, even in those situations where it is usually otherwise.
8. Lightning is well known to destroy and to change the polarity of the mariner's needle.
Dr. Franklin was not contented with the bare observation of these important resemblances. He availed himself of many curious discoveries which he had made of electrical laws. In particular, having observed that electricity was drawn off at a great distance, and without the least violence of action, by a sharp metallic point, he proposed to philosophers to erect a tall mast or pole on the highest part of a building, and to furnish the top of it with a fine metallic point, properly insulated, with a wire leading to an insulated apparatus for exhibiting the common electrical appearances. To the whole of this contrivance he gave the name of thunder-rod, which it still retains. He had not a proper opportunity of doing this himself at the time of writing his dissertation in a letter from Philadelphia to the Royal Society of London; but the contents were so scientific, and so interesting, that in a few weeks time they were known over all Europe. His directions were followed in many places. In particular, the French academicians, encouraged by the presence of their monarch, and the great satisfaction which he expressed at the repetition of Dr. Franklin's most instructive experiments, which discovered and established the theory of positive and negative electricity, as it is still received, were eager to execute his orders, making his grand experiment, which promised so fairly to bring this tremendous operation of nature not only within the pale of science, but within the management of human power.
But, in the mean time, Dr. Franklin, impatient of delay, and perhaps incited by the honourable desire of well-deserved fame, put his own scheme in practice. His inventive mind suggested to him a most ingenious method of presenting a point to a thunder cloud at a very great distance from the ground. This was by fixing his point on the head of a paper kite, which the wind should raise to the clouds, while the wet string that held it should serve for a conductor of the electricity. We presume that it was with a palpitating heart that Dr. Franklin, unknown to the neighbours, and accompanied only by his son, went into the fields, and sent up his messenger that was to bring him such news from the heavens. He told a person, who repeated it in the hearing of the present writer, that when he saw the fibres of the cord raise themselves up like huge bristles, he uttered a deep sigh, and would have wished that moment of joy to have been his last. He obtained but a few faint sparks from his apparatus that day; but returned to his house in a state of perfect happiness, now feeling that his name was never to die. Thus did the soap bubble, and the paper kite, from being the playthings of children, become, in the hands of Newton, and of Franklin, the means of acquiring immortal honour, and of doing the most important service to society.
We may justly consider this as one of the greatest of philosophical discoveries, and as doing the highest honour to the inventor; for it was not a suggestion from an accidental observation, but arose from a scientific comparison of facts, and a sagacious application of the doctrine of positive and negative electricity; a doctrine wholly Dr. Franklin's, and the result of the most acute and discriminating observation. It was this alone that suggested the whole; and by explaining to his satisfaction the curious property of sharp points, gave him the courage to handle the thunderbolt of Jove.
It is then a point fully ascertained, that thunder and lightning are the electric snap and spark, as much superior to our puny imitations as we can conceive from the immense extent of the instruments in the hands of Nature. If, says Dr. Franklin, a conductor one foot thick and five feet long will produce such snaps as agitate the whole human frame, what may we not expect from a surface of 10,000 acres of electrified clouds? How loud must be the explosion? how terrible the effects?
This discovery immediately directed the attention of electrical philosophers to the state of the atmosphere with respect to electricity; and in this also Dr. Franklin led the way. He immediately erected his thunder rods; and they have been imitated all over the world, with many alterations or improvements, according to the different views and skill of their authors. It is needless to insist here on their construction. They have been described in the article Electricity (Encycl.); and any person well acquainted with its theory, as laid down in the Supplementary article Electricity, will be at no loss to accommodate his own construction to his situation and purposes.
Dr. Franklin took the lead, as we have already observed, in this examination of the electrical state of the atmosphere. He seldom found it without giving signs of electricity, and this was generally negative. See Phil. Trans. Vol. XLVIII. p. 358. and 785.
Mr. Canton repeated those experiments, and found the same results; both, however, found that the electricity would frequently change from positive to negative, and from negative to positive, in very short spaces of time, as different portions of clouds or air passed the thunder-rod.
We must here remark, that our acquaintance with the laws of electricity sufficiently informs us, that the observed electricity of our thunder rod may frequently be of a different kind from that of the cloud which excites the appearance at our apparatus. We know that air, like glass, is a non-conductor; and that when it is brought into any state of electricity, either by communication, or by mere induction, it will remain in that state for some time, and that it always changes its electricity per se. A positive cloud, in the higher regions of the atmosphere, will render the air immediately below it negative, and a stratum below that positive. If the thunder rod be in this positive stratum, it will exhibit positive electricity; but if the cloud be considerably nearer, the rod, by being in the adjoining negative stratum, may show a negative electricity, which will exceed the positive electricity which the distant positive cloud would have induced on its lower end by mere position, had the intervening air been away. This excess of negative electricity must depend on the degree in which the surrounding stratum of air has been rendered negative. If this has been the almost instantaneous effect of the presence of the positive cloud, it cannot be rendered so negative as to produce negative electricity in the lower end. But if the stratum of air has for some considerable time accompanied the positive cloud, its negative electricity has been increasing, and some would remain, even if the cloud were removed. We must, at all times, consider the thunder rod as affected by all the electricity in its neighbourhood. The distant positive cloud would at any rate render the lower end of the rod positive, without communication, by merely displacing the electricity in the rod itself, just as the north pole of a lodestone would make the remote end of a soft iron rod a north pole. In like manner, the negative stratum of air immediately adjoining to the positive cloud would make the lower end of the rod negative, without communication. A positive stratum of air below this would have the contrary effect. The appearances, then, at the end of the rod, must be the result of the prevalence of one of these above the others; and many intervening circumstances must be understood before we can infer with certainty the state of a cloud from the appearances at the lower end of the apparatus. It would, therefore, be a most instructive addition to a thunder rod to have an electroscope at both ends. If they show the same kind of electricity, we may be assured that it is by communication, and is the same with that of the surrounding stratum of air: But if they show opposite electricities (which is generally the case), then we learn that it is by position or induction. We recommend this to the careful attention of the philosopher.
In this way we perfectly explain an appearance which puzzled both of the above-mentioned observers. When a single low cloud approached the rod, the electroscopé would show positive electricity, but negative when the cloud was in the zenith, and positive again when it had passed by. We also learn from this the cause of Dr Franklin's disappointment in his expectations of very remarkable phenomena by means of his kite. He imagined that it would be vastly superior to the apparatus which he had recommended to the philosophers of Europe. But the string of the kite, traversing several strata in different states of electricity, served as a conductor between them, and he could only obtain the surplus which might be nothing, even when the clouds were strongly electrified.
The most copious and curious observations on the electrical state of the atmosphere are those by Professor Beccaria of Turin. He had connected the tops of several steeples of the city by insulated wires. He did the same thing at a monastery on a high hill in the neighbourhood. Each of these collected the electricity of a separate stratum of considerable extent. He frequently found these two strata in opposite states of strong electricity.
