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1762: CCIV: TO DAVID HUME - Benjamin Franklin, The Works of Benjamin Franklin, Vol. III Letters and Misc. Writings 1753-1763 [1904]Edition used:The Works of Benjamin Franklin, including the Private as well as the Official and Scientific Correspondence, together with the Unmutilated and Correct Version of the Autobiography, compiled and edited by John Bigelow (New York: G.P. Putnam’s Sons, 1904). The Federal Edition in 12 volumes. Vol. III (Letters and Misc. Writings 1753-1763).
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CCIVTO DAVID HUMELondon, 24 January, 1762. Dear Sir:—In compliance with my Lord Marischal’s request, communicated to me by you, when I last had the pleasure of seeing you, I now send you what at present appears to me to be the shortest and simplest method of securing buildings, &c., from the mischiefs of lightning. Prepare a steel rod five or six feet long, half an inch thick at its biggest end, and tapering to a sharp point; which point should be gilt to prevent its rusting. Let the big end of the rod have a strong eye or ring of half an inch diameter: Fix this rod upright to the chimney or highest part of the house, by means of staples, so as it may be kept steady. Let the pointed end be upwards, and rise three or four feet above the chimney or building that the rod is fixed to. Drive into the ground an iron rod of about an inch diameter, and ten or twelve feet long, that has also an eye or ring in its upper end. It is best that the rod should be at some distance from the foundation of the building, not nearer than ten feet, if your ground will allow so much. Then take as much length of iron rod of about half an inch diameter, as will reach from the eye in the rod above, to that in the rod below; and fasten it securely to those rods, by passing its ends through the rings, and bending those ends till they likewise form rings. This length of rod may either be in one or several pieces. If in several, let the ends of the pieces be also well hooked to each other. Then close and cover every joint with lead, which is easily done, by making a small bag of strong paper round the joint, tying it close below, and then pouring in the melted lead; it being of use in these junctures, that there should be considerable quantity of metalline contact between piece and piece. For, if they were only hooked together and so touched each other but in points, the lightning, in passing through them, might melt and break them where they join. The lead will also prevent the weakening of the joints by rust. To prevent the shaking of this rod by the wind, you may secure it by a few staples to the building, till it comes down within ten feet of the ground, and thence carry it off to your ground rod; near to which should be planted a post, to support the iron conductor above the heads of people walking under it. If the building be large and long, as an hundred feet or upwards, it may not be amiss to erect a pointed rod at each end, and form a communication by an iron rod between them. If there be a well near the house, so that you can by such a rod form a communication from your top rod to the water, it is rather better to do so than to use the ground rod above mentioned. It may also be proper to paint the iron, to render it more durable by preserving it from rust. A building thus guarded will not be damaged by lightning, nor any person or thing therein killed, hurt, or set on fire. For either the explosion will be prevented by the operation of the point, or, if not prevented, then the whole quantity of lightning exploded near the house, whether passing from the cloud to the earth, or from the earth to the cloud, will be conveyed in the rods. And though the iron be crooked round the corner of the building, or make ever so many turns between the upper and lower rod, the lightning will follow it, and be guided by it, without affecting the building. I omit the philosophical reasons and experiments on which this practice is founded, for they are many, and would make a book. Besides they are already known to most of the learned throughout Europe. In the American British colonies many houses have been, since the year 1752, guarded by these principles. Three facts have only come to my knowledge of the effects of lightning on such houses. If I have not been explicit enough in my directions, I shall, on the least intimation, endeavour to supply the defect. I am, &c.B. Franklin. CCVTO E. KINNERSLEYLondon, 20 February, 1762. Sir:—I received your ingenious letter of the 12th of March last, 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 vapor ascending from electrized water, 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 ceiling, hung opposite to, and within an inch of, the bullet. I then sprinkled the heated plate with water, which arose fast from it in vapor. If vapor 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 electrized. 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 vapor. 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; and I was present at one made by Dr. Hadley, then Professor of Chemistry at Cambridge, 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 sixty-five, the state of warmth in the common air, to seven, which is twenty-two 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. 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 shown me my mistake. Mr. Wilson had formerly acquainted us that red-hot glass would conduct electricity; 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, charged and then sealed hermetically, might retain its electricity for ever; but having afterwards some suspicion that possibly that subtile 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 place 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 October 31st, 1760, viz.: “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 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 phial, 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 phial 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 two hundred and ten, sufficient to render the extreme thin glass in a Florence flask permeable even to a shock. Lord Charles Cavendish, by a very ingenious experiment, has found the heat of four hundred requisite to render thicker glass permeable to the common current. “A glass tube (see Plate I.), 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. The wire A being electrified by a machine, before the heat was applied, the corks at E separated, at first upon the principle of the Leyden phial. 
