a bright and vivid flash of fire, suddenly appearing in the atmosphere, and commonly disappearing in an instant, sometimes attended with clouds and thunder, and sometimes not.

The phenomena of lightning are always surprising, and sometimes very terrible; neither is there any kind of natural appearance in which there is more diversity, not two flashes being ever observed exactly similar to one another. In a serene sky, the lightning, in this country at least, almost always hath a kind of indistinct appearance without any determinate form, like the sudden illumination of the atmosphere occasioned by firing a quantity of loose gunpowder; but when accompanied with thunder, it is well defined, and hath very often a zig-zag form. Sometimes it makes only one angle, like the letter V; sometimes it hath several branches, and sometimes it appears like the arch of a circle. But the most formidable and destructive form which lightning is ever known to assume is that of balls of fire. The motion of these is very often easily perceptible to the eye; but wherever they fall, much mischief is occasioned by their bursting, which they always do with a sudden explosion, like that of fire-arms. Sometimes they will quietly run along, or rest for a little upon anything, and then break into several pieces, each of which will explode; or the whole ball will burst at once, and produce its mischievous effects only in one place. The next to this in its destructive effects is the zig-zag kind; for that which appears like indistinct flashes, whose form cannot be readily observed, is seldom or never known to do hurt.—The colour of the lightning also indicates in some measure its power to do mischief; the palest and brightest flashes being most destructive; such as are red or of a darker colour, commonly doing less damage.

A very surprising property of lightning, the zig-zag kind especially, when near, is its seeming omnipresence. If two persons are standing in a room looking different ways, and a loud clap of thunder accompanied with zig-zag lightning happens, they will both distinctly see the flash, not only by that indistinct illumination of the atmosphere which is occasioned by fire of any kind; but the very form of the lightning itself, and every angle it makes in its course, will be as distinctly perceptible, as though they had looked directly at the cloud from whence it proceeded. If a person happened at that time to be looking on a book, or other object which he held in his hand, he would distinctly see the form of the lightning between him and the object at which he looked. This property seems peculiar to lightning, and to belong to no other kind of fire whatever.

The effects of lightning are generally confined within a small space; and are seldom similar to those which accompany explosions of gun-powder, or of inflammable air in mines. Instances of this kind, however, have occurred; the following is one of the most remarkable of which we have any distinct account.

"August 2d 1763, about six in the evening, there arose at Anderlecht, about a league from Brussels, a conflict of several winds borne upon a thick fog. This conflict lasted four or five minutes, and was attended with a frightful hissing noise, which could be compared to nothing but the yellings of an infinite number of wild beasts. The cloud then opening, discovered a kind of very bright lightning, and in an instant the roofs of one side of the houses were carried off and dispersed at a distance; above 1000 large trees were broke off, some near the ground, others near the top, some torn up by the roots; and many both of the branches and tops carried to the distance of 60, 100, or 120 paces; whole coppices were laid on one side, as corn is by ordinary winds. The glass of the windows which was most exposed was shivered to pieces. A tent in a gentleman's garden was carried to the distance of 4000 paces; and a branch torn from a large tree, struck a girl in the forehead as she was coming into town, at the distance of 40 paces from the trunk of the tree, and killed her on the spot."

These terrible effects seem to have been owing to the prodigious agitation in the air, occasioned by the emission of such a vast quantity of lightning at once; or perhaps to the agitation of the electric fluid itself, which is still more dangerous than any concussion of the atmosphere; for thunder-storms will sometimes produce most violent whirlwinds, such as are by the best philosophers attributed to electricity, nay, even occasion an agitation of the waters of the ocean itself, and all this too—after the thunder and lightning have ceased.—Of this we have the following instances.

"Great Malvern, October 16th 1761. On Wednesday last, we had the most violent thunder ever known in the memory of man. At a quarter past four in the afternoon, I was surprised with a most shocking and dismal noise; 100 forges (the nearest resemblance I can think of,) were they all at work at once, could scarce equal it. I ran to the fore-door, and calling my eye upon the side of the hill about 400 yards to the south-west of my house, there appeared a prodigious smoke, attended with the same violent noise. I ran back into the house, and cried out, a volcano (for so I thought) had burst out of the hill; but I had no sooner got back again, than I found it had descended, and was passing on within about 100 yards of the south end of my house. It seemed to rise again in the meadow just below it; and continued its progress to the east, rising in the same manner for four different times, attended with the same dismal noise as at first; the air being filled with a nauseous and sulphurous smell. I saw it gradually decrease till quite extinguished in a turnip-field about a quarter of a mile below my house. The turnip-leaves, with leaves of trees, dirt, sticks, &c. filled the air, and flew higher than any of these hills. The thunder ceased before this happened, and the air soon afterwards became calm and serene."—The vast column of smoke mentioned in the above letter was so large, that a physician of eminence at Worcester saw it in its progress down the hill, about a mile from Fenchurch, which is above 20 miles from Malvern.—In August 1763, a most violent storm of thunder, rain, and hail, happened at London, which did damage in the adjacent country, to the amount of 50,000 l. Hailstones fell of an immense size, from two to ten inches circumference; but the most surprising circumstance was the sudden flux and reflux of the tide in Plymouth pool, exactly corresponding with the like agi- agitation in the same place, at the time of the great earthquake at Lisbon.