The following general observations are made out from a comparison of a vast variety of more particular ones made in different places:
1. The air is almost always electrical, especially in the daytime and dry weather; and the electricity is generally positive. It does not become negative, unless by winds from places where it rains, snows, or is foggy.
2. The moisture of the air is the constant conductor of its electricity in clear weather.
3. When dark or wet weather clears up, the electricity is always negative. If it has been very moist, and dries very fast, the electricity is very intense, and diminishes when the air attains its greatest dryness; and there may continue long stationary, by a supply of air in a drying state from distant places.
4. If, while the sky overcasts in the zenith, only a high cloud is formed, without any secondary clouds under it, and if this cloud is not the extension of another which rains in some remote place, the electricity (if any) is always positive.
5. If the clouds, while gathering, are shaped like locks of wool, and are in a state of motion among each other; or if the general cloud is forming far aloft, and stretches down like descending smoke, a frequent positive electricity prevails, more intense as the changes in the atmosphere are quicker; and its intensity predicts the great quantity of snow or rain which is to follow.
6. When an extensive, thin, level cloud forms, and darkens the sky, we have strong positive electricity.
7. Low thick fogs, rising into dry air, carry up so much electricity as to produce sparks at the apparatus. If the fog continues round the apparatus without rising, the electricity fails.
8. When, in clear weather, a cloud passes over the apparatus, low and tardy in its progress, and far from any other, the positive electricity gradually diminishes, and returns when the cloud has gone over.
9. When many white clouds gather over head, continually uniting with and parting from each other, and thus form a body of great extent, the positive electricity increases.
10. In the morning, when the hygrometer indicates dryness equal to that of the preceding day, positive electricity obtains, even before sunrise.
11. As the sun gets up, this electricity increases; more remarkably if the dryness increases. It diminishes in the evening.
12. The midday electricity, of days equally dry, is proportional to the heat.
13. Winds always lessen the electricity of a clear day, especially if damp; therefore they do not electrify the air by friction on solid bodies.
14. In cold seasons, with a clear sky and little wind, a considerable electricity arises after sunset, at dew falling.
The same happens in temperate and warm weather.
If, in the same circumstances, the general dryness of the air is less, the electricity is also less.
15. The electricity of dew, like that of rain, depends on its quantity. This electricity of dew may be imitated by electrifying the air of a close room (not too dry), and filling a bottle with very cold water, and setting it in the upper part of the room. As the damp condenses on its sides, an electrometer will show very vivid electricity.
Such a collection of observations, to be fit for inference, requires very nice discrimination. It is frequently difficult to discover electricity in damp air, though it is then generally strongest; because the insulation of the apparatus is hurt by the dampness. To make the observation with accuracy, requires a portable apparatus, whose insulation can be made good at all times. With such apparatus we shall never miss observing electricity in fogs, or during snow.
There is a very curious phenomenon, which may be frequently observed in Edinburgh, and no doubt in other towns similarly situated. In a clear day of the tickliest month fog... month of May, an easterly wind frequently brings a fog with it, which advances from the sea in a dense body; and when it comes up the High-street, it chills the body exceedingly, while it does not greatly affect the thermometer. Immediately before its gaining the street, one feels like a tickling on the face, as if a cobweb had fallen on it, and naturally puts up his hand, and rubs the face. We have never found this to fail, and have often been amused with seeing every person rubbing his face in his turn. The writer of this article has observed the same thing at St Peterburgh, in a summer's evening, when a low fog came on about ten o'clock.
The general appearances of a thunder storm are nearly as follow:
For the most part the wind is gentle, or it is calm. A low dense cloud begins in a place previously clear; this increases fast in size; but this is only upwards, and in an arched form, like great bags of cotton. The lower surface of the cloud is commonly level, as if it rested on a glass plane.
Soon after appear numberless small ragged clouds, like flakes of cotton teased out. These are moving about in various uncertain directions, and continually changing their ragged shape. This change, however, is generally by augmentation. Whatever occasions the precipitation of the dissolved water seems to gain ground. As these clouds move about, they approach each other, and then stretch out their ragged arms toward each other. This is not by an augmentation, but by a real bending of their tatters towards the other cloud. They seldom come into contact; but after coming very near in some parts, they as plainly recede, either in whole, or by bending their arms away from each other.
But during this confused motion, the whole mass of small clouds approaches the great one above it; and when near it, the clouds of the lower mass frequently coalesce with each other before they finally coalesce with the upper cloud: But as frequently the upper cloud increases without them. Its lower surface, from being level and smooth, now becomes ragged, and its tatters stretch down towards the others, and long arms are extended towards the ground. The heavens now darken space; the whole mass sinks down; wind arises, and frequently shifts in squalls; small clouds are now moving swiftly in various directions; lightning now darts from cloud to cloud. A spark is sometimes seen co-existent through a vast horizontal extent, of a crooked shape, and of different brilliancy in its different parts. Lightning strikes between the clouds and the earth—frequently in two places at once. A continuation of these snaps rarifies the cloud; and in time it dissipates. This is accompanied by heavy rain or hail; and then the upper part of the clouds is high and thin.
During this progress of the storm, the thunder rod is strongly electrified; chiefly when the principal cloud is over head. The state of the electricity frequently changes from positive to negative—almost every flash, however distant, occasions a sudden start of the electroscope, and then a change of the electricity. When the cloud is more uniform, the electricity is so too.
The question now is, In what manner does the air acquire this electricity? How come its different parts to be in different states, and to retain this difference for a length of time? and how is the electric equilibrium restored with that rapidity, and to that extent, that we observe in a thunder storm? For we know that air is a very imperfect conductor, and transmits electricity to small distances only, and very slowly. We shall mention several circumstances, which are known facts in electricity, and must frequently concur, at least, with the other causes of this grand phenomenon.
Air is rendered electrical in a great variety of ways.
1. All operations which excite electricity in other bodies have the same effect on air. It is electrified by friction. When blown on any body, such as glass, &c., that body exhibits electricity by a sensible electrocope. We therefore conclude that the air has acquired the opposite electricity from this rubber. A glass vessel, exhausted of air, and broken in the dark, gives a loud crack, and a very sensible flash of light. An air-gun, discharged (without a bell) in the dark, does the same. Blowing on an electric with a pair of bellows never fails to excite it. In short, the facts to this purpose are numberless.
2. Electricity is produced by a number of chemical operations, which are continually going on. The melting and freezing of electric bodies in contact with each other, such as chocolate in its moulds, wax-candles in their moulds, sealing-wax, &c. Nay, it is highly probable that any body, in passing from its fluid to its solid form, or the contrary, is electrical. This is the case when a solution of Glauber's salt, or of nitre, in water, is made to crystallize all at once by agitation.
