To Ebenezer Kinnersley, with Associated Papers
MSS not found; reprinted from Experiments and Observations on Electricity, 1769 edition, pp. 397–425.9
When Franklin included this letter to Kinnersley in the 1769 edition of Experiments and Observations on Electricity he placed immediately after it two accounts of lightning strokes in South Carolina, which he had mentioned to Kinnersley, and his own remarks on the second of these incidents. Almost certainly the accounts had come to him in 1761 or early 1762 with a letter (not found) from some friend in South Carolina, probably Dr. John Lining. Although their dates would indicate that they should have appeared in the previous volume of this series, they are placed here, after the letter to Kinnersley, chiefly because they relate so closely to matters about which he was writing his friend that it seems more useful to depart from the normal chronological arrangement in this instance and to place them, as he himself did, with his own letter.
London, Feb. 20, 1762.
I received your ingenious letter of the 12th of March last,1 and thank you cordially for the account you give me of the new experiments you have lately made in Electricity. It is a subject that still affords me pleasure, though of late I have not much attended to it.
Your second experiment, in which you attempted, without success, to communicate positive electricity by vapour ascending from electrised water,2 reminds me of one I formerly made, to try if negative electricity might be produced by evaporation only. I placed a large heated brass plate, containing four or five square feet, on an electric stand; a rod of metal, about four feet long, with a bullet at its end, extended from the plate horizontally. A light lock of cotton, suspended by a fine thread from the cieling, hung opposite to, and within an inch of the bullet. I then sprinkled the heated plate with water, which arose fast from it in vapour. If vapour should be disposed to carry off the electrical, as it does the common fire from bodies, I expected the plate would, by losing some of its natural quantity, become negatively electrised. But I could not perceive, by any motion in the cotton, that it was at all affected; nor by any separation of small cork-balls suspended from the plate, could it be observed that the plate was in any manner electrified. Mr. Canton here has also found, that two tea-cups, set on electric stands, and filled, one with boiling, the other with cold water, and equally electrified, continued equally so, notwithstanding the plentiful evaporation from the hot water. Your experiment and his agreeing, show another remarkable difference between electric and common fire. For the latter quits most readily the body that contains it, where water, or any other fluid, is evaporating from the surface of that body, and escapes with the vapour. Hence the method long in use in the east, of cooling liquors, by wrapping the bottles round with a wet cloth, and exposing them to the wind. Dr. Cullen, of Edinburgh, has given some experiments of cooling by evaporation;3 and I was present at one made by Dr. Hadley, then professor of chemistry at Cambridge,4 when, by repeatedly wetting the ball of a thermometer with spirit, and quickening the evaporation by the blast of a bellows, the mercury fell from 65, the state of warmth in the common air, to 7, which is 22 degrees below freezing; and, accordingly, from some water mixed with the spirit, or from the breath of the assistants, or both, ice gathered in small spicula round the ball, to the thickness of near a quarter of an inch. To such a degree did the mercury lose the fire it before contained, which, as I imagine, took the opportunity of escaping, in company with the evaporating particles of the spirit, by adhering to those particles.
Your experiment of the Florence flask, and boiling water, is very curious.5 I have repeated it, and found it to succeed as you describe it, in two flasks out of three. The third would not charge when filled with either hot or cold water. I repeated it, because I remembered I had once attempted to make an electric bottle of a Florence flask, filled with cold water, but could not charge it at all; which I then imputed to some imperceptible cracks in the small, extremely thin bubbles, of which that glass is full, and I concluded none of that kind would do. But you have shewn me my mistake. Mr. Wilson had formerly acquainted us, that red hot glass would conduct electricity;6 but that so small a degree of heat as that communicated by boiling water, would so open the pores of extremely thin glass, as to suffer the electric fluid freely to pass, was not before known. Some experiments similar to yours, have, however, been made here, before the receipt of your letter, of which I shall now give you an account.