Instances are also to be found, where lightning, by its own proper force, without any assistance from those less common agitations of the atmosphere or electric fluids, hath thrown stones of immense weight to considerable distances; torn up trees by the roots, and broke them in pieces; shattered rocks; beat down houses, and set them on fire, &c. All these, however, are to be considered as the more unusual phenomena of lightning; its common mode of action being entirely similar to that of a charged Leyden vial, where the electric matter discharges itself from a substance positively electrified to one that is negatively so. The identity of electric matter and lightning seems now, indeed, so well established, that there is not the least foundation for seeking any other solution of the phenomena of lightning, than what may be obtained by comparing them with those of our electrical experiments. The different forms of the flashes are all exemplified in those of electrical sparks. Where the quantity of electricity is small, and consequently incapable of striking at any considerable distance, the spark appears straight, without any curvature or angular appearance; but where the electricity is very strong, and of consequence capable of striking an object at a pretty considerable distance, it assumes a crooked or zig-zag form. This is always the case with Mr Nairn's very powerful machines, the conductors of which are six feet in length and one foot in diameter. Sparks may be taken from them at the distance of 16, 17, or even 20 inches; and all of these put on the angular zig-zag form of lightning. The reason of this appearance, both in these sparks and in lightning, is, that the more fluid electric matter has a tendency to pass through the denser and less fluid atmosphere with great rapidity; and in fact, this is the way in which all the more fluid substances pass through those that are less so, at least when their velocity becomes considerable.—If bubbles of air or steam pass very gently up through water, their course from the bottom to the top of the vessel will differ very little, if at all, from a straight line; but when they are impelled by a considerable force, as in air blown from a bellows, or the bubbles of steam which arise in boiling water, their course is then marked by waved and crooked lines, and the deflection of the bubbles to the right or left, will be precisely in proportion to their ascending velocity, and to the weight of the water by which they are resisted.

In the case of air blown through water, however, or steam ascending from the bottom of a vessel of boiling water, though the course of the bubbles is waved and crooked, we never observe it to be angular as in lightning. The reason of this is, that there is no proportion between the capacity of the air for yielding to the impetus of lightning, and the velocity with which the latter is moved. From Mr Robins's experiments in gunnery, it appears, that the air cannot yield with a velocity much greater than 1200 feet in a second, and that all projectiles moving with a greater degree of velocity meet with a violent resistance. But if we suppose lightning to move only with one half the velocity of light, that is, near 100,000 miles in a second, its motion in the fluid atmosphere will meet with a resistance very little inferior to what air would meet with in passing through the most solid bodies. The smallest difference of the resistance of the atmosphere on either side, must determine the lightning to that side; and in its passage to that new place where the resistance is least, it must pass on in a straight line, making an angle with its former course, because the atmosphere is altogether incapable of yielding with such rapidity as the electric matter requires, and therefore resists like a solid rock. The case is otherwise in the former examples: for though a small difference in the resistance forces the bubbles of air or steam to deviate from side to side, yet there is always a considerable proportion between the capacity of water for yielding, and that force by which the bubbles urge it to yield; so that though it does make a resistance sufficient to prevent the bubbles from moving in a straight line, yet it also perceptibly yields at all times, and therefore the track of the bubbles is formed by a number of curves and not angles.

Hence we may understand the reason why the zig-zag kind of lightning is so dangerous, namely, because it must overcome a very violent resistance of the atmosphere; and wherever that resistance is in the smallest degree lessened, there it will undoubtedly strike, and at a very considerable distance too. It is otherwise with that kind which appears in flashes of no determinate form. The electric matter of which these are composed, is evidently dissipated in the air by some conducting substances which are present there; and of consequence, though a man, or other conducting body, happened to be very near such a flash, he would not be struck or materially injured by it, tho' a zig-zag flash, in such circumstances, would have probably discharged its whole force upon him.

The most destructive kind of lightning, however, as we have already observed, is that which assumes the form of balls. These are produced by an exceeding form of great power of electricity gradually accumulated till balls, the resistance of the atmosphere is no longer able to confine it. In general, the lightning breaks out from the electrified cloud by means of the approach of some conducting substance; either a cloud, or some terrestrial substance: but the fire-balls seem to be formed, not because there is any substance at hand to attract the electric matter from the cloud, but because the electricity is accumulated in such quantity that the cloud itself can no longer contain it. Hence such balls fly off slowly, and have no particular destination. Their appearance indicates a prodigious commotion and accumulation of electricity in the atmosphere, without a proportionable disposition in the earth to receive it. This disposition, however, we know is perpetually altered by a thousand circumstances, and the place which first becomes most capable of admitting electricity will certainly receive a fire-ball. Hence this kind of lightning has been known to move slowly backwards and forwards in the air for a considerable space of time, and then suddenly to fall on one or more houses, according to their being more or less affected with an electricity opposite to that of the ball at the time. It will also run along the ground, break into several parts, and produce several explosions at the same time.

It is very difficult to imitate lightning of this kind in our electrical experiments. The only cases in which Lightning: it hath been done in any degree are those in which Dr Priestley made the explosion of a battery pass for a considerable way over the surface of raw flesh, water, &c. In these cases, if, while the electric flash passed over the surface of the flesh, it had been possible to interrupt the metallic circuit by taking away the chain, the electric matter discharged from the battery would have been precisely in the situation of one of the fireballs above-mentioned; i.e., it would have been at a loss for a conductor. The negative side of the battery was the place of its destination; but to that it would not have easily got, because of the great quantity of atmosphere which lay in its way, and the incapacity of the neighbouring bodies to receive it. But if, while the electric matter was thus stationary for want of a conductor, if any person standing near, or touching the negative side of the battery, presented a finger to that seemingly inoffensive luminous body, he would instantly be struck very violently; because a free communication being now made by means of his body, the powers by which the electric fluid is impelled from one place to another would instantly urge it upon him. But if we suppose a person, who hath no communication with the battery, to present his finger to the same body, he may perhaps receive a slight spark from it; but not a shock of any consequence, because there is not a perfect communication by means of his body with the place to which the electric fire is destined.