The solution of bodies in their menstrua is, in like manner, productive of electricity in many cases. Thus iron or chalk, while dissolving in the sulphuric acid, produce negative electricity in the mixture, and positive in the electric vapours which arise from them.
A most copious source of electricity is the conversion of water into elastic steam by violent heats. When this is done in a proper apparatus, the electricity of the liquid is negative, and the vapour is positive. But if this be accompanied by a decomposition of the water, the liquid is sometimes strongly negative. Thus, when water evaporates suddenly from a red hot silver cup, the cup is strongly negative; but if from clean red hot iron, so that the iron is calcined; and inflammable air produced, the iron is positive. If the decomposition of the water is sufficiently copious to do more than compensate for the negative electricity produced by the mere expansion of the water into steam, the electricity is positive; but not otherwise. Water expanded from a piece of red hot coal always gives negative electricity, and this frequently very strong. These experiments should always be made in metallic vessels. If made in glass vessels, the glass takes a charge, which expends the produced electricity, and remains nearly neutral, so that the production of electricity is not observed. These facts are to be found among many experiments of Mr Saussure. But there is here a very wide field of new inquiry, which cannot fail of being very instructive, and particularly in the present question. We see some of the effects very distinctly in several phenomena of thunder and lightning. Thus, the great eruptions of Aetna and Vesuvius are always accompanied by forked lightnings, which are seen darting among the volumes of emitted smoke and steam. Here is a very copious conversion of water into elastic steam; and here also it is most reasonable to expect a copious decomposition of water, Thunder-water, by the iron and coalsy matters, which are exposed to the joint action of fire and water. These two electricities will be opposite; or when not opposite, will not be equal: in either of which cases, we have vast masses of steam in states fit for flashing into each other.
A fact more to our purpose is, that if a silk or linen cloth, of a downy texture, be moistened or damped, and hung before a clear fire to dry, the fibres bristle up, and on bringing the finger, or a metal knob, near them, they are plainly attracted by it. We found them negatively electric. This shews that the simple solution of water in air produces electricity. And this is the chief operation in Nature connected with the state of the atmosphere. It is thus that the watery vapours from all bodies, and particularly the copious exudation of plants, disappear in our atmosphere. There can be no doubt but that the opposite electricity will be produced by the precipitation of this vapour; that is, by the formation of clouds in clear air. When damp, but clear air in one vessel expands into an adjoining vessel, from which the air has been exhausted, a cloud appears in both, and a delicate electrometer is affected in both vessels; but our apparatus was not fitted for ascertaining the kind of electricity produced. Here then is another unexplored field of experiment. We got two vessels made, having diaphragms of thin silk. These were damped, and set into two tubs of water, of very different temperatures. Dry air was then blown through them, and came from their spouts saturated with water. The spouts were turned toward each other. Being of very different temperatures, the streams produced a cloud upon mixing together, and a strong negative electricity was produced. We even found that an electrometer, placed in a vessel filled with condensed air, was affected when this air was allowed to rush out by a large hole.
Lastly, we know that the tourmaline, and many of the columnar crystals, are rendered electrical by merely heating and cooling. Nay, Mr. Canton found that dry air became negative by heating, and positive by cooling, even when it was not permitted to expand or contract.
When water is precipitated, and forms a cloud, it is reasonable to expect that it will have the electricity of the air from which it is precipitated. This may be various, but in general negative: For the heat by which the air was enabled to dissolve the water made it negative; and much more the friction on the surface of the earth. But as heat caused it to dissolve the water, cold will make it precipitate it; and we should therefore expect that the air will be in the state in which it was when it took up the water. But if it be cooled so fast as to precipitate it in the form of rain, or snow, or hail, we may expect positive electricity. Accordingly, in summer, hail flowers always show strong positive electricity; so does snow when falling dry.
Here, then, are copious sources of atmospheric electricity. The mere expansion and condensation of the air, and still more the solution and precipitation of watery vapours in it, are perhaps sufficient to account for all the inequality of electric state that we observe in the atmosphere.
The masses of air thus differently constituted are evidently disposed in strata. The clouds are seen to be so. These clouds are not the strata, but the boundaries of strata; which, from the very nature of things, are in different states with respect to the subjection or precipitation of water. When two such strata are thus adjoining, they will slowly act on each other's temperature, and by mixing will form a thin stratum of cloud (sphere) along their mutual confines. If the one stratum has any motion relative to the other, and be in the smallest degree disturbed, they will mix to a greater depth in each; and this mixture will not be perfectly uniform. The extreme mobility of air will greatly increase this jumble of the adjoining parts of the two strata, and will give the cloud a greater thickness. If the jumble has been very great, so as to push one of them through the other, we shall have great towering clouds, perhaps pervading the whole thickness of the stratum of air. We take these clouds to be like great foggy bladders, superficially opaque where they have come into contact with the surrounding stratum of air, but transparent within.
When the wind, or stratum in motion, does not push all the quietest air before it, it generally gets over it, and then flows along its upper side, and, by a partial mixing, produces a fleecy cloud, as already described. We may observe here, by the way, that the motion of those fleecy clouds is by no means a just indication of the motion of the stratum; it is nearly the motion composed of the half of the motions of the two.
This is in all probability the state of the atmosphere, consisting of strata of clear air many hundred yards thick, separated from each other by thin fleeces of strata of clouds, which have been produced by the mixture of the two adjoining strata. This is no fancy; for we actually see the sky separated by strata of clouds at a great distance from each other. And we see that these strata maintain their situations, without farther admixture, for a long time, the bounding clouds continuing all the while to move in different directions. In the year 1759, during the siege of Quebec, a hard gale blew one day from the westward, which made it almost impracticable to send a number of provision-boats to our troops stationed above the town. While the men were tugging hard at the oars against the wind, and hardly advancing, though the tide of flood favoured them, the French threw some bombs to destroy the boats. One of these burst in the air, near the top of its flight, which was about a quarter of a mile high. The round ball of smoke produced by the explosion remained in the same spot for above seven minutes, and disappeared by gradual diffusion. The lower air was moving to the eastward at least 30 feet per second.
In 1783, when a great fleet rendezvoused in Leith Roads, the ships were detained by an easterly wind, which had blown for six weeks without intermission. The sky was generally clear; sometimes there was a thin fleece of clouds at a great height, moving much more slowly in the same direction with the wind below. During the last eight days, the upper current was from the westward, as appeared by the motion of the upper clouds. High towering clouds came down the river, with a little rain; the strata were jumbled, and the whole atmosphere grew hazy and uniform; then came thunder, and heavy rain, and the wind below shifted to the westward.
Thus it is sufficiently evinced, that the atmosphere frequently consists of such strata, well distinguished from each each other; their appearance and progress leave us no room to doubt but that they come from different quarters, and had been taken up or formed at different places, and in different circumstances, and therefore differing in respect of their electrical states.