I formerly had an opinion that a Leyden bottle, charg’d and then seal’d hermetically, might retain its electricity for ever; but having afterwards some suspicion that possibly that subtil fluid might, by slow imperceptible degrees, soak through the glass, and in time escape, I requested some of my friends, who had conveniences for doing it, to make trial, whether, after some months, the charge of a bottle so sealed would be sensibly diminished. Being at Birmingham, in September 1760, Mr. Bolton of that place7 opened a bottle that had been charged, and its long tube neck hermetically sealed in the January preceding. On breaking off the end of the neck, and introducing a wire into it, we found it possessed of a considerable quantity of electricity, which was discharged by a snap and spark. This bottle had lain near seven months on a shelf, in a closet, in contact with bodies that would undoubtedly have carried off all its electricity, if it could have come readily through the glass. Yet as the quantity manifested by the discharge was not apparently so great as might have been expected from a bottle of that size well charged, some doubt remained whether part had escaped while the neck was sealing, or had since, by degrees, soaked through the glass. But an experiment of Mr. Canton’s, in which such a bottle was kept under water a week, without having its electricity in the least impaired, seems to show, that when the glass is cold, though extremely thin, the electric fluid is well retained by it. As that ingenious and accurate experimenter made a discovery, like yours, of the effect of heat in rendering thin glass permeable by that fluid, it is but doing him justice to give you his account of it, in his own words, extracted from his letter to me, in which he communicated it, dated Oct. 31, 1760, viz.8
“Having procured some thin glass balls, of about an inch and a half in diameter, with stems, or tubes, of eight or nine inches in length, I electrified them, some positively on the inside, and others negatively, after the manner of charging the Leyden bottle, and sealed them hermetically. Soon after I applied the naked balls to my electrometer, and could not discover the least sign of their being electrical; but holding them before the fire, at the distance of six or eight inches, they became strongly electrical in a very short time, and more so when they were cooling. These balls will, every time they are heated, give the electrical fluid to, or take it from other bodies, according to the plus or minus state of it within them. Heating them frequently, I find will sensibly diminish their power; but keeping one of them under water a week, did not appear in the least degree to impair it. That which I kept under water, was charged on the 22d of September last, was several times heated before it was kept in water, and has been heated frequently since, and yet it still retains its virtue to a very considerable degree. The breaking two of my balls accidentally, gave me an opportunity of measuring their thickness, which I found to be between seven and eight parts in a thousand of an inch.
“A down feather, in a thin glass ball, hermetically sealed, will not be affected by the application of an excited tube, or the wire of a charged vial, unless the ball be considerably heated; and if a glass pane be heated till it begins to grow soft, and in that state be held between the wire of a charged vial, and the discharging wire, the course of the electrical fluid will not be through the glass, but on the surface, round by the edge of it.”
By this last experiment of Mr. Canton’s, it appears, that though by a moderate heat, thin glass becomes, in some degree, a conductor of electricity, yet, when of the thickness of a common pane, it is not, though in a state near melting, so good a conductor as to pass the shock of a discharged bottle. There are other conductors which suffer the electric fluid to pass through them gradually, and yet will not conduct a shock. For instance, a quire of paper will conduct through its whole length, so as to electrify a person, who, standing on wax, presents the paper to an electrified prime conductor; but it will not conduct a shock even through its thickness only; hence the shock either fails, or passes by rending a hole in the paper. Thus a sieve will pass water gradually, but a stream from a fire engine would either be stopped by it, or tear a hole through it.
It should seem, that to make glass permeable to the electric fluid, the heat should be proportioned to the thickness. You found the heat of boiling water, which is but 210, sufficient to render the extreme thin glass in a Florence flask permeable even to a shock. Lord Charles Cavendish,9 by a very ingenious experiment, has found the heat of 400 requisite to render thicker glass permeable to the common current.
“A glass tube,1 of which the part CB was solid, had wire thrust in each end, reaching to B and C.
“A small wire was tied on at D, reaching to the floor, in order to carry off any electricity that might run along upon the tube.
“The bent part was placed in an iron pot, filled with iron filings; a thermometer was also put into the filings; a lamp was placed under the pot; and the whole was supported upon glass.
“But after the part CB of the tube was heated to 600, the corks continued to separate, though you discharged the electricity by touching the wire at E, the electrical machine continuing in motion.
“Upon letting the whole cool, the effect remained till the thermometer was sunk to 400.”
It were to be wished, that this noble philosopher would communicate more of his experiments to the world, as he makes many, and with great accuracy.
You know I have always look’d upon and mentioned the equal repulsion in cases of positive and of negative electricity, as a phaenomenon difficult to be explained.2 I have sometimes, too, been inclined, with you, to resolve all into attraction; but besides that attraction seems in itself as unintelligible as repulsion, there are some appearances of repulsion that I cannot so easily explain by attraction; this for one instance. When the pair of cork balls are suspended by flaxen threads, from the end of the prime conductor, if you bring a rubbed glass tube near the conductor, but without touching it, you see the balls separate, as being electrified positively; and yet you have communicated no electricity to the conductor, for, if you had, it would have remained there, after withdrawing the tube; but the closing of the balls immediately thereupon, shews that the conductor has no more left in it than its natural quantity. Then again approaching the conductor with the rubbed tube, if, while the balls are separated, you touch with a finger that end of the conductor to which they hang, they will come together again, as being, with that part of the conductor, brought to the same state with your finger, i.e. the natural state. But the other end of the conductor, near which the tube is held, is not in that state, but in the negative state, as appears on removing the tube; for then part of the natural quantity left at the end near the balls, leaving that end to supply what is wanting at the other, the whole conductor is found to be equally in the negative state. Does not this indicate that the electricity of the rubbed tube had repelled the electric fluid, which was diffused in the conductor while in its natural state, and forced it to quit the end to which the tube was brought near, accumulating itself on the end to which the balls were suspended? I own I find it difficult to account for its quitting that end, on the approach of the rubbed tube, but on the supposition of repulsion; for, while the conductor was in the same state with the air, i.e. the natural state, it does not seem to me easy to suppose, that an attraction should suddenly take place between the air and the natural quantity of the electric fluid in the conductor, so as to draw it to, and accumulate it on the end opposite to that approached by the tube; since bodies, possessing only their natural quantity of that fluid, are not usually seen to attract each other, or to affect mutually the quantities of electricity each contains.