Hence we may account for the seemingly capricious nature of lightning of all kinds, but especially of that kind which appears in the form of balls. Sometimes it will strike trees, high houses, steeples, and towers, without touching cottages, men, or other animals, who are in the neighbourhood. In such cases, people would be apt to say that the neighbourhood of these higher objects prevented the others from the stroke; but with little reason, since low houses, men walking in the fields, cattle, nay, the surface of the earth itself, have all been struck, while high trees and steeples in the neighbourhood have not been touched. In like manner, fire-balls have passed very near certain persons without hurting them, while they have, as it were, gone considerably out of their way to kill others. The reason of all this is, that in thunder-storms there is constantly a certain zone of earth considerably under the surface, which the lightning desires (if we may use the expression) to strike, because it hath an electricity opposite to that of the lightning itself. Those objects, therefore, which form the most perfect conductors between the electrified clouds and that zone of earth, will be struck by the lightning, whether they are high or low; and because we know not the conducting quality of the different terrestrial substances, the superstitious are apt to ascribe strokes of lightning to the Divine vengeance against particular persons, whereas it is certain that this fluid, as well as others, acts according to invariable rules from which it is never known to depart.

the time of severe thunder-storms, is supposed to proceed from the earth, as well as from the clouds; but this fact hath never been well ascertained, and indeed from the nature of the thing it seems very difficult to be ascertained; for the motion of the electric fluid is so very quick, that it is altogether impossible to determine, by means of our senses, whether it goes from the earth or comes to it. In fact, there are in this country many thunder-storms in which it doth not appear that the lightning touches any part of the earth, and consequently can neither go to it nor come out from it. In these cases, it flashes either from an electrified cloud to one endowed with an opposite electricity, or merely into those parts of the atmosphere which are ready to receive it. But if not only the clouds, but the atmosphere all the way betwixt them and the earth, and likewise for a considerable space above the clouds, are electrified one way, the earth must then be struck. The reason of this will appear from a consideration of the principles laid down under the article Electricity, sect. vi. It there appears, that the electric fluid is altogether incapable either of accumulation or diminution in any particular part of space. What we call electricity is only the motion of this fluid made perceptible to our senses. Positive electricity is when the current of electric matter is directed from the electrified body. Negative electricity is when the current is directed towards it.

Let us now suppose, that a positively electrified cloud is formed over a certain part of the earth's surface. The electric matter flows out from it first into the atmosphere all round; and while it is doing so, the atmosphere is negatively electrified. In proportion, however, as the electric current pervades greater and greater portions of the atmospheric space, the greater is the resistance to its motion, till at last the air becomes positively electrified as well as the cloud; and then both act together as one body. The surface of the earth then begins to be affected, and it silently receives the electric matter by means of the trees, grass, &c., which grow upon its surface, till at last it becomes positively electrified also, and begins to send off a current of electricity from the surface downwards. The causes which at first produced the electricity of the clouds, (and which are treated of under the article Thunder,) still continuing to act, the power of the electric current becomes inconceivably great. The danger of the thunder-storm now begins; for as the force of the lightning is directed to some place below the surface of the earth, it will certainly dart towards that place, and shatter every thing to pieces which resists its passage. The benefit of conducting-rods will now be evident: for we are sure that the electric matter will in all cases take the way where it meets with the least resistance; and this is through the substance, or rather over the surface, of metals. In such a case, therefore, if there happen to be a house furnished with a conductor directly below the cloud, and at the same time a zone of negatively electrified earth not very far below the foundation of the house, the conductor will almost certainly be struck, but the building will be unhurt. If the house wants a conductor, the lightning will nevertheless strike in the same place, in order to get at the negatively electrified zone above-mentioned; but the building will now be damaged, because the materials of it cannot readily conduct the electric fluid.

We will now be able to enter into the dispute. Whether the preference is due to knobbled or pointed conductors for preserving buildings from strokes of lightning. Ever since the discovery of the identity of electricity... Electricity and lightning; it hath been allowed by all parties, that conductors of some kind are in a manner essentially necessary for the safety of buildings in those countries where thunder-storms are very frequent. The principle on which they act hath been already explained; namely, that the electric fluid, when impelled by any power, always goes to that place where it meets with the least resistance, as all other fluids also do. As metals, therefore, are found to give the least resistance to its passage, it will always choose to run along a metallic rod, in preference to a passage of any other kind. We must, however, carefully consider a circumstance which seems to have been too much overlooked by electricians in their reasonings concerning the effects of thunder-rods; namely, that lightning, or electricity, never strikes a body merely for the sake of the body itself, but only because by means of that body it can readily arrive at the place of its destination. When a quantity of electricity is collected from the earth, by means of an electric machine, a body communicating with the earth will receive a strong spark from the prime conductor. The body receives this spark, not because it is itself capable of containing all the electricity of the conductor and cylinder, but because the natural situation of the fluid being disturbed by the motion of the machine, a stream of it is sent off from the earth. The natural powers, therefore, make an effort to supply what is thus drained off from the earth; and as the individual quantity which comes out is most proper for supplying the deficiency, as not being employed in any natural purpose, there is always an effort made for returning it to the earth. No sooner, then, is a conducting body, communicating with the earth, presented to the electrical machine, than the whole effort of the electricity is directed against that body, not merely because it is a conductor, but because it leads to the place where the fluid is directed by the natural powers by which it is governed, and at which it would find other means to arrive, though that body were not to be presented. That this is the case, we may very easily satisfy ourselves, by presenting the very same conducting substance in an insulated state to the prime conductor of the machine; for then we shall find, that only a very small spark will be produced. In like manner, when lightning strikes a tree, a house, or a thunder-rod, it is not because these objects are high, or in the neighbourhood of the cloud; but because they communicate with some place below the surface of the ground, against which the impetus of the lightning is directed; and at that place the lightning would certainly arrive, though none of the above-mentioned objects had been interposed.