The consequence of their continuing long together would be a gradual but slow progress of their electricity to a state of equilibrium. The air is perhaps never in a perfectly dry state, and its moisture will cause the electricity to diffuse itself gradually. It is not beyond the power of our mathematics to ascertain the progress of this approximation to the electric equilibrium. We see something very like it in the curious experiments of Piccarda with mirror plates laid together, and charged by means of a coating on the outer plates. These plates were found to consist of alternate strata of positive and negative electricity, which gradually penetrated through the plates, and coalesced till they were reduced to two strata; perhaps in time the electricity would have disappeared entirely by these two also coalescing. In the same manner there would be a flow transposition of sensible electricity through these strata without any sensible appearances. If any collateral causes should make a part more damp than the rest, there would be a more brisk transference through it, accompanied with faint flashes of lambeant lightning.
But thunder requires a rapid communication, and a reformation of electric equilibrium in an instant, and to a vast extent. The means for this are at hand, furnished by Nature. The strata of charged air are furnished with a coating of cloud. The lower stratum is coated on the under side by the earth.
When a jumble is made in any of the strata, a precipitation of vapour must generally follow. Thus a conductor is brought between the electrical coatings. This will quickly enlarge, as we see that in our little imitations the knobs of our conductors instantaneously arrange any particles of dust which chance to lie in the way, in such a manner as to complete the line of conduct, and occasion a spark to fly to a much greater distance than it would have leaped if no dust had been interposed. We have often procured a discharge between two knobs which were too far asunder, by merely breathing the damp air between them. In this manner the interposed cloud immediately attracts other clouds, grows ragged by the passage of electricity through clear air, where it causes a precipitation by altering the natural equilibrium of its electricity; for a certain quantity of electricity may be necessary for air's holding a certain quantity of vapour. Accordingly we see in a thunderstorm that small clouds continually and suddenly form in parts formerly clear. Whatever causes thunder, does in fact promote this precipitation.
These clouds have the electricity of the surrounding air, and must communicate it to others in an opposite state, and within reach. They must approach them, and must afterwards recede from them, or from any that are in the same state of electricity with themselves. Hence their ragged forms, and the familiar form of the under surface of the great cloud; hence their continual and capricious shifting from place to place; they are carriers, which give and take between the other clouds, and they may become stepping stones for the general discharge.
If a small cloud form a communication with the ground, and the great cloud be positive or negative, we must have a complete discharge, and all the electrical phenomena, with great violence; for this coating of vapour is abundantly complete for the purpose. It consists of small vehicles, which are sufficiently near each other for discharging the whole air that is in their interstices. A phial coated with amalgam is by no means fully coated. If we hold it between the eye and the light, we shall see that it is only covered with a number of detached points of amalgam, which looks like a cobweb. Yet this glass is almost completely discharged by a single spark, the residuum being hardly perceptible.
The general scene of thunder is the heavens; and it is by no means a frequent case that a discharge is made into the earth. The air intervening between the earth and the lowest coating is commonly very much confused in consequence of the hills and dales, which, by altering the currents of the winds, toss up the inferior parts, and mix them with those above. This generally keeps the earth pretty much in the same electrical state as the lowest stratum of clouds.
Nor are the great thunder storms in general instances which are of the restoration of equilibrium between two strata immediately incumbent on each other. They seem, for the most part, to be strokes between two parcels of air which are horizontally distant. This, however, we do not affirm with great confidence. Our chief reason for thinking so is, that in these great storms the spark or shaft of forked lightning is directed horizontally, and sometimes is seen at once through an extent of several miles.
The nature of this spark has not, we think, been properly considered. It is simply compared to a long arc of electrical spark, which we conceive to be drawn thro' the pure air, and is considered as marking the actual transference of electricity from one end to the other. But this we doubt very much. We are certain of having the longest observed shafts of lightning at one and the same instant stretching horizontally, though with many capricious zigzags and lateral sputterings, at least five miles. We cannot conceive this to have been the striking distance, because the greatest vertical distance of the strata is not the half of this. We rather think that it is a simultaneous range of discharges, each accompanied with light, differently bright, according to the electrical capacity of the cloud into which it is made; and if there is a real transference of electric matter on this occasion (which we do not affirm), it is only of a small quantity from one cloud to the next adjoining. This we think confirmed by the sound of thunder. It is not a snap, incomparably louder than our loudest snap from coated glass; but a long continued, rumbling, and very unequal noise. There is no doubt but that this snap was almost simultaneous through the whole extent of the spark; but its different parts are conveyed to our ear in time, and are therefore heard by us in succession; and it is not an uniform roar, but a rumbling noise, unequally loud, according as the different parts of the spark are indeed differently loud. We should hear a noise of the same kind if we stood at one end of a long line of foldiers, who discharged their musquets (differently loaded) in the same instant. When any part of the spark is very near us, and is not very diffuse, the snap begins with great smartness, and continues for some time, not unlike the violent tearing of a piece of strong silk; after which it becomes more and more mel- Thunder, low as it comes from a greater distance. We do not, however, affirm, that the whole extensive spark and snap are co-existent or simultaneous. The cloud is, in all probability, but an indifferent conductor, and even a sensible time may elapse during the propagation of the spark to a great distance. Beccaria observed this in a line of 250 feet of chain, lying loosely on the ground, consisting of near 6000 links. He thought that it employed a full second; but when the chain was gently stretched, the communication seemed instantaneous.
We cannot help thinking that even the electrical spark between two metal knobs is of the same kind. Not a quantity of luminous matter which issues from the one and goes to the other, but a light that is excited or produced in different material interjacent particles of air or other interposed matter. The angular and sputtering form is quite incompatible with the motion of a simple luminous point. Nay, our chemical knowledge here comes in aid, and obliges us to speculate about the manner in which this light is produced. Whence does it come? It may be produced by two knobs of ice. We know that water consists of vital and inflammable air, which have already emitted the light which made an ingredient of their composition. The spark therefore does not come from the ice. Is it then from the air? If so, perhaps water is produced, or rather something else, for there is not always inflammable air at hand to compose water. Yet the transference of electricity has decomposed the air, or has robbed it of part of its light. The remainder may not be water; but it is no longer air. Is not this confirmed by the peculiar smell which always accompanies electric sparks? and the peculiar taste, not unlike the taste felt on the tongue when it is touched by the zinc in the experiments on Galvanism? Even the fine pencil of light which flows from a point positively electrified, appears through a magnifying glass to consist, not of luminous lines, but of lines of luminous points. And these points are of different brilliancy and different colour, both of which are incessantly changing. And be it farther observed, that these lines are curves, diverging from each other, and convex to the axis. This circumstance indicates a mutual repulsion, arising, in all probability, from the expansion of the air. And, lately, no spark nor light of any kind can be obtained in a space perfectly void of air.