There are likewise appearances of repulsion in other parts of nature. Not to mention the violent force with which the particles of water, heated to a certain degree, separate from each other, or those of gunpowder, when touch’d with the smallest spark of fire, there is the seeming repulsion between the same poles of the magnet, a body containing a subtle moveable fluid, in many respects analagous to the electric fluid. If two magnets are so suspended by strings, as that their poles of the same denomination are opposite to each other, they will separate, and continue so; or if you lay a magnetic steel bar on a smooth table, and approach it with another parallel to it, the poles of both in the same position, the first will recede from the second, so as to avoid the contact, and may thus be push’d (or at least appear to be push’d) off the table. Can this be ascribed to the attraction of any surrounding body or matter drawing them asunder, or drawing the one away from the other? If not, and repulsion exists in nature, and in magnetism, why may it not exist in electricity? We should not, indeed, multiply causes in philosophy without necessity; and the greater simplicity of your hypothesis would recommend it to me, if I could see that all appearances might be solved by it. But I find, or think I find, the two causes more convenient than one of them alone. Thus I would solve the circular motion of your horizontal stick, supported on a pivot, with two pins at their ends, pointing contrary ways, and moving in the same direction when electrified, whether positively or negatively: When positively, the air opposite to the points being electrised positively, repels the points; when negatively, the air opposite the points being also, by their means, electrised negatively, attraction takes place between the electricity in the air behind the heads of the pins, and the negative pins, and so they are, in this case, drawn in the same direction that in the other they were driven. You see I am willing to meet you half way, a complaisance I have not met with in our brother Nollet, or any other hypothesis-maker, and therefore may value myself a little upon it, especially as they say I have some ability in defending even the wrong side of a question, when I think fit to take it in hand.
What you give as an established law of the electric fluid, “That quantities of different densities mutually attract each other, in order to restore the equilibrium”3 is, I think, not well founded, or else not well express’d. Two large cork balls, suspended by silk strings, and both well and equally electrified, separate to a great distance. By bringing into contact with one of them, another ball of the same size, suspended likewise by silk, you will take from it half its electricity. It will then, indeed, hang at a less distance from the other, but the full and the half quantities will not appear to attract each other, that is, the balls will not come together. Indeed, I do not know any proof we have, that one quantity of electric fluid is attracted by another quantity of that fluid, whatever difference there may be in their densities. And, supposing in nature, a mutual attraction between two parcels of any kind of matter, it would be strange if this attraction should subsist strongly while those parcels were unequal, and cease when more matter of the same kind was added to the smallest parcel, so as to make it equal to the biggest. By all the laws of attraction in matter, that we are acquainted with, the attraction is stronger in proportion to the increase of the masses, and never in proportion to the difference of the masses. I should rather think the law would be, “That the electric fluid is attracted strongly by all other matter that we know of, while the parts of that fluid mutually repel each other.” Hence its being equally diffused (except in particular circumstances) throughout all other matter. But this you jokingly call “electrical orthodoxy.” It is so with some at present, but not with all; and, perhaps, it may not always be orthodoxy with any body. Opinions are continually varying, where we cannot have mathematical evidence of the nature of things; and they must vary. Nor is that variation without its use, since it occasions a more thorough discussion, whereby error is often dissipated, true knowledge is encreased, and its principles become better understood and more firmly established.
Air should have, as you observe, “its share of the common stock of electricity, as well as glass, and, perhaps, all other electrics per se.”4 But I suppose, that, like them, it does not easily part with what it has, or receive more, unless when mix’d with some nonelectric, as moisture for instance, of which there is some in our driest air. This, however, is only a supposition; and your experiment of restoring electricity to a negatively electrised person, by extending his arm upwards into the air, with a needle between his fingers, on the point of which light may be seen in the night, is, indeed, a curious one. In this town the air is generally moister than with us, and here I have seen Mr. Canton electrify the air in one room positively, and in another, which communicated by a door, he has electrised the air negatively. The difference was easily discovered by his cork balls, as he passed out of one room into another. Pere Beccaria, too, has a pretty experiment, which shews that air may be electrised.5 Suspending a pair of small light balls, by flaxen threads, to the end of his prime conductor, he turns his globe some time, electrising positively, the balls diverging and continuing separate all the time. Then he presents the point of a needle to his conductor, which gradually drawing off the electric fluid, the balls approach each other, and touch, before all is drawn from the conductor; opening again as more is drawn off, and separating nearly as wide as at first, when the conductor is reduced to the natural state. By this it appears, that when the balls came together, the air surrounding the balls was just as much electrised as the conductor at that time; and more than the conductor, when that was reduced to its natural state. For the balls, though in the natural state, will diverge, when the air that surrounds them is electrised plus or minus, as well as when that is in its natural state and they are electrised plus or minus themselves. I foresee that you will apply this experiment to the support of your hypothesis, and I think you may make a good deal of it.