The fallacy of that kind of reasoning generally employed concerning the use of thunder-rods, will now be sufficiently apparent. Because a point presented to an electrified body in our experiments, always draws off the electricity in a silent manner; therefore Dr Franklin and his followers have concluded, that a pointed conductor will do the same thing to a thunder-cloud, and thus effectually prevent any kind of danger from a stroke of lightning. Their reasoning on this subject, they think, is confirmed by the following fact among many others. "Dr Franklin's house at Philadelphia was furnished with a rod extending nine feet above the top of the chimney. To this rod was connected a Lightning wire of the thickness of a goose-quill, which descended through the well of the stair-case; where an interruption was made, so that the ends of the wire, to each of which a little bell was fixed, were distant from each other about six inches; an insulated brass ball hanging between the two bells. The author was one night waked by loud cracks, proceeding from his apparatus in the stair-case. He perceived, that the brass ball, instead of vibrating as usual between the bells, was repelled and kept at a distance from both; while the fire sometimes passed in very large quick cracks directly from bell to bell; and sometimes in a continued dense white stream, seemingly as large as his finger; by means of which the whole stair-case was enlightened, as with sun-shine, so that he could see to pick up a pin.—From the apparent quantity of electric matter of which the cloud was thus evidently robbed, by means of the pointed rod (and of which a blunt conductor would not have deprived it), the author conceives, that a number of such conductors must considerably lessen the quantity of electric fluid contained in any approaching cloud, before it comes to near as to deliver its contents in a general stroke."

For this very reason, Mr Benjamin Wilson and his followers, who constitute the opposite party, have determined that the use of pointed conductors is utterly unsafe. They say, that in violent thunder-storms the whole atmosphere is full of electricity; and that attempts to exhaust the vast quantity there collected, are like attempting to clear away an inundation with a shovel, or to exhaust the atmosphere with a pair of bellows. They maintain, that though pointed bodies will effectually prevent the accumulation of electricity in any substance; yet if a non-electrified body is interposed between a point and the conductor of an electrical machine, the point will be struck at the same moment with the non-electrified body, and at a much greater distance than that at which a knob would be struck. They affirm also, that, by means of this silent solicitation of the lightning, inflammable bodies, such as gun-powder, tinder, and Kunckel's phosphorus, may be set on fire; and for these last facts they bring decisive experiments. From all this, say they, it is evident that the use of pointed conductors is unsafe. They solicit a discharge to the place where they are; and as they are unable to conduct the whole electricity in the atmosphere, it is impossible for us to know whether the discharge they solicit may not be too great for our conductor to bear; and consequently all the mischiefs arising from thunder-storms may be expected, with this additional and mortifying circumstance, that this very conductor hath probably solicited the fatal stroke, when without it the cloud might have passed harmlessly over our heads without striking at all.

Here the reasoning of both parties is equally wrong. They both proceed on this erroneous principle, that in thunder-storms the conductor will always solicit a discharge, or that at such times all the elevated objects on the surface of the earth are drawing off the electricity of the atmosphere; but this cannot be the case, unless the electricity of the earth and of the atmosphere is of a different kind. Now, it is demonstrable, that until this difference between the electricity of the atmosphere and of the surface of the earth ceases, there cannot Lightning cannot be a thunder-storm. When the atmosphere begins to be electrified either positively or negatively, the earth, by means of the inequalities and moisture of its surface, but especially by the vegetables which grow upon it, absorbs that electricity, and quickly becomes electrified in the same manner with the atmosphere. This absorption, however, ceases in a very short time, because it cannot be continued without setting in motion the whole of the electric matter contained in the earth itself. Alternate zones of positive and negative electricity will then begin to take place below the surface of the earth, for the reasons mentioned under the article Electricity, sect. vi. §9. Between the atmosphere and one of these zones, the stroke of the lightning always will be. Thus, supposing the atmosphere is positively electrified, the surface of the earth will, by means of trees, &c., quickly become positively electrified also; we shall suppose to the depth of 10 feet. The electricity cannot penetrate farther on account of the resistance of the electric matter in the bowels of the earth. At the depth of 10 feet from the surface, therefore, a zone of negatively-electrified earth begins, and to this zone the electricity of the atmosphere is attracted; but to this it cannot get, without breaking through the positively-electrified zone which lies uppermost, and shattering to pieces every bad conductor which comes in its way. We are very sure, therefore, that in whatever places the outer-zone of positively-electrified earth is thinner, there the lightning will strike whether a conductor happens to be present or not. If there is a conductor, either knobbed or sharp-pointed, the lightning will indeed infallibly strike it; but it would also have struck a house situated on that spot without any conductor; and though the house had not been there, it would have struck the surface of the ground itself.—Again, if we suppose the house with its conductor to stand on a part of the ground where the positively-electrified zone is very thick, the conductor will neither silently draw off the electricity, nor will the lightning strike it, though perhaps it may strike a much lower object, or even the surface of the ground itself, at no great distance; the reason of which undoubtedly is, that there the zone of positively-electrified earth is thinner, than where the conductor was.