All these circumstances concur in explaining the nature of the shaft of forked lightning. It is a series of appearances excited in the intervening medium, and which produce some chemical change in it. Thunder, when it strikes a house, always leaves a peculiar smell. Inflammable air has also a peculiar and very disagreeable smell. The smell produced by electricity greatly resembles the smell produced by striking two pieces of quartz together.
Mr Deuc supposes that the electrical spark, as it is exhibited in thunder, is always accompanied by the decomposition of air now so familiarly known, and that this is the origin of the deluge of rain which commonly signifies the storm. But this is not in the smallest degree probable. The decomposition extends surely no farther than where the light is separated; and we should no more expect a deluge of rain, even if we had inflammable air ready at hand, than we expect drops of water in our electrical experiments. Something different from water follows this decomposition, total or partial, of the vital air; and the water which we do observe to accompany thunder, is no more than what we should expect from the copious precipitation of water in a cloudy form. Mr Saufure's observations assure us, that the particles of a cloud are vehicles. Indeed no person who has looked narrowly at a fog, or has observed how large the particles are of the cloud which forms in a receiver when we suddenly diminish the density of the air, and who observes how slowly these particles descend, can doubt of their being hollow vehicles. We cannot perhaps explain their formation; but there they are. We can hardly conceive them receiving the commotion which accompanied the spark without collapsing by the agitation. Perhaps the very cessation of their electricity may produce this effect. They will therefore no longer float in the air, but fall, and unite, and come to the ground in rain. We may expect this rain to be copious, for it is the produce of two strata of clouds. It greatly contributes to the putting an end to the storm, by passing through the strata, and helping to restore the equilibrium.
One may at first expect that a single clap of thunder will restore the equilibrium of any extent of clouds, and how then we require an explanation of their frequent repetition before this is accomplished. This is not difficult, and some time, the fact is a confirmation of the above theory, which is considerably different from the generally received notions of the subject. We consider the stratum of clear air as the charged electric; positive on one side, and negative on the other, and coated with conducting clouds. When the discharge is made, the state of electricity is indeed changed through the whole stratum, but the equilibrium is by no means completed. The stratum is perhaps a quarter of a mile in thickness. The discharge does not immediately affect all this; but does it superficially, leaving the rest unbalanced. It is like the reticulum which is left in a Leyden phial when the discharge has been made by means of a spark drawn at a distance. It is still more like the reticulum of the discharge of a Leyden phial that is coated only in patches on one side. Each of these patches discharge what is immediately under it, and round it to a certain small distance, but leaves a part beyond this still charged. This redundant electricity gradually diffuses itself into the spaces just now discharged; and, after some considerable time has elapsed, another discharge may be made. In like manner, the electricity remaining in the interior of the stratum diffuses itself; comes within the action of the coating, and may be again discharged by a clap of thunder. We have a still better parallel to this in Beccaria's experiments with two or more plates of glass laid together. After the first discharge, the internal surfaces will exhibit certain electricity. Lay the plates together, and, after some time, the electricity of the inner surfaces will be different, and another discharge may be obtained.
Magnetism affords the best illustration of this. If a magnet be brought near a piece of soft iron, lying below a paper on which iron filings are lightly thrown, it will instantly induce a north pole on one end, and a south pole on the other; and this will be distinctly observed by the way in which these filings will arrange themselves. But if, instead of soft iron, we place a bar of hard tempered steel, the south pole will be, but a small small matter removed from the north pole; but by con- tinuing the magnet long in the same place, the distribu- tion of magnetism in the piece of hard steel will gra- dually advance along the bar, and after a long time the neutral point will be almost in the middle of the bar, and the south pole will be at the farther end. See Magnetism, in this Suppl.
We said that the clouds were the usual scenes of the violent electric phenomena. We imagine that the great- est part of the thunder strokes which have been felt have been of the kind which Lord Mahon, now Lord Stanhope, calls the returning stroke. If two clouds A and B are incumbent over the plain a and b; and if A be positive and B negative, the earth will be main- tained in a negative state at a, and a positive state at b. If the discharge be now made between the clouds A and B, the electricity must instantly rush up through a conductor at a, and down through one at b, and each place will have a stroke. The same thing will happen if the negative cloud B is above the positive cloud A, but not in so great a degree; for the negative elec- tricity at a will now be much less than in the other case, because it is induced only by the prevalence of the po- sitive cloud A over the more remote negative cloud B.
This returning stroke explains, much better than we can by any direct stroke, the capricious effects of thun- der. A person at Vienna received a terrible shock by having his hand on a thunder-rod during a violent ex- plosion which he saw above three miles distant. Sparks are observed at thunder rods at every the most distant flash of lightning.
Beccaria has a different theory of thunder. He im- agines that the different parts of the earth are in diffe- rent states of electricity, and that the clouds are the restoring conductors. But this does not accord with what we know of electricity. The earth is too good a conductor, that Dr Watson could not observe any time lost in communicating the electricity to the distance of more than four miles. It is very true, that the earth is almost always in a state of very unequal, and even opposite, electricity in its different parts; but this arises from the variety of clouds strongly electrified in the opposite way. This induces electricity, or disturbs the natural uniform diffusion of electricity, just as the bringing magnets or lodestones into the neighbourhood of a piece of iron, without touching it, renders it mag- netical in its different parts. While they continue in their places, the piece of iron will be magnetical, and differently so in its different parts.
Such are the thoughts which occur to us on this subject. But we by no means affirm that we have given a full account of the procedure of Nature; we have only pointed out several necessary consequences of the known laws of electricity, and of its production in the atmosphere by means of natural operations which are continually going on. These must operate, and pro- duce an electrical state of the atmosphere greatly resem- bling what we observe; and we have shown, from the acknowledged doctrines of electricity, how this want of equilibrium may be removed, and must be removed, by the same operations of Nature. The equilibrium must be restored by means of the conducting coating furnish- ed by the clouds. But there may be the least consider- able of Nature's resources; and the subject is still an unexplored field, in the examination of which we may hope to make great progress, in consequence of our daily increasing knowledge of the chemical state of the atmosphere.
Knowledge is valuable chiefly as it is useful. No Dr Frank- lin ever saw the propriety of this apothegm more strongly than Dr Franklin, or more assiduously adhered to it in the course of a long and studious life. How- ever greatly we may admire his sagacity, penetration, thunder, and logical discrimination, in the discoveries he has made in the science of electricity, and his discovery of the identity of electricity and thunder, we must acknow- ledge infinitely greater obligations to him for putting it in our power to ward off the fatal, and formerly inevi- table stroke, of this awful agent in the hands of Na- ture.
Dr Franklin considers the earth as performing the office of a conductor in restoring the electric equilib- rium of the atmosphere, which has been disturbed by the incessant action of the unrestrained powers of Na- ture.