It was a curious enquiry of yours, Whether the electricity of the air, in clear dry weather, be of the same density at the height of two or three hundred yards, as near the surface of the earth; and I am glad you made the experiment.6 Upon reflection, it should seem probable, that whether the general state of the atmosphere at any time be positive or negative, that part of it which is next the earth will be nearer the natural state, by having given to the earth in one case, or having received from it in the other. In electrising the air of a room, that which is nearest the walls, or floor, is least altered. There is only one small ambiguity in the experiment, which may be cleared by more trials; it arises from the supposition that bodies may be electrised positively by the friction of air blowing strongly on them, as it does on the kite and its string. If at some times the electricity appears to be negative, as that friction is the same, the effect must be from a negative state of the upper air.
I am much pleased with your electrical thermometer, and the experiments you have made with it.7 I formerly satisfied myself by an experiment with my phial and syphon, that the elasticity of the air was not increased by the mere existence of an electric atmosphere within the phial; but I did not know, till you now inform me, that heat may be given to it by an electric explosion. The continuance of its rarefaction, for some time after the discharge of your glass jar and of your case of bottles, seem to make this clear. The other experiments on wet paper, wet thread, green grass, and green wood, are not so satisfactory; as possibly the reducing part of the moisture to vapour, by the electric fluid passing through it, might occasion some expansion which would be gradually reduced by the condensation of such vapour. The fine silver thread, the very small brass wire, and the strip of gilt paper, are also subject to a similar objection, as even metals, in such circumstances, are often partly reduced to smoke, particularly the gilding on paper.
But your subsequent beautiful experiment on the wire, which you made hot by the electric explosion, and in that state fired gunpowder with it, puts it out of all question, that heat is produced by our artificial electricity, and that the melting of metals in that way, is not by what I formerly called a cold fusion. A late instance here, of the melting a bell-wire, in a house struck by lightning, and parts of the wire burning holes in the floor on which they fell, has proved the same with regard to the electricity of nature. I was too easily led into that error by accounts given, even in philosophical books, and from remote ages downwards, of melting money in purses, swords in scabbards, &c. without burning the inflammable matters that were so near those melted metals. But men are, in general, such careless observers, that a philosopher cannot be too much on his guard in crediting their relations of things extraordinary, and should never build an hypothesis on any thing but clear facts and experiments, or it will be in danger of soon falling, as this does, like a house of cards.
How many ways there are of kindling fire, or producing heat in bodies! By the sun’s rays, by collision, by friction, by hammering, by putrefaction, by fermentation, by mixtures of fluids, by mixtures of solids with fluids, and by electricity. And yet the fire when produced, though in different bodies it may differ in circumstances, as in colour, vehemence, &c. yet in the same bodies is generally the same. Does not this seem to indicate that the fire existed in the body, though in a quiescent state, before it was by any of these means excited, disengaged, and brought forth to action and to view? May it not constitute part, and even a principal part, of the solid substance of bodies? If this should be the case, kindling fire in a body would be nothing more than developing this inflammable principle, and setting it at liberty to act in separating the parts of that body, which then exhibits the appearances of scorching, melting, burning, &c. When a man lights an hundred candles from the flame of one, without diminishing that flame, can it be properly said to have communicated all that fire? When a single spark from a flint, applied to a magazine of gunpowder, is immediately attended with this consequence, that the whole is in flame, exploding with immense violence, could all this fire exist first in the spark? We cannot conceive it. And thus we seem led to this supposition, that there is fire enough in all bodies to singe, melt, or burn them, whenever it is, by any means, set at liberty, so that it may exert itself upon them, or be disengaged from them. This liberty seems to be afforded it by the passage of electricity through them, which we know can and does, of itself, separate the parts even of water; and perhaps the immediate appearances of fire are only the effects of such separations? If so, there would be no need of supposing that the electric fluid heats itself by the swiftness of its motion, or heats bodies by the resistance it meets with in passing through them. They would only be heated in proportion as such separation could be more easily made. Thus a melting heat cannot be given to a large wire in the flame of a candle, though it may to a small one; and this not because the large wire resists less that action of the flame which tends to separate its parts, but because it resists it more than the smaller wire; or because the force being divided among more parts, acts weaker on each.
This reminds me, however, of a little experiment I have frequently made, that shews, at one operation, the different effects of the same quantity of electric fluid passing through different quantities of metal. A strip of tinfoil, three inches long, a quarter of an inch wide at one end, and tapering all the way to a sharp point at the other, fixed between two pieces of glass, and having the electricity of a large glass jar sent through it will not be discomposed in the broadest part; towards the middle will appear melted in spots; where narrower, it will be quite melted; and about half an inch of it next the point will be reduced to smoke.