We must also observe, that the Franklinians make their pointed conductors to be of too great consequence. To the houses on which they are fixed, no doubt, their importance is very great; but in exhausting a thunder-cloud of its electricity, their use must appear trifling; and to insist on it, ridiculous. Innumerable objects, as trees, grass, &c. are all conspiring to draw off the electricity, as well as the conductor, if it could be drawn off; but of effecting this there is an impossibility, because they have the same kind of electricity with the clouds themselves. The conductor hath not even the power of attracting the lightning a few feet out of the direction which it would choose of itself. Of this we have a most remarkable and decisive instance, in what happened to the magazine at Purfleet in Essex, on May 15, 1777. That house was furnished with a pointed conductor, raised above the highest part of the building; nevertheless, about six in the evening of the abovementioned day, a flash of lightning struck an iron cramp in the corner of the wall, considerably lower than the top of the conductor, and only 46 feet in a sloping line distant from the point.—This produced a long dispute with Mr Wilson concerning the propriety of using pointed conductors; and, by the favour of his majesty, he was enabled to construct a more magnificent electrical apparatus than any private person could be supposed to erect at his own expense, and of which some account is given under the article Electricity, n° 78. The only new experiments, however, which this apparatus produced, were, the firing of gunpowder by the electric aura, as it is called; and a particularly violent shock which a person received when he held a small pointed wire in his hand, upon which the conductor was discharged. We must observe, that the electrified surface of the conductor was 620 feet; and we can have but little idea of the strength of sparks from a conductor of this magnitude, supposing it properly electrified. Six turns of the wheel made the discharge felt through the whole body like the strong shock of a Leyden vial; and nobody chose to make the experiment when the conductor had received a higher charge. A very strong shock was felt, as already observed, when this conductor was discharged upon a pointed wire held in a person's hand, even though the wire communicated with the earth; which was not felt, or but very little, when a knopped wire was made use of.—To account for this difference may, perhaps, puzzle electricians; but with regard to the use of blunt or pointed thunder-rods, both experiments seem quite inconclusive. Though a very great quantity of electric matter silently drawn off will fire gunpowder, this only proves that a pointed conductor ought not to pass through a barrel of gunpowder; and if a person holding a pointed wire in his hand received a strong shock from Mr Wilson's great conductor, it can thence only be inferred, that in the time of thunder nobody ought to hold the conductor in their hands; both which precautions common-sense would dictate without any experiment. From the accident at Purfleet, however, the disputants on both sides ought to have seen, that, with regard to lightning, neither points nor knobs can attract. Mr Wilson surely had no reason to condemn the pointed conductor for soliciting the flash of lightning, seeing it did not strike the point of the conductor, but a blunt cramp of iron; neither have the Franklinians any reason to boast of its effect in silently drawing off the electric matter, since all its powers were neither able to prevent the flash, nor to turn it 46 feet out of its way. The matter of fact is, the lightning was determined to enter the earth at the place where the board-house stands, or near it. The conductor fixed on the house offered the easiest communication; but 46 feet of air intervening between the point of the conductor and the place of explosion, the resistance was less through the blunt cramp of iron, and a few bricks moistened with rain-water, to the side of the metallic conductor, than through the 46 feet of air to its point; for the former was the way in which the lightning actually passed.

Mr Wilson and his followers seem also mistaken in supposing that a pointed conductor can solicit a greater discharge than what would otherwise happen. Allowing the quantity of electricity in the atmosphere during the time of a thunder-storm to be as great In a late publication on the subject of electricity by Lord Mahon, we find a new kind of lightning made mention of, which he is of opinion may give a fatal stroke, even though the main explosion was at a considerable distance; a mile, for instance, or more. This he calls the electrical returning stroke; and exemplifies it in the following manner, from some experiments made with a very powerful electrical machine, the prime conductor of which (six feet long, by one foot diameter) would generally, when the weather was favourable, strike into a brass ball connected with the earth, to the distance of 18 inches, or more. In the following account this brass ball, which we shall call A, is supposed to be constantly placed at the striking distance; so that the prime conductor, the instant that it becomes fully charged, explodes into it.

Another large conductor, which we shall call the second conductor, is suspended, in a perfectly insulated state, farther from the prime conductor than the striking distance, but within its electrical atmosphere—at the distance of six feet, for instance. A person standing on an insulating stool touches this second conductor very lightly with a finger of his right hand; while, with a finger of his left hand, he communicates with the earth, by touching very lightly a second brass ball fixed at the top of a metallic stand, on the floor, and which we shall call B.

While the prime conductor is receiving its electricity, sparks pass (at least if the distance between the two conductors is not too great) from the second conductor to the insulated person's right hand; while similar and simultaneous sparks pass out from the finger of his left hand into the second metallic ball B, communicating with the earth. These sparks are part of the natural quantity of electric matter belonging to the second conductor, and to the insulated person; driven from them into the earth, through the ball B, and its stand, by the elastic pressure or action of the electrical atmosphere of the prime conductor. The second conductor and the insulated person are hereby reduced to a negative state.