He observes that the usual preference will be given to the best conductors. In this respect, a metal rod far surpasses the brick, stone, timber, and other materials which compose our buildings, especially when they are dry, as is usually the case in the thundery season. He therefore advises us to place metallic conductors in the way of the atmospheric electricity, in those places where it is most likely to strike, and to continue them down to the moist earth, at some depth under the sur- face. Nay, as it has been found that thunder has not in every instance struck the highest parts of buildings, he advises to raise the metallic conductors to some con- siderable height above the building, the more certain- ly to invite the electricity to take this course.
To ensure success, he observes that the electrical dis- charges shock dissipate water, and even metallic conductors, for con- ductors too small. He therefore advises to make the striking con- ductor at least half an inch square, none of that size having ever been destroyed, though smaller have, by the thunder; yet even these had conducted the thun- der to the ground with perfect safety to the building.
No part of a conductor must terminate in the build- ing; for the electricity accumulates exceedingly at the remote extremities of all long rods, and tends to fly off with great force, especially if another conductor is near. This aids the accumulation, by acquiring at its upper end an electricity opposite to that of the lower end of the other; and this effect, produced by the influence of a positive cloud, makes the upper and negative end of the lower portion of a divided conductor draw more electricity to the lower end of the upper portion. This redundant electricity, strongly attracted by the nega- tive lower portion, flies off with great violence through the air; or if surrounded with any matter capable of conversion into elastic vapour by heat, bursts it with ir- refrangible force. Thus the thunder, acting on the vane spindle of St Bride's steeple in London, sprung from its lower end to the upper end of an iron window bar, and burst the stone in which it was fixed, by expanding the moisture into steam. In like manner it burst the stone at the lower end of this bar, to make its way to an iron cramp which connected the opposite sides of the steeple; from this it struck to another cramp; and so from Thunder cramp to cramp, till it reached the gutter-leaks of the church, bursting and throwing off the stonework in many places.
All interruptions must therefore be carefully avoided, and the whole must be made as much as possible one continued metal rod.
Farther, Dr Franklin observing the singular property which sharp points possess of drawing off the electricity in silence, advises us to finish our conductor with a fine point of gilt copper, which cannot be blunted by rust.
But as thus raising the conductor, and pointing it, are so many invitations to the thunder to take this course; and as we cannot be certain that the quantity thus invited may not be more than what the rod can conduct with safety—it has appeared to Dr Wilson, and other able electricians, that it will be safer to give abundance of conduct to what may unavoidably visit us, without inviting what might otherwise have gone harmlessly by.
This was attentively considered by Dr Franklin, Dr Watson, Mr Canton, Dr Wilson, and others, met as a committee of the Royal Society, at the desire of the Board of Ordnance, to contrive a conductor for the powder magazine at Purfleet.
We think that the theory of induced electricity, founded on Dr Franklin's discoveries, and confirmed by all the later inventions of the electrophorus, condenser, &c., will decide this question in the most satisfactory manner.
When a cloud positively electrified comes over a building, it renders it negatively electrical in all its parts, if of conducting materials, and even the ground on which it stands. This effect is more remarkably produced if the structure is of a tall and slender shape, like a steeple or a rod. Therefore the external electrical fluid is attracted by the building with greater force than if it had consisted of materials less conductive. A discharge will therefore be made through it in preference to any neighbouring building, because it is more eminently negative. For the same reason, if there are two buildings equal and similar, one of them being a good conductor, and the other being a less perfect one, the perfect conductor, becoming more powerfully negative, the cloud will become more strongly positive over this house than over the other, and the stroke will be made through it.
The same thing must obtain in a perfect conductor continued from the top to the foundation of a house, built of worse conducting materials. The conductor becoming more eminently negative than any other part of the building, the electric fluid will be more strongly attracted by it, accumulated in its neighbourhood, and will all be discharged through it, so long as it is able to conduct.
If the building is of great extent, the proximity of one part of the building to the thunder cloud may produce an accumulation of electrical fluid in its neighbourhood, in preference to a more perfect, but remote, conductor. But when the distances from the cloud are not very unequal, the accumulation will always be in the neighbourhood of the perfect conductor; and this will determine the discharge that way. The accumulation in the neighbourhood of the rod will be small indeed, when the rod is small; but then it is dense, and the whole of electric phenomena show that it is the density, and not the quantity, of accumulation which produces the violent tendency to fly off: it is this alone which makes it impossible to confine electricity in a body which terminates in a sharp point.
For the same reason, bodies of the same materials and shape will increase the accumulation in the adjoining part of the cloud in proportion as they are nearer to it, or more advanced beyond the rest of the building.
And bodies of slender shape, and pointed, will produce this accumulation in their neighbourhood in a still more remarkable degree, and determine the course of the discharge with still greater certainty.
But it is evident that a metallic rod, no higher than the rest of the building, may occasion an accumulation in the adjoining part of a near thunder cloud sufficient to produce a discharge, when the building itself, consisting of imperfect conductors, would not have provoked the discharge at all. It may therefore be doubted whether we have derived any advantage from the conductor.
To judge properly of this, we must consider houses as they really are, consisting of different materials, in very different shapes and situations; and particularly as having many large pieces of metal in their construction, in various positions with regard to the cloud, the ground, and to each other. Suppose all the rest of the building to be of non-conducting materials. When a positive thunder cloud comes overhead, every piece of metal in the building becomes electrical, without having received anything as yet from the cloud; that end of each which is nearest the cloud becoming negative, and the remote end positive. But, moreover, the electricity of one increases the electricity of its neighbour. Then the most elevated becomes more strongly attractive at its upper end than it would have been had the others been away; and therefore produces a greater accumulation in the nearer part of the thunder cloud than it would otherwise have done, and it will receive a spark. By this its lower end becomes more overcharged, and this makes the upper end of the next more undercharged, and the spark is communicated to it, and so on to the ground; which would not have happened without this succession of conductors. Thus it is easy to conceive, that the accumulation in the cloud is just insufficient to produce a discharge—While things are in this state, just ready to snap, should a man chance to pass under a bell wire, or under a lutrine hanging by a chain, his body will immediately augment the positive electricity of the lower end of the conductor above him, and thus will augment the negative electricity of its upper end. This again will produce the same effect in the conductor above it: and thus each conductor becomes more overcharged at its lower end, and more undercharged at the upper end. Before this, every thing was just ready to snap. All will now strike at once. The cloud will be discharged through the house, and the man will be the sacrifice, the whole discharge being made through his body. This needs no demonstration for any well-informed electrician. Those who have only such a knowledge of the theory as can be gathered from the writings of Priestley, Cavallo, and other popular authors, may convince themselves of the truth of what is here delivered in the following manner.