You were not mistaken in supposing that your account of the effect of the pointed rod, in securing Mr. West’s house from damage by a stroke of lightning, would give me great pleasure.8 I thank you for it most heartily, and for the pains you have taken in giving me so complete a description of its situation, form, and substance, with the draft of the melted point. There is one circumstance, viz. that the lightning was seen to diffuse itself from the foot of the rod over the wet pavement, which seems, I think, to indicate, that the earth under the pavement was very dry, and that the rod should have been sunk deeper, till it came to earth moister and therefore apter to receive and dissipate the electric fluid. And although, in this instance, a conductor formed of nail rods, not much above a quarter of an inch thick, served well to convey the lightning, yet some accounts I have seen from Carolina,9 give reason to think, that larger may be sometimes necessary, at least for the security of the conductor itself, which, when too small, may be destroyed in executing its office, though it does, at the same time, preserve the house. Indeed, in the construction of an instrument so new, and of which we could have so little experience, it is rather lucky that we should at first be so near the truth as we seem to be, and commit so few errors.
There is another reason for sinking deeper the lower end of the rod, and also for turning it outwards under ground to some distance from the foundation; it is this, that water dripping from the eaves falls near the foundation, and sometimes soaks down there in greater quantities, so as to come near the end of the rod though the ground about it be drier. In such case, this water may be exploded, that is, blown into vapour, whereby a force is generated that may damage the foundation. Water reduced to vapour, is said to occupy 14,000 times its former space. I have sent a charge through a small glass tube, that has borne it well while empty, but when filled first with water, was shattered to pieces and driven all about the room: Finding no part of the water on the table, I suspected it to have been reduced to vapour; and was confirmed in that suspicion afterwards, when I had filled a like piece of tube with ink, and laid it on a sheet of clean paper, whereon, after the explosion, I could find neither any moisture nor any sully from the ink. This experiment of the explosion of water, which I believe was first made by that most ingenious electrician father Beccaria, may account for what we sometimes see in a tree struck by lightning, when part of it is reduced to fine splinters like a broom; the sap vessels being so many tubes containing a watry fluid, which when reduced to vapour, rends every tube length-ways. And perhaps it is this rarefaction of the fluids in animal bodies killed by lightning or electricity, that by separating its fibres, renders the flesh so tender, and apt so much sooner to putrify. I think too, that much of the damage done by lightning to stone and brick walls, may sometimes be owing to the explosion of water, found, during showers, running or lodging in the joints or small cavities or cracks that happen to be in the walls.
Here are some electricians that recommend knobs instead of points on the upper end of the rods, from a supposition that the points invite the stroke.1 It is true that points draw electricity at greater distances in the gradual silent way; but knobs will draw at the greatest distance a stroke. There is an experiment that will settle this. Take a crooked wire of the thickness of a quill, and of such a length as that one end of it being applied to the lower part of a charged bottle, the upper may be brought near the ball on the top of the wire that is in the bottle. Let one end of this wire be furnished with a knob, and the other be gradually tapered to a fine point. When the point is presented to discharge the bottle it must be brought much nearer before it will receive the stroke, than the knob requires to be. Points besides tend to repel the fragments of an electrised cloud, knobs draw them nearer. An experiment which I believe I have shewn you, of cotton fleece hanging from an electrised body, shows this clearly when a point or a knob is presented under it.
You seem to think highly of the importance of this discovery, as do many others on our side of the water.2 Here it is very little regarded; so little, that though it is now seven or eight years since it was made publick, I have not heard of a single house as yet attempted to be secured by it.3 It is true the mischiefs done by lightning are not so frequent here as with us, and those who calculate chances may perhaps find that not one death (or the destruction of one house) in a hundred thousand happens from that cause, and that therefore it is scarce worth while to be at any expence to guard against it. But in all countries there are particular situations of buildings more exposed than others to such accidents, and there are minds so strongly impressed with the apprehension of them, as to be very unhappy every time a little thunder is within their hearing; it may therefore be well to render this little piece of new knowledge as general and as well understood as possible, since to make us safe is not all its advantage, it is some to make us easy. And as the stroke it secures us from might have chanced perhaps but once in our lives, while it may relieve us a hundred times from those painful apprehensions, the latter may possibly on the whole contribute more to the happiness of mankind than the former.
Your kind wishes and congratulations are very obliging. I return them cordially; being with great regard and esteem, My dear Sir, Your affectionate friend, and most obedient humble servant,
Accounts from Carolina (mention’d in the foregoing Letter) of the effects of Lightning, on two of the Rods commonly affix’d to Houses there, for securing them against Lightning.
Charles-town, Nov. 1, 1760.