At length, however, the prime conductor, having acquired its full charge, suddenly strikes into the ball A, of the first metallic stand, placed for that purpose at the striking distance of 17 or 18 inches. The explosion being made, and the prime conductor suddenly robbed of its electric atmosphere, its pressure or action on the second conductor, and on the insulated person, as suddenly ceases; and the latter instantly feels a smart returning stroke, though he has no direct or visible communication (except by the floor) either with the striking or struck body, and is placed at the distance of five or six feet from both of them. This returning stroke is evidently occasioned by the sudden re-entrance of the electric fire naturally belonging to his body and to the second conductor, which had before been expelled from them by the action of the charged prime conductor upon them; and which returns to its former place, lightning the instant that action or elastic pressure ceases. The author shows, that there can be no reason to suppose that the electrical discharge from the prime conductor should, in this experiment, divide itself at the instant of the explosion, and go different ways, so as to strike the second conductor and insulated person in this manner, and at such a distance from it.

When the second conductor and the insulated person are placed in the densest part of the electrical atmosphere of the prime conductor, or just beyond the striking distance, the effects are still more considerable; the returning stroke being extremely severe and pungent, and appearing considerably sharper than even the main stroke itself, received directly from the prime conductor. This circumstance the author alleges as an unanswerable proof that the effect which he calls the returning stroke, was not produced by the main stroke being any wise divided at the time of the explosion, since no effect can ever be greater than the cause by which it is immediately produced.—Having taken the returning stroke eight or ten times one morning, he felt a considerable degree of pain across his chest during the whole evening, and a disagreeable sensation in his arms and wrists all the next day.

We come now to the application of this experiment, and of the doctrine deduced from it, to what passes in natural electricity, or during a thunder-storm; in which there is reason to expect similar effects, but on a larger scale—a scale so large indeed, according to the author's representation, that persons and animals may be destroyed, and particular parts of buildings may be considerably damaged, by an electrical returning stroke, occasioned even by some very distant explosion from a thunder-cloud;—possibly at the distance of a mile or more.

It is certainly easy to conceive, that a charged extensive thunder-cloud must be productive of effects similar to those produced by the author's prime conductor. Like it, while it continues charged, it will, by the superinduced elastic electrical pressure of its atmosphere—to use the author's own expression—drive into the earth a part of the electric fluid naturally belonging to the bodies which are within the reach of its widely extended atmosphere; and which will therefore become negatively electrical. This portion too of their electric fire, as in the artificial experiment, will, on the explosion of the cloud, at a distance, and the cessation of its action upon them, suddenly return to them; so as to produce an equilibrium, and restore them to their natural state.

To this theory, the authors of the Monthly Review have given the following answer: "We cannot, however, agree with the ingenious author, with respect to viewers, the greatness of the effects, or of the danger to be apprehended from the returning stroke in this case; as we think his estimate is grounded on an erroneous foundation.—Since (says he) the density of the electrical atmosphere of a thunder-cloud is so immense, when compared to the electrical density of the electrical atmosphere of any prime conductor, charged by means of any electrical apparatus whatsoever; and since a returning stroke, when produced by the sudden removal of even the weak elastic electrical pressure of the electrical atmosphere of a charged prime conductor, may be extremely strong, as we Lightning: we have seen above; it is mathematically evident, that, when a returning stroke comes to be produced by the sudden removal of the very strong elastic electrical pressure of the electrical atmosphere of a thunder-cloud powerfully charged; the strength of such a returning stroke must be enormous.

"If indeed the quantity of electric fluid naturally contained in the body of a man, for instance, were immense, or indefinite, the author's estimate between the effects producible by a cloud, and those caused by a prime conductor, might be admitted. But surely an electrified cloud,—how great forever may be its extent, and the height of its charge, when compared with the extent and charge of a prime conductor—cannot expel from a man's body (or any other body) more than the natural quantity of electricity which it contains. On the sudden removal, therefore, of the pressure by which this natural quantity had been expelled, in consequence of the explosion of the cloud into the earth; no more (at the utmost) than his whole natural stock of electricity can re-enter his body (c). But we have no reason to suppose that this quantity is so great, as that its sudden re-entrance into his body should destroy or even injure him.

"In the experiment above described, the insulated person receives into his body, at the instant of the returning stroke, not only all that portion of his own natural electric-fire which had been expelled from it; but likewise transmits through it, at the same instant, in consequence of his peculiar situation, all the electric fire of which the large second conductor had been robbed; and which must necessarily re-pass through his body, to arrive at that conductor. To render the case somewhat parallel, in natural electricity, the man's body must be so peculiarly circumstanced, supposing him to be in a house, that the electric matter which has been expelled from the house into the earth, by the pressure of an extensive thunder-cloud, could not return back into the building, on the explosion of the cloud at a distance, without passing through his body: a case not likely to happen, unless the house were insulated (like the second conductor in the preceding experiment), and his body became the channel through which alone the house could have its electric matter restored to it: it appears much more probable that the electric matter returns to the house through the same channels by which it before infallibly passed out, and with equal silence, tho' more suddenly.

"In the case of a man who is abroad, and in an open field, during the time of an explosion;—as he is unconnected with other masses of matter above him, no more than the precise quantity of electric fire, which had been before expelled from his body, will suddenly return into it at the instant of a distant explosion: and that this quantity is not usually very large, may be inferred from many considerations.

(c) "We suppose the person not to be so situated, that the returning fire of other bodies must necessarily pass through his body.

(d) "The author does indeed produce a living evidence, in the case of a person at Vienna, who, he has been credibly informed, received an electrical shock, by having held one hand accidentally in contact with an interrupted metallic conducting rod, at the instant that a thunder-cloud exploded at the distance, as was conjectured, of above half an English mile. He likewise observes, that a 'very strong, bright, and sudden stroke' (or spark) of electrical fire has been seen, by several electricians, to pass in the interval, or interruption, purposely left in the conducting rod of a house at the instant of a distant explosion; and 'when it was fully proved, by the sharp point of the conductor not being melted, or even tinged,' that the conductor itself had not been struck.—These observations, however, do not by any means prove the magnitude, or dangers, of the returning stroke, but merely its existence; which we do not contest."