In dry weather, and the most favourable circumstances for good electrical experiments, let a very large globe, globe, smoothly covered with metal, and well insulated, be as highly electrified as possible, without exposing it to a rapid dissipation. To ensure this circumstance (which is important) let it be electrified till it begins to sputter, and note the state of the electrometer. Discharge this electricity, and electrify it to about half of this intensity. Provide three or four insulated metal conductors, about three inches long and an inch diameter, terminated by hemispheres, and all well polished.
Having electrified the globe, as above directed, bring one of the insulated conductors slowly up to it, and note its distance when it receives a spark. In doing this, take care that there be no conducting body near the remote end of the insulated conductor. It will be best to push it gradually forward by means of a long glass rod. Withdraw the conductor, discharge its electricity, restore the globe to its former electricity, indicated by an electrometer, and repeat this experiment till the greatest striking distance is exactly discovered. Now set another of the insulated conductors about half an inch behind the first, and push them forward together, by a glass rod, till a spark is obtained. The striking distance will be found greater than before. Then repeat this last experiment, with this difference, that the two conductors are pushed forward by taking hold of the remote one. The striking distance will be found much greater than before. Lastly, push forward the two conductors, the remote one having a wire communicating with the ground, till they are a small matter without the striking distance; and, leaving them in this situation, take any little conducting body, such as a brass ball fixed on the end of a glass rod, and pass it briskly through between the globe and the nearest conductor, or through between the two conductors, taking care that it touch neither of them in the passage. It will be seen that, however swift the passage is made, there will be a discharge through all the four bodies. The inference from this is obvious and demonstrative.
A very remarkable instance of this fact was seen at the chapel in Tottenham Court Road, London. A man, going into the chapel by the east door, was killed by the thunder, which came down from the little bell-house, along the bell-wire, and the rod of the clock pendulum, from the end of which it leaped to some iron work above the door, and from thence, from nail to nail, till it reached the man's head.
This interruption of conduct, which is almost unavoidable in the construction of any building, is the cause of most of the accidents that are recorded; for when the ends of those communicating conductors are included in materials of less conducting power, the electricity, in making its way to the next in a very dense state, never fails to explode every thing which can be converted into elastic vapour by heat. There is always a sufficient quantity of moisture in the stone or brickwork for this purpose; and most vegetable substances contain moisture or other expandible matter. The stone, brick, or timber, is burnt; and thrown to a considerable distance; or if kept together by a weight of wall, the wall is shattered. It is worth remarking that although no force whatever seems able to prevent this explosion, the quantity of matter exploded is extremely small; for the stones are never thrown to a greater distance than they would have been by two or three grains of gunpowder properly confined.
All these accidents will be prevented by giving a sufficient uninterrupted conduct; and it is proper to make use of such a conductor, although it may invite many discharges which would not otherwise happen. So long as the conductor is sufficient for the purpose, there seems to be no doubt of the propriety of this maxim.
But the most serious objection remains. As we are thunder certain that these conductors, whether raised above the rod will building or not, will produce discharges through them even when which otherwise would not have happened, and as we are quite uncertain whether the quantity contained in a able to discharge the thunder cloud may not greatly exceed what the thunder rod can conduct without being dissipated in smoke, thunder it seems very dangerous thus to invite a stroke which our conductor may not be able to discharge. In particular, it is reasonable to believe that the strata of electrified clouds which come near the earth lose much of their electricity by passing over the sharp points of trees, &c., while those which are much higher may retain their electricity undiminished, and pass on. May it not therefore happen, that our conductor will invite a fatal stroke, which would have gone harmlessly by?
The doubt is natural, and it is important.
Let us suppose a very extensive and highly electrified cloud, in a positive state, to come within such a distance from a building as just not to strike it, if unprovided with a conductor, but which will most certainly strike the same building furnished with a conductor; and let the electricity be so great that the conductor shall be dissipated in smoke before even a small part of it is discharged.—What will be the fate of the building? We believe that it will be perfectly safe.
However rapid we may suppose that motion by which electricity is communicated, it is still motion, and time elapses during the propagation. The cloud is discharged, not in a very instant, but in a very short time. Part of the cloud is therefore discharged, while it explodes the conductor, and the electricity of the remainder is now too weak (by our supposition) to strike the building no longer furnished with a conductor. This must be the case, however large and powerful the cloud may be, and however small the conductor.
But suppose that the cloud has come so near as to strike the building unprovided with a conductor. Then as much will be discharged through the building as it can conduct; and if the quantity be too great, the building will be destroyed; but let a conductor (though insufficient) be added. The discharge will be made through it as long as it lasts, and the remainder only will be discharged through the house, surely with much less danger than before.
The truth of these conclusions from theory is fully verified by fact. When the church of Newbury in New England was struck by lightning in 1755, a bell wire, no bigger than a knitting needle, conducted the thunder with perfect safety to the building as far down the steeple as the wire reached, though the stroke was so great that the wire had been exploded, and no part of it remained, but only a mark along the wall occasioned by its smoke. From the termination of the wire to the ground the steeple was exceedingly shattered, and stones of great weight were thrown out from the foundation (where they were probably moister) to the distance of 20 and 30 feet. Another remarkable instance happened in the summer palace at St Petersburg. A Heyduk and a soldier of a foot regiment were standing sentinels at the door of the jewel-chamber; the Heyduk, with his scimitar resting on his arm, was carelessly leaning on the soldier, who had his musket shouldered. Both were struck down by lightning; and the soldier was killed, his left leg scorched, and his shoes burst. The Heyduk had received no damage, but felt himself tripped up, as if a great dog had run against him. A narrow strip of gold lace, which was sewed along the seam of his jacket and pantaloons breeches, reaching to his shoes, had been exploded on the left side. This seems to have been his protection. In all probability, the stroke came to both along the musket (or perhaps to the Heyduk along the scimitar). The Heyduk had a complete, though insufficient, conductor, and was safe. The soldier had not, and was killed. The push felt by the former probably arose from the explosion of the lace.
It seems therefore plain that metallic conductors are always a protection; that advancing them above the building increases their protection; and that pointing them may sometimes enable them to diminish a stroke, by discharging part of the electricity silently.