———It is some Years since Mr. Raven’s Rod was struck by lightning.4 I hear an account of it was published at the time, but I cannot find it. According to the best information I can now get, he had fix’d to the outside of his chimney a large iron Rod, several feet in length, reaching above the chimney; and to the top of this rod the points were fixed. From the lower end of this rod, a small brass wire was continued down to the top of another iron rod driven into the earth. On the ground-floor in the chimney stood a gun, leaning against the back wall, nearly opposite to where the brass wire came down on the outside. The lightning fell upon the points, did no damage to the rod they were fix’d to; but the brass wire, all down till it came opposite to the top of the gun-barrel, was destroyed.* There the lightning made a hole through the wall or back of the chimney, to get to the gun-barrel,† down which it seems to have pass’d, as, although it did not hurt the barrel, it damaged the butt of the stock, and blew up some bricks of the hearth. The brass wire below the hole in the wall remain’d good. No other damage, as I can learn, was done to the house. I am told the same house had formerly been struck by lightning, and much damaged, before these rods were invented.———
Mr. William Maine’s Account of the Effects of Lightning on his Rod, dated at Indian Land, in South Carolina, Aug. 28, 1760.6
———I had a set of electrical points, consisting of three prongs, of large brass wire tipt with silver, and perfectly sharp, each about seven inches long; these were riveted at equal distances into an iron nut about three quarters of an inch square, and opened at top equally to the distance of six or seven inches from point to point, in a regular triangle. This nut was screwed very tight on the top of an iron rod of above half an inch diameter, or the thickness of a common curtain rod, composed of several joints, annexed by hooks turned at the ends of each joint, and the whole fixed to the chimney of my house by iron staples. The points were elevated (a),7 six or seven inches above the top of the chimney; and the lower joint sunk three feet in the earth, in a perpendicular direction.
Thus stood the points on Tuesday last about five in the evening, when the lightning broke with a violent explosion on the chimney, cut the rod square off just under the nutt, and I am persuaded, melted the points, nut, and top of the rod, entirely up; as after the most diligent search, nothing of either was found (b), and the top of the remaining rod was cased over with a congealed solder. The lightning ran down the rod, starting almost all the staples (c), and unhooking the joints, without affecting the rod (d), except on the inside of each hook where the joints were coupled, the surface of which was melted (e), and left as cased over with solder. No part of the chimney was damaged (f), only at the foundation (g), where it was shattered almost quite round, and several bricks were torn out (h). Considerable cavities were made in the earth quite round the foundation, but most within eight or nine inches of the rod. It also shattered the bottom weather-board (i), at one corner of the house, and made a large hole in the earth by the corner post. On the other side of the chimney, it ploughed up several furrows in the earth, some yards in length. It ran down the inside of the chimney (k), carrying only soot with it; and filled the whole house with its flash (l), smoke, and dust. It tore up the hearth in several places (m), and broke some pieces of china in the beaufet (n). A copper tea kettle standing in the chimney was beat together, as if some great weight had fallen upon it (o); and three holes, each about half an inch diameter, melted through the bottom (p). What seems to me most surprising is, that the hearth under the kettle was not hurt, yet the bottom of the kettle was drove inward, as if the lightning proceeded from under it upwards (q), and the cover was thrown to the middle of the floor (r). The fire dogs, an iron loggerhead, an Indian pot, an earthen cup, and a cat, were all in the chimney at the time unhurt, though great part of the hearth was torn up (s). My wife’s sister, two children, and a Negro wench, were all who happened to be in the house at the time: The first, and one child, sat within five feet of the chimney; and were so stunned, that they never saw the lightning nor heard the explosion; the wench, with the other child in her arms, sitting at a greater distance, was sensible of both; though every one was so stunn’d that they did not recover for some time; however it pleased God that no farther mischief ensued. The kitchen, at 90 feet distance, was full of Negroes, who were all sensible of the shock; and some of them tell me, that they felt the rod about a minute after, when it was so hot that they could not bear it in hand.
The foregoing very sensible and distinct account may afford a good deal of instruction relating to the nature and effects of lightning, and to the construction and use of this instrument for averting the mischiefs of it. Like other new instruments, this appears to have been at first in some respects imperfect; and we find that we are, in this as in others, to expect improvement from experience chiefly: But there seems to be nothing in the account, that should discourage us in the use of it; since at the same time that its imperfections are discovered, the means of removing them are pretty easily to be learnt from the circumstances of the account itself; and its utility upon the whole is manifest.
One intention of the pointed rod, is, to prevent a stroke of lightning. (See pages 126, 162.9) But to have a better chance of obtaining this end, the points should not be too near to the top of the chimney or highest part of the building to which they are affixed, but should be extended five or six feet above it; otherwise their operation in silently drawing off the fire (from such fragments of cloud as float in the air between the great body of cloud and the earth) will be prevented. For the experiment with the lock of cotton hanging below the electrified prime conductor, shews, that a finger under it, being a blunt body, extends the cotton, drawing its lower part downwards; when a needle with its point presented to the cotton, makes it fly up again to the prime conductor; and that this effect is strongest, when as much of the needle as possible appears above the end of the finger; grows weaker as the needle is shortened between the finger and thumb; and is reduced to nothing when only a short part below the point appears above the finger. Now it seems the points of Mr. Maine’s rod were elevated only (a) six or seven inches above the top of the chimney; which, considering the bulk of the chimney and the house, was too small an elevation. For the great body of matter near them would hinder their being easily brought into a negative state by the repulsive power of the electrised cloud, in which negative state it is that they attract most strongly and copiously the electric fluid from other bodies, and convey it into the earth.