But after the part CB of the tube was heated to six hundred, 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 four hundred.” 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 looked upon and mentioned the equal repulsion, in cases of positive and of negative electricity, as a phenomenon difficult to be explained. 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, shows 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—that is, 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—that is, 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 touched with the smallest spark of fire, there is the seeming repulsion between the same poles of the magnet, a body containing a subtile movable fluid in many respects analogous 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 pushed (or at least appear to be pushed) 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 would be solved by it. But I find, or think I find, the two causes more convenient than one of them alone. Thus I might 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 electrized positively, repels the points; when negatively, the air opposite the points also, by their means, electrized 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 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,”1 is, I think, not well founded, or else not well expressed. 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 anybody. 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 increased, 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.” But I suppose that, like them, it does not easily part with what it has, or receive more, unless when mixed with some non-electric, 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 electrized 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 electrized the air negatively. The difference was easily discovered by his cork balls, as he passed out of one room into another. Père Beccaria, too, has a pretty experiment, which shows that air may be electrized. Suspending a pair of small light balls, by flaxen threads, to the end of his prime conductor, he turns his globe some time, electrizing 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 widely 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 electrized 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 electrized plus or minus, as well as when that is in its natural state and they are electrized 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 inquiry 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. 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 electrizing 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 electrized 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. I formerly satisfied myself, by an experiment with my phial and siphon, that the electricity 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, seems 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 vapor, by the electric fluid passing through it, might occasion some expansion which would be gradually reduced by the condensation of such vapor. 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 of 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 color, 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 continue a 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 a 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 shows, 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. 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 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 vapor, whereby a force is generated that may damage the foundation. Water reduced to vapor is said to occupy fourteen thousand 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 vapor, 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 the 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 watery fluid, which, when reduced to vapor rends every tube lengthwise. And perhaps it is this rarefaction of the fluid in animal bodies killed by lightning or electricity, that, by separating its fibres, renders the flesh so tender, and apt so much sooner to putrefy. 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. 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 may 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 electrized cloud, knobs draw them nearer. An experiment, which, I believe, I have shown you, of cotton fleece hanging from an electrized 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. Here it is very little regarded; so little, that, though it is now seven or eight years since it was made public, I have not heard of a single house as yet attempted to be secured by it. 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 expense 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.1 I return them cordially; being, with great regard and esteem, my dear Sir, your affectionate friend and most obedient humble servant, B. Franklin. Accounts from Carolina (mentioned in the foregoing Letter) of the Effects of Lightning on two of the Rods commonly affixed to Houses there, for securing them against Lightning. Charleston, 1 November, 1760. . . . “It is some years since Mr. Raven’s rod was struck by lightning. 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 fixed 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 fixed to; but the brass wire, all down till it came opposite to the top of the gun-barrel, was destroyed.1 There the lightning made a hole through the wall, or back of the chimney, to get to the gun-barrel,2 down which it seems to have passed, 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 remained 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 damage, before these rods were invented.” Mr. William Maine’s Account of the Effects of the Lightning on his Rod, dated at Indian Land, in South Carolina, August 28, 1760. . . . “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 about 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) 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 nut, 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 buffet (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 the 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 a great part of the hearth was torn up (s). My wife’s sister, two children, and a negro wench were all who happened 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 stunned that they did not recover for some time; however, it pleased God that no farther mischief ensued. The kitchen, at ninety 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.” Remarks by Benjamin FranklinThe 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 Vol. II., page 431; Vol. III., page 77.) 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 shows, that a finger under it, being a blunt body, extends the cotton, drawing its lower point 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 electrized 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 supra, page 79.) Where the quantity of the electric fluid passing is too great for the conductor through 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 confined 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 learn 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 showing, 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 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 supra, page 388.) 