Dr Franklin having formed all his notions of thunder from his pre-established theory, and having seen the principal phenomena so conformable to it, was naturally led to expect this conformity in cases which he could not easily examine precisely by experiment. Accordingly, in his first dissertation, he affirmed that a fine point always discharges a thunder cloud silently, and at a great distance. The analogous experiments in artificial electricity are so beautiful and so perspicuous, that this confidence in the protecting power of fine points is not surprising; and this confidence was rendered almost complete by a most singular case which fell under his own observation. He was awakened one night by loud cracks in his hair case, as if some person had been lashing the waistcoating with a great horse-whip. He thought it so, and got up in anger to chide the idle fool. On looking out at his chamber door, he saw that the disturbance proceeded from electric explosions at some interruptions of his conductor. He saw the electricity pass, sometimes in bright sparks, producing those loud twacks, and sometimes in a long continued stream of dense white dazzling light as big as his finger, illuminating the hair-case like sunshine, and making a loud noise like a cutter's wheel. Had the cloud (says he) retained all this till it came within striking distance, the consequences would have been inconceivably dreadful. Yet not long after this he found that he had been in a mistake; for the house of Mr Watt in Philadelphia, furnished with a finely pointed conductor, was struck by a terrible clap of thunder, and the point of the conductor was melted down about two inches. This is perhaps the only instance on record of a finely-pointed conductor being struck. The board-room at the powder magazine at Purfleet was indeed struck, though provided with a conductor; but the stroke was through another part of the building. St Peter's church, Cornhill, has been eight times struck between 1772 and 1787; while St Michael's, in its neighbourhood, and much higher, has never had a stroke since 1772, when it was furnished with an excellent pointed conductor by Mr Nairne.
Dr Franklin having seen the above exception to his Thunder rule, and reflected on it, acknowledges that there are cases where a pointed conductor may be struck, viz. when it serves as a stepping stone, to complete a canal of conveyance already near completed. A small cloud may sometimes serve as a stepping stone (like the man coming under a lustre) for the electricity to come out of a great cloud, and discharge through the pointed conductor. Whenever it comes to the striking distance from the conductor, it will explode at once; whereas the great cloud itself must have come nearer, and had its force gradually diminished. It is remarkable that a point employed in this way in artificial electricity, must be brought nearer to another body than a ball need be, before it can receive a stroke. The difference is about one third of the whole. Nairne found, that a ball one nine-tenths inches in diameter, exploded at the distance of nine inches, and a point at six inches distance.
We must also observe that a pointed conductor can have no advantage over a blunt one in the case of a returning stroke; which is perhaps the most common of any. This depends on another discharge, which is made perhaps at a great distance. This was most distinctly the case in the instance mentioned some time ago, of the person at Vienna who had a shock from a thunder rod by an explosion far distant. This thunder rod was a very fine one, furnished with five gilt points.
Still, however, this property of sharp points was greatly overrated by Dr Franklin, and those who took all his theories of electricity from the simple discoveries of his sagacious mind. Unfortunately Dr Franklin had not cultivated mathematical knowledge; and, ever eager after discovery, and ardent in all his pursuits, his wonderful penetration carried him through, and seldom allowed him to rest long on false conclusions. He was certainly one of the greatest philosophers; and a little erudition would perhaps have brought him side by side with Newton. It was reserved, however, for Lord C. Cavendish and for Æpinus, to subject the investigations of Franklin to number and measure. By studying what they have written on the subject, or even the view which we have given of their theory in the article Electricity (Suppl.), the reader will be fully convinced, that a point has little or no advantage over a ball, with respect to a thunder cloud which is brought to the thunder rod by a brisk wind; although, when it comes slowly up during an almost perfect calm, it may discharge all that can be discharged without a snap. The complication in a point is indeed very great, but the quantity complicated is moderate; and therefore its action, at any considerable distance, is but trifling. All this is fully verified by Dr Wilson's judicious experiments in the Pantheon. He had a prodigious quantity of electrified surface suspended there, and made a pointed apparatus come to its striking distance with a motion which he could regulate and measure. And he found that with the very moderate velocity of twelve feet in a second, he never failed of procuring a very smart stroke. The experiments made in the usual way by the partisans of sharp points (for it became a matter of indecent party) were numberless, and decidedly in their favour. The great and just authority of Dr Franklin, who was one of the committee, procured them still more consideration, or at least hindered people from seeing the force of Dr Wilson's reasoning. It is somewhat surprising, prizing, that Dr Wilson, a lover of mathematical learning, and a good judge, as appears from his publication of the papers of Mr Robins, did not himself see the full force of his own experiments. He had not surely studied either Lepinus or Cavendish. He indeed frequently says, that the state of the electricity in a thunder cloud, and in coated glass, is exceedingly different; and that the first extends its sensible influence much farther than the last, when both have the same quantity of electricity.
But he seems not to have formed to himself any adequate notion of the difference. Had he done this, he would have seen that he has disposed his great electrified surface very improperly. It should have been collected much nearer his pointed apparatus, that this might, if possible, have been within the sphere of attraction of every part of his artificial cloud. He would then have found results, some of which would have been much more favourable to his own general opinion, while others would have exhibited the peculiarities of the sharp point in a more showy manner than anything we have seen.
Reasoning from the true theory of coated glass, we shall learn that, when the glass is exceedingly thin, the accumulation of electricity, or the charge, will be exceedingly great; while the external appearance, or apparent energy, of the electricity may be hardly sensible, and will extend to a very small distance. Thus, a circular plate of coated glass, six inches in diameter and one-twentieth thick, when electrified so as to make an electrometer diverge 50 degrees, contains about 60 times as much electricity as a brass plate, of the same diameter, electrified to the same degree; and these two will have the same influence on an electrometer placed at a distance from them, and will give a spark nearly at the same distance. The spark from the coated glass will be bright, and will give a shock; while that from the brass plate will be trifling. The cause of the equality of influence is, that the positive electricity of the one side of the coated glass is almost balanced by the negative electricity of the other side, and the unbalanced part is about \( \frac{1}{12} \)th of the whole. If we now take a brass plate of 46 inches in diameter, and electrify it to the same degree with the coated glass, we shall find that it will require the same number of turns of the machine to bring it to this state, or to charge the coated glass. They contain the same quantity of electricity, and the spark of both will give the same shock. But this large plate will have a much wider influence; a person coming within ten feet of it will feel his hair bend towards it, and feel like a cobweb on his face.
It may be farther demonstrated that the power of influence of a point to attract the electricity to a given degree from the large plate, is vastly smaller than its power to attract it to the same degree from the coated plate. This
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(A) In the terrible thunder stroke on Leven House in Scotland, the two great streams of electricity had taken the course of the vents which had been most in use, but not to get at the iron work, for it had branched off from the vents, at a great distance from the bottom. The chief conductors through the building had been various gilded mouldings, gilded leather hangings, gilded screens, picture frames, and the foil of mirrors. In this progress the steps have been so many, and so capricious, that no line of progress can be traced, according to any principle. The thunder seems to have electrified at once the whole of the leaden roof, and, besides the two main tracks along the vents, to have afterwards darted at every metal thing in its way. The lowest point of the track was a leaden water cistern; which, however, received no damage; but a thick stone wall was burst through to get at it. THUS, in sea language, a word used by the pilot in directing the helmsman or steersman to keep the ship in her present situation when failing with a scant wind, so that she may not approach too near the direction of the wind, which would shiver her sails, nor fall to leeward, and run farther out of her course.