(b) Nothing of the points, &c. could be found. This is a common effect. (See page 163.1) Where the quantity of the electric fluid passing is too great for the conductor thro’ which it passes, the metal is either melted, or reduced to smoke and dissipated; but where the conductor is sufficiently large, the fluid passes in it without hurting it. Thus these three wires were destroyed, while the rod to which they were fixed, being of greater substance, remained unhurt; its end only, to which they were joined, being a little melted, some of the melted part of the lower ends of those wires uniting with it, and appearing on it like solder.
(c) (d) (e) As the several parts of the rod were connected only by the ends being bent round into hooks, the contact between hook and hook was much smaller than the rod; therefore the current through the metal being confin’d in those narrow passages, melted part of the metal, as appeared on examining the inside of each hook. Where metal is melted by lightning, some part of it is generally exploded; and these explosions in the joints appear to have been the cause of unhooking them; and, by that violent action, of starting also most of the staples. We learned from hence, that a rod in one continued piece is preferable to one composed of links or parts hooked together.
(f) No part of the chimney was damaged; because the lightning passed in the rod. And this instance agrees with others in shewing, that the second and principal intention of the rods is obtainable, viz. that of conducting the lightning. In all the instances yet known of the lightning’s falling on any house guarded by rods, it has pitched down upon the point of the rod; and has not fallen upon any other part of the house. Had the lightning fallen on this chimney, unfurnished with a rod, it would probably have rent it from top to bottom, as we see, by the effects of the lightning on the points and rod, that its quantity was very great; and we know that many chimneys have been so demolished. But no part of this was damaged, only (f) (g) (h) at the foundation, where it was shattered and several bricks torn out. Here we learn the principal defect in fixing this rod. The lower joint being sunk but three feet into the earth, did not it seems go low enough to come at water, or a large body of earth so moist as to receive readily from its end the quantity it conducted. The electric fluid therefore thus accumulated near the lower end of the rod, quitted it at the surface of the earth, dividing in search of other passages. Part of it tore up the surface in furrows, and made holes in it: Part entered the bricks of the foundation, which being near the earth are generally moist, and, in exploding that moisture, shattered them. (See page 415.2) Part went through or under the foundation, and got under the hearth, blowing up great part of the bricks (m) (s), and producing the other effects (o) (p) (q) (r). The iron dogs, loggerhead and iron pot were not hurt, being of sufficient substance, and they probably protected the cat. The copper tea kettle being thin, suffered some damage. Perhaps, tho’ found on a sound part of the hearth, it might at the time of the stroke have stood on the part blown up, which will account both for the bruising and melting.
That it ran down the inside of the chimney (k) I apprehend must be a mistake. Had it done so, I imagine it would have brought something more than soot with it; it would probably have ripp’d off the pargetting, and brought down fragments of plaister and bricks. The shake, from the explosion on the rod, was sufficient to shake down a good deal of loose soot. Lightning does not usually enter houses by the doors, windows, or chimneys, as open passages, in the manner that air enters them: Its nature is, to be attracted by substances, that are conductors of electricity; it penetrates and passes in them, and, if they are not good conductors, as are neither wood, brick, stone nor plaister, it is apt to rend them in its passage. It would not easily pass thro’ the air from a cloud to a building, were it not for the aid afforded it in its passage by intervening fragments of clouds below the main body, or by the falling rain.
It is said that the house was filled with its flash (1). Expressions like this are common in accounts of the effects of lightning, from which we are apt to understand that the lightning filled the house. Our language indeed seems to want a word to express the light of lightning as distinct from the lightning itself. When a tree on a hill is struck by it, the lightning of that stroke exists only in a narrow vein between the cloud and the tree, but its light fills a vast space many miles round; and people at the greatest distance from it are apt to say, “the lightning came into our rooms through our windows.” As it is in itself extreamly bright, it cannot, when so near as to strike a house, fail illuminating highly every room in it through the windows; and this I suppose to have been the case at Mr. Maine’s; and that, except in and near the hearth, from the causes abovementioned, it was not in any other part of the house; the flash meaning no more than the light of the lightning. It is for want of considering this difference, that people suppose there is a kind of lightning not attended with thunder. In fact there is probably a loud explosion accompanying every flash of lightning, and at the same instant; but as sound travels slower than light, we often hear the sound some seconds of time after having seen the light; and as sound does not travel so far as light, we sometimes see the light at a distance too great to hear the sound.
(n) The breaking some pieces of china in the beaufet, may nevertheless seem to indicate that the lightning was there: But as there is no mention of its having hurt any part of the beaufet, or of the walls of the house, I should rather ascribe that effect to the concussion of the air, or shake of the house by the explosion.