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, though 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 ripped off the pargeting, and brought down fragments of plaster 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 plaster, it is apt to rend them in its passage. It would not easily pass through the air from a cloud to a building, were it not for the aid afforded 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 up with its flash (l). 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 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 window.” As it is in itself extremely 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 above mentioned, 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 buffet, may nevertheless seem to indicate that the lightning was there; but as there is no mention of its having hurt any part of the buffet, 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. CCVITO MISS MARY STEVENSONMonday morning, 8 March, 1762. Dear Polly:—Your good mamma has just been saying to me that she wonders what can possibly be the reason she has not had a line from you in so long a time. I have made no complaint of that kind, being conscious that, by not writing myself, I have forfeited all claim to such favor, though no letters give me more pleasure, and I often wish to hear from you; but indolence grows upon me with years, and writing grows more and more irksome to me. Have you finished your course of philosophy? No more doubts to be resolved? No more questions to ask? If so, you may now be at full leisure to improve yourself in cards. Adieu, my dear child, and believe me ever your affectionate friend, B. Franklin. P. S.—Respects to Mrs. Tickell, &c. Mamma bids me tell you she is lately much afflicted and half a cripple with the rheumatism. I send you two or three French Gazettes de Médecine, which I have just received from Paris, wherein is a translation of the extract of a letter you copied out for me. You will return them with my French letters on Electricity, when you have perused them. CCVIITO MISS MARY STEVENSONLondon, 22 March, 1762. I must retract the charge of idleness in your studies, when I find you have gone through the doubly difficult task of reading so big a book, on an abstruse subject, and in a foreign language. In answer to your question concerning the Leyden phial. The hand that holds the bottle receives and conducts away the electric fluid that is driven out of the outside by the repulsive power of that which is forced into the inside of the bottle. As long as that power remains in the same situation, it must prevent the return of what it had expelled; though the hand would readily supply the quantity if it could be received. Your affectionate friend, B. Franklin. CCVIIITO MRS. DEBORAH FRANKLINLondon, 24 March, 1762. My Dear Child:—I condole with you most sincerely on the death of our good mother,1 being extremely sensible of the distress and affliction it must have thrown you into. Your comfort will be, that no care was wanting on your part towards her, and that she had lived as long as this life could afford her any rational enjoyment. It is, I am sure, a satisfaction to me, that I cannot charge myself with having ever failed in one instance of duty and respect to her during the many years that she called me son. The circumstances attending her death were indeed unhappy in some respects; but something must bring us all to our end, and few of us shall see her length of days. My love to brother John Read, and sister and cousin Debby, and young cousin Johnny Read, and let them all know that I sympathize with them all affectionately. This I write in haste, Mr. Beatty having just called on me to let me know, that he is about to set out for Portsmouth, in order to sail for America. I am finishing all business here in order for my return, which will either be in the Virginia fleet, or by the packet of May next; I am not yet determined which. I pray God grant us a happy meeting. We are all well, and Billy presents his duty. Mr. Strahan has received your letter, and wonders he has not been able to persuade you to come over. Mrs. Stevenson desires her compliments; she expected Sally would have answered her daughter’s letter, that went with the gold needle. I have received yours by the last packet, and one from our friend Mr. Hughes. I will try to write a line to him if I have time. If not, please to tell him I will do all I can to serve him in his affair. Acquaint Mr. Charles Norris that I send him a gardener in Bolitho’s ship. The particulars of your letters I shall answer by the same ship. I can now only add that I am, as ever, my dear Debby, your affectionate husband, B. Franklin. CCIXFROM DAVID HUME TO B. FRANKLINEdinburgh, 10 May, 1762. Dear Sir:—I have a great many thanks to give you for your goodness in remembering my request, and for the exact description, which you sent me of your method of preserving houses from thunder. I communicated it to our Philosophical Society, as you gave me permission; and they desire me to tell you, that they claim it as their own, and intend to enrich with it the first collection, which they may publish. The established rule of our Society is, that, after a paper is read to them, it is delivered by them to some member, who is obliged, in a subsequent meeting, to read some paper of remarks upon it. It was communicated to our friend, Mr. Russel; who is not very expeditious in finishing any undertaking; and he did not read his remarks till the last week, which is the reason why I have been so late in acknowledging your favor. Mr. Russel’s remarks, besides the just praises of your invention, contained only two proposals for improving it. One was that in houses where the rain-water is carried off the roof by a lead pipe, this metallic body might be employed as a conductor to the electric fire, and save the expense of a new apparatus. Another was, that the wire might be carried down to the foundation of the house, and be thence conveyed below ground to the requisite distance, which would better secure it against accidents. I thought it proper to convey to you these two ideas of so ingenious a man, that you might adopt them, if they appear to you well founded. I am very sorry that you intend soon to leave our hemisphere. America has sent us many good things, gold, silver, sugar, tobacco, indigo, &c.; but you are the first philosopher, and indeed the first great man of letters, for whom we are beholden to her. It is our own fault that we have not kept him; whence it appears that we do not agree with Solomon, that wisdom is above gold, for we take care never to send back an ounce of the latter which we once lay our fingers upon. I saw yesterday our friend Sir Alexander Dick, who desires me to present his compliments to you. We are all very unwilling to think of your settling in America, and that there is some chance for our never seeing you again; but no one regrets it more than does, Dear Sir, |