"When a person standing on the ground holds a lightning pair of Mr Canton's balls in his hand, while a highly charged thunder-cloud is suspended over his head; the angle made by the balls indicates the electrical state of that person, or the quantity of natural electricity of which his body is at that time deprived, by the action of the (positively) charged cloud hanging over him. But we have never seen the repulsion of the balls so considerable, as to furnish any just apprehensions that the return of his natural electric matter, however sudden, could be attended with injury to him: nor would he be sensible of any commotion on the balls suddenly coming together; tho' a spark might undoubtedly be perceived, at that instant, were he insulated, and placed in the same manner with the author when he tried the above-related experiment.

"The author nevertheless observes, that there have been instances of persons who have been killed by natural electricity, having been found with their shoes torn, and with their feet damaged by the electrical fire; but who have not had, over their whole body, any other apparent marks of having been struck by lightning." He adds, "if a man walking out of doors were to be killed by a returning stroke, the electrical fire would rush into that man's body thro' his feet, and his feet only; which would not be the case, were he to be killed by any main stroke of explosion, either positive or negative."

"It would be no difficult task, we think, to account for these appearances in a different manner; were all the circumstances attending the case minutely ascertained: but without interrogating the dead on this subject, we may more satisfactorily appeal to the experience of the living (p), to shew, that though the returning stroke must take place, in all thunder-storms, in some degree or other; yet it is not of that alarming magnitude which the author ascribes to it. If, in any particular thunder-storm, a man in the open fields could be killed, at the instant of a distant explosion, merely by the return of his own electric fire, which had before been driven out of his body; surely numerous observations of persons who had experienced the returning stroke, in lighter degrees, would be familiar; and scarce a great thunder-storm must have occurred, in which one person or another must not, at the instant of an explosion, have felt the effects of the returning stroke, in some degree or other—from that of a violent concussion, to that of a slight and almost imperceptible pulsation. But no observations of this kind are known to us; nor have we ever heard of any person's experiencing any kind of electrical commotion in a thunder-storm, except such as have either been directly struck, or have happened to be in the very near neighbourhood of the spot where the explosion took place.

"The author has been aware of this objection, which which he proposes, and endeavours to remove; but his answer to it amounts to little more than what has been already quoted from him; that is, to a simple estimate of the enormous difference between the electrical density, or the elastic electrical pressure, of the atmosphere of an extensive thunder-cloud, and that of a charged prime conductor. We have already observed, that this is not the proper method of estimating their different effects, when these two causes, how unequal forever in power, are considered as exerting that power on bodies containing a limited, and comparatively small, quantity of electric matter.

"We have been induced to discuss this subject thus particularly, with a view to quiet the minds of the timorous; as the author's extension of his principles, respecting the returning stroke in artificial electricity, to what passes in natural electricity, holds out a new, and, in our opinion, groundless subject of terror to those who, in the midst of their apprehensions, have hitherto only dreaded the effects of a thunder-storm when it made near approaches to them; but who, if this doctrine were believed, would never think themselves in security while a thunder-cloud appeared in sight, unless sheltered in a house furnished with proper conductors:—for we should not omit to remark, that a subsequent observation tends to diminish their fears, by shewing that high and pointed conductors tend to secure both persons and buildings, against the various effects of any returning stroke whatever, as well as of the main stroke.

"Indeed various parts of this work, besides those immediately appropriated to the subject, tend to prove the utility of high and pointed conductors, in preference to those which terminate in a ball, or rounded end. Towards the end of the performance, the author discusses this matter very particularly; and enumerates the 'necessary requisites' in erecting them, in number 11; to every one of which, tho' we have taken the liberty to differ from him on another subject, we readily subscribe. As this matter is of a popular nature, and on a subject generally interesting, we shall transcribe this list; adding a short explanation to particular articles.—These requisites (says the author) are 11 in number:

1stly, That the rod be made of such substances, as are, in their nature, the best conductors of electricity.

2ndly, That the rod be uninterrupted, and perfectly continuous.—This is a very material circumstance. One entire piece of metal cannot perhaps be had; but it is not sufficient that the rods, of which the conductor consists, be sensibly in contact; they should be pressed into actual contact by means of nuts and screws, with a thin piece of sheet-lead between the shoulders of the joints.

3rdly, That it be of a sufficient thickness.—A copper rod half an inch square, or an iron rod one inch square, or one of lead two inches square, are thought fully sufficient by the author.

4thly, That it be perfectly connected with the common stock.—That is, it should be carried deep into the earth, which is frequently dry near the surface; and then continued in a horizontal direction, so as to have the farther extremity dipped, should this be practicable, into water, at the distance of 10 yards or more from the foundation.

Vol. VI.

5thly, That the upper extremity of the rod be as lightning-acutely pointed as possible.—This termination should be of copper; or rather a very fine and exceedingly acute needle of gold should be employed, which will not materially add to the expense.

6thly, That it be very finely tapered:—so that the upper extremity may be a cone, the diameter of the base of which may bear an extremely small proportion to its height; for instance, that of one to forty.

