Imágenes de página

of a candle, though it may to a small one ; | Water reduced to vapour, is said to occupy and this not because the large wire resists 14,000 times its former space. I have sent a less that action of the flame which tends to charge through a small glass tube, that has separate its parts, but because it resists it borne it well while empty, but when filled more than the smaller wire; or because the first with water, was shattered to pieces and force being divided among more parts acts driven all about the room :-finding no part weaker on each.

of the water on the table, I suspected it to This reminds me, however, of a little ex- have been reduced to vapour ; and was conperiment I have frequently made, that shows, firmed in that suspicion afterwards, when I at one operation, the different effects of the had filled a like piece of tube with ink, and same quantity of electric fluid passing through laid it on a sheet of clean paper, whereon, after different quantities of metal. · A strip of tin- the explosion, I could find neither any moisfoil, three inches long, a quarter of an inch ture nor any sully from the ink. This expewide at one end, and tapering all the way to riment of the explosion of water, which I bea sharp point at the other, fixed between two lieve was first made by that most ingenious pieces of glass, and having the electricity of electrician, father Beccaria, may accuunt for a large glass jar sent through it, will not be what we sometimes see in a tree struck by discomposed in the broadest part; towards lightning, when part of it is reduced to fine the middle will appear melted in spots; splinters like a broom; the sap vessels being where narrower, it will be quite melted ; and so many tubes containing a watery fluid, about half an inch of it next the point will be which, when reduced to vapour, rends every reduced to smoke.

tube lengthways. And perhaps it is this raYou were not mistaken in supposing that refaction of the fluids in animal bodies killed your account of the effect of the pointed rod, by lightning or electricity, that, by separating in securing Mr. West's house from damage its fibres, renders the flesh so tender, and apt by a stroke of lightning, would give me so much sooner to putrefy. I think too, that great pleasure. I thank you for it most much of the damage done by lightning to heartily, and for the pains. you have taken in stone and brick walls may sometimes be giving me so complete a description of its si- owing to the explosion of water, found, during tuation, form, and substance, with the draft of showers, running or lodging in the joints of the melted point. There is one circumstance, small cavities or cracks that happen to be in viz. that the lightning was seen to diffuse it. the walls. self from the foot of the rod over the wet pave Here are some electricians that recommend ment, which seems, I think, to indicate that knobs instead of points on the upper end of the earth under the pavement was very dry, the rods, from a supposition that the points inand that the rod should have been sunk deep- vite the stroke. It is true that points draw er, till it came to earth moister, and therefore electricity at greater distances in the gradual apter to receive and dissipate the electric flu- silent way; but knobs will draw at the greatid. And although, in this instance, a conduct- est distance a stroke. There is an experior formed of nail rods, not much above a quar- ment that will settle this. Take a crooked ter of an inch thick, served well to convey wire of the thickness of a quill, and of such a the lightning, yet some accounts I have seen length as that one end of it being applied to from Carolina, give reason to think that larger the lower part of a charged bottle, the upper may be sometimes necessary, at least for the may be brought near the ball on the top of security of the conductor itself, which when the wire that is in the bottle. Let one end too small, may be destroyed in executing its of this wire be furnished with a knob, and the office, though it does, at the same time, pre- other may be gradually tapered to a fine point. serve the house. Indeed, in the construction When the point is presented to discharge the of an instrument so new, and of which we bottle, it must be brought much nearer before could have so little experience, it is rather it will receive the stroke, than the knob relucky that we should at first be so near the quires to be. Points besides tend to repel the truth as we seem to be, and commit so few fragments of an electrised cloud, knobs draw

them nearer. An experiment, which I beThere is another reason for sinking deeper lieve I have shown you, of cotton fleece the lower end of the rod, and also for turning hanging from an electrised body, shows this it outwards under ground to some distance clearly when a point or a knob is presented from the foundation; it is this, that water under it. dripping from the eaves falls near the founda You seem to think highly of the importance tion, and sometimes soaks down there in of this discovery, as do many others on our greater quantities, so as to come near the end side of the water. Here it is very little reof the rod, though the ground about it be dri- garded; so little, that though it is now seven er. In such case, this water may be exploded, or eight years since it was made public, I that is, blown into vapour, whereby a force is have not heard of a single house as yet atgenerated, that may damage the foundation. ) tempted to be secured by it. It is true the


mischiefs done by lightning are not so fre- remained good. No other damage, as I can quent here as with us, and those who calcu- learn, was done to the house. I am told the late chances may perhaps find that not one same house had formerly been struck by lightdeath (or the destruction of one house) in a ning, and much damaged, before these rods hundred thousand happens from that cause, were invented.”. 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 Mr. William Maine's Account of the Effects of buildings more exposed than others to such

of the Lightning on his Rod, dated at In

dian Land, in South Carolina, August accidents, and there are minds so strongly im

28, 1760. pressed with the apprehension of them, as to be very unhappy every time a little thunder "I HAD a set of electrical points, consistis within their hearing; it may therefore being of three prongs, of large brass wire tipt well to render this little piece of new know- with silver, and perfectly sharp, each about ledge as general and as well understood as seven inches long; these were rivetted at possible, since to make us safe is not all its equal distances into an iron nut about three advantage, it is some to make us easy. And quarters of an inch square, and opened at top as the stroke it secures us from might have equally to the distance of six or seven inches chanced perhaps but once in our lives, while from point to point, in a regular triangle. it may relieve us a hundred times from those This nut was screwed very tight on the top painful apprehensions, the latter may possibly of an iron rod of above half an inch diameter, on the whole contribute more to the happiness or the thickness of a common curtain-rod, of mankind than the former.