Thus, to me it appears, that the house and its inhabitants were saved by the rod, though the rod itself was unjointed by the stroke; and that, if it had been made of one piece, and sunk deeper in the earth, or had entered the earth at a greater distance from the foundation, the mentioned small damages (except the melting of the points) would not have happened.
9. The letter to Kinnersley is printed as Letter XXXVIII and the papers associated with it follow immediately as Letters XXXIX and XL. These documents also appear with the same numbers in the 1774 edition of Exper. and Obser., pp. 405–35.
1. See above, IX, 282–93. Many of the matters BF discusses are in response to parts of Kinnersley’s letter. Footnote references to the appropriate pages are given below.
2. Above, IX, 283.
3. BF had mentioned Dr. William Cullen’s experiments on evaporation in a letter to Dr. John Lining, April 14, 1757; see above, VII, 184. Cullen’s report is in the Edinburgh Philosophical Society’s Essays and Observations, Philosophical and Literary, II (1755), 145–75.
5. Above, IX, 283–4.
6. A Treatise on Electricity (London, 1750), pp. 143–4.
7. Matthew Boulton (1728–1809), engineer, inherited in 1759 his father’s silver-stamping and piercing plant at Birmingham and soon after BF’s visit founded the important works at Soho. Subsequently he became associated with James Watt in developing the steam engine and provided the working capital that made Watt’s great achievement possible. He had many scientific interests and was elected F.R.S. in 1785. DNB. On BF’s visit to Birmingham in 1760, see above, IX, 231 n.
8. What follows here is the full text of Canton’s letter printed above, IX, 240.
9. Charles Cavendish (c. 1693–1783), 3d son of the 2d Duke of Devonshire; M.P., 1725–41; F.R.S., 1727; awarded the Copley Medal, 1757, for his experiments on thermometers. He was a scientist of distinction but has been overshadowed by his son Henry Cavendish (1731–1810), later associated with BF on the Purfleet Committee of 1772. A. Wolf, A History of Science, Technology, and Philosophy in the Eighteenth Century (2d edit., 1961), pp. 225, 243–4, 313–15. The quotation from one of his papers BF gives here apparently appeared in print for the first time in 1769, when BF published this letter to Kinnersley. The only publication of an entire paper by Charles Cavendish appears to be one on thermometers in Phil. Trans., L (1757), 300–10.
1. In Exper. and Obser., 1769 edit., these words are followed by “(See Plate VI.),” obviously referring to the plate printed on the facing page and reproduced here on p. 43, although the plate in question is number “VII.” The reference was corrected in the 1774 edition.
2. Kinnersley had written that he had begun to be doubtful of the “Doctrine of Repulsion in Electrised Bodies”; above, IX, 284. For an analysis of this discussion, see I. Bernard Cohen, Franklin and Newton (Phila., 1956), pp. 531–4.
3. Above, IX, 284.
6. Above, IX, 285.
7. Above, IX, 286–9.
8. Above, IX, 291–3.
9. For these accounts see the two extracts reprinted immediately below.
1. This is BF’s first mention of a disagreement which developed into a major controversy among electrical scientists in Great Britain. It will be dealt with in considerable detail in connection with the so-called Purfleet Committee of 1772.
2. The final paragraph of Kinnersley’s letter (above, IX, 293) had extolled BF’s “important Discovery” of the lightning rod: “May the Benefit thereof be diffused over the whole Globe. May it extend to the latest Posterity of Mankind; and make the Name of Franklin like that of Newton, immortal.”
3. For Lord Marischal’s interest in the lightning rod, however, see above, p. 17, and below, pp. 81, 82–3. It is not certain whether he intended to use it on his residence at Neuchâtel or on his family home, Keith Hall, in Scotland.
4. In his letter to Hume mentioning this incident, BF referred to “the house of Mr. Raven, at John’s Island, near Charlestown, South Carolina.” Above, p. 19. The community of Johns Island is near the Stono River and about ten miles west of Charleston. A John Raven was a member in 1750 of the Charleston Library Company of which Dr. John Lining was president. Another John Raven, possibly a son, died in 1764, aged 38. So. Car. Hist. and Geneal. Mag., XXIII (1922), 168; XXIX (1928), 240.
5. BF doubtless added this and the next note in printing this account in 1769.
6. William Maine (d. 1776) came to So. Car. from Ireland and in about 1756 purchased Jericho Plantation, about 22 miles west of Charleston. Here he planted mulberry trees as part of a silkworm project. Transactions of the Huguenot Society of South Carolina, no. 64 (1959), pp. 34–5. John R. Todd and Francis M. Hutson, Prince William’s Parish and Plantations (Richmond, 1935), p. 215.
7. This and other italicised letters were apparently inserted by BF when he printed this account in order to provide reference points for his comments in the “Remarks” that follow.
8. BF probably wrote these remarks in 1769 specifically for inclusion in Exper. and Obser.
9. This and other page references below are to the 1769 edition of Exper. and Obser. The two passages cited here are found in the present edition, V, 78; VI, 98–9.
1. Above, VI, 100.
2. Above, p. 51.