7thly, That it be extremely prominent:—that is, 8, 10, or 15 feet at least above the highest parts of the building. The author lays great stress on this circumstance; in consequence of the law above-mentioned, deduced by him from his experiments, relating to electric atmospheres. According to this law, the density of an electric atmosphere (the negative atmosphere, for instance, of the roof of a house, &c. while a positively charged cloud hangs over it) diminishes in the inverse ratio of the square of the distance from the surface of the body to which that atmosphere belongs. Accordingly, if the rod project 12 feet into this atmosphere, it will reach to a part of it four times less dense than if the rod projected only to half that distance, or six feet;—and to a part one hundred and forty-four times rarer, than if it projected only one foot.

8thly, That each rod be carried, in the shortest convenient direction, from the point at its upper end, to the common stock.

9thly, That there be neither large nor prominent bodies of metal upon the top of the building proposed to be secured, but such as are connected with the conductor, by some proper metallic communication.

10thly, That there be a sufficient number of high and pointed rods.—On edifices of great importance, especially magazines of gun-powder, the author thinks these ought never to be above 40 or 50 feet asunder.

11thly, That every part of the rods be very substantially erected."

The author declares that he has never been able to hear of a single instance, nor does he believe that any can be produced, of an high, tapering, and acutely pointed metallic conductor, having ever, in any country, been struck by lightning; if it had all the necessary requisites abovementioned, especially the second and fourth.

On the whole, it seems to be pretty certain, that use of both pointed and knopped metallic conductors do have the power of preserving any body placed at a small distance from them from being struck by lightning. This they do, not because they can attract the lightning far out of its way, but because the resistance to its passage is always least on that side where they are; and as pointed conductors diminish the resistance more considerably than blunt ones, they seem in all cases to be preferable.—It appears, however, that a single conductor, whether blunt or pointed, is not capable of securing all the parts of a large building from strokes of lightning; and therefore several of them will be required for this purpose: but to what distance their influence extends, hath not been determined, nor does it seem easily capable of being ascertained.

It now remains only to explain some of the more uncommon appearances and effects of lightning. One thing some of these is, that it is frequently observed to kill all-times kills ternately: that is, supposing a number of people alternately standing... Lightning standing in a line; if the first person was killed, the second perhaps would be safe; the third would be killed, and the fourth safe; the fifth killed, &c.—Effects of this kind are generally produced by the most violent kind of lightning; namely, that which appears in the form of balls, and which are frequently seen to divide themselves into several parts before they strike. If one of these parts of a fire-ball strikes a man, another will not strike the person who stands immediately close to him; because there is always a repulsion between bodies electrified the same way. Now, as all these parts into which the ball breaks have all the same kind of electricity, it is evident that they must for that reason repel one another; and this repulsion is so strong, that a man may be interposed within the stroke of two of them, without being hurt by either.

The other effect of lightning is mentioned under the article JERUSALEM, where those who attempted to rebuild the temple had the marks of crosses impressed upon their garments and bodies. This may reasonably be thought to arise from the same cause to which the angular appearance of lightning in the air is owing; namely, its violent impetus and velocity, together with the opposition of the atmosphere. A small stroke of lightning, sometimes indeed a very considerable one, cannot always enter the substance of terrestrial bodies, even when it touches them, for reasons already given. In this case it runs along their surface, and, as in its motion it is perpetually resisted by the atmosphere, it undoubtedly has the same angular motion which we often perceive in the atmosphere. If in this situation it happens to touch the human skin, or a garment, especially of linen, as being a conductor, it will undoubtedly leave a mark upon it; and this mark being of a zig-zag form, might, in the above instance, have been either taken for the exact form of a cross by the beholders, or have suggested that idea in relating the story to make it appear more wonderful.

These observations may serve to give some idea of the nature of lightning, and its operations after it appears in its proper form and bursts out from the cloud; but for an account of its original formation, and of the powers by which the clouds are at first electrified, and their electricity kept up notwithstanding many successive discharges of lightning, and the quantity of electric matter continually carried off by the rain, &c. see the article Thunder.

Artificial Lightning. Before the discoveries of Dr Franklin concerning the identity of electricity and lightning, many contrivances were invented in order to represent this terrifying phenomenon in miniature: the combustion of phosphorus in warm weather, the accension of the vapour of spirit of wine evaporated in a close place, &c. were used in order to support the hypothesis which at that time prevailed; namely, that lightning was formed of some sulphurous, nitrous, or other combustible vapours, floating in long trains in the atmosphere, which by some unaccountable means took fire, and produced all the destructive effects of that phenomenon. These representations, however, are now no more exhibited; and the only true artificial lightning is universally acknowledged to be the discharge of electric matter from bodies in which it is artificially set in motion by our machines.

LIGHTFOOT (John), a very learned English divine in the 17th century, was educated in Christchurch, Cambridge. Sir Rowland Cotton, knight, took him into his family as his chaplain, and engaged him in the study of the Hebrew language. He refused to travel; but changed his resolution, being importuned by the people of Stone in Staffordshire to be their minister. From hence he removed to Hornsey near London, for the sake of Sion-college library, where he discharged the duties of his function, and prosecuted his rabbinical studies, till June 1642, when he retired to London, was chosen minister of St Bartholomew's behind the Exchange, and appointed one of the assembly of divines in 1643. August 26, 1645, he preached, before the house of commons, a sermon printed at London the same year, in which he recommends to the parliament a Review and Survey of the translation of the bible, and to hasten the settling of the church. In 1655, he was chosen vice-chancellor of the university of Cambridge. He was collated to a prebend in the cathedral of Ely by Sir Orlando Bridgeman, then keeper of the great seal. He published several valuable works, particularly "The Harmony of the Old, and the Harmony of the New Testament," &c. He died in 1675, aged 74.