composed of several joints, annexed by hooks Your kind wishes and congratulations are turned at the ends of each joint, and the whole very obliging. I return them cordially ;-be fixed to the chimney of my house by iron staing, with great regard and esteem,

ples. The points were elevated (a) six or B. FRANKLIN.” seven inches above the top of the chimney;

and the lower joint sunk three feet in the Effects of Lightning in Carolina, earth, in a perpendicular direction. Referred to in the preceding Letter of the effive in the evening, when the lightning broke

Thus stood the points on Tuesday last about fects of Lightning on two of the rods commonly affixed to houses there, for securing them with a violent explosion on the chimney, cut against Lightning.

the rod square off just under the nut, and I am

persuaded, melted the points, nut, and top of "CHARLESTON, Nov 1, 1760.

the rod; entirely up; as after the most diligent It is some years since Mr. Ra- search, nothing of either was found (6,) and the ven's rod was struck by lightning. I hear top of the remaining rod was cased over with an account of it was published at the time, a congealed solder. The lightning ran down but I cannot find it. According to the best the rod, starting almost all the staples (ç) and information I can now get, he had fixed to the unhooking the joints without affecting the rod outside of his chimney a largę iron rod, seve- (d,) except on the inside of each hook where ral feet in length, reaching above the chim- the joints were coupled, the surface of which ney; and to the top of this rod the points was melted (e,) and left as cased over with were fixed. From the lower end of this rod, solder. No part of the chimney was damaged a small brass wire was continued down to the (f,) only at the foundation (8, where it was top of another iron rod driven into the earth. shattered almost quite round, and several On the ground-floor in the chimney stood a bricks were torn out (h.) Considerable cavities gun, leaning against the back wall, nearly were made in the earth quite round the founopposite to where the brass wire came down dation, but most within eight or nine inches on the outside. The lightning fell upon the of the rod. It also shattered the bottom weapoints, did no damage to the rod they were ther-board (i,) at one corner of the house, and fixed to; but the brass wire, all down till it made a large hole in the earth by the corner came opposite to the top of the gun-barrel, post. On the other side of the chimney, it was destroyed.* There the lightning made ploughed up several furrows in the earth, a hole through the wall or back of the chim. some yards in length. It ran down the inside ney, to get to the gun-barrel,t down which it of the chimney (k,) carrying only soot with seems to have passed, as, although it did not it; and filled the whole house with its flash, hurt the barrel, it damaged the butt of the (1) smoke, and dust. It tore up the hearth stock, and blew up some bricks of the hearth. in several places (m,) and broke some pieces The brass wire below the hole in the wall of china in the bæufet (n.) A copper tea

kettle standing in the chimney was beat to* A proof that it was not of sufficient substance to gether, as if some great weight had fallen conduct with safety to itself (though with safety so far upon it (0;) and three holes, each about half | A more substantial conductor

an inch diameter, melted through the bottom

(p.). What seems to me the most surprising weaker as the needle is shortened between the is, that the hearth under the kettle was not finger and thumb; and is reduced to nothing hurt, yet the bottom of the kettle was drove when only a short part below the point apinward, as if the lightning proceeded from pears above the finger. Now it seems the under it upwards (9.) and the cover was thrown points of Mr. Maine's rod were elevated only to the middle of the floor (r.) The fire (a) six or seven inches above the top of the dogs, an iron loggerhead, an Indian pot, an chim ; which, considering the bulk of the earthen cup, and a cat, were all in the chim-chimney and the house, was too small an eleney at the time unhurt, though a great part of vation. For the great body of the matter near the hearth was torn up (s.) My wife's sister, them would hinder their being easily brought two children, and a negro wench, were all into a negative state by the repulsive power who happened to be in the house at the time: of the electrised cloud, in which negative state the first, and one child sat within five feet of it is that they attract most strongly and copithe chimney; and were so stunned, that they ously the electric fluid from other bodies, and never saw the lightning nor heard the explo- convey it into the earth. sion; the wench, with the other child in her (6) Nothing of the points, fc. could be arms, sitting at a greater distance, was sensi- found. This is a common effect. (See page ble of both; though every one was so stunned 297.) Where the quantity of the electric that they did not recover for some time; how. fluid passing is too great for the conductor ever it pleased God that no farther mischief through which it passes, the metal is either ensued. The kitchen, at 90 feet distance, melted, or reduced to smoke and dissipated; was full of negroes, who were all sensible of but where the conductor is sufficiently large, the shock; and some of them tell me, that they the fluid passes in it without hurting it. Thus felt the rod about a minute after, when it was these three wires were destroyed, while the so hot that they could not bear it in hand. rod to which they were fixed, being of greater

substance, remained unhurt; its end only, to

which they were joined, being a little melted, Remarks by Dr. Franklin.

some of the melted part of the lower ends of The foregoing very sensible and distinct those wires uniting with it, and appearing on account may

afford a good deal of instruction it like solder. relating to the nature and effects of lightning, (c) (d) (e) As the several parts of the rod and to the construction and use of this instru- were connected only by the ends being bent ment for averting the mischiefs of it. Like round into hooks, the contact between hook other new instruments, this appears to have and hook was much smaller than the rod; been at first in some respects imperfect; and therefore the current through the metal being we find that we are, in this as in others, to confined in those narrow passages, melted part expect improvement from experience chiefly : of the metal, as appeared on examining the but there seems to be nothing in the account, inside of each hook. Where metal is melted that should discourage us in the use of it; by lightning, some part of it is generally exsince at the same time that its imperfections ploded; and these explosions in the joints apare discovered, the means of removing them pear to have been the cause of unhooking are pretty easily to be learnt from the circum- them; and, by that violent action, of starting stances of the account itself; and its utility also most of the staples. We learn from upon the whole is manifest.

hence, that a rod in one continued piece is One intention of the pointed rod, is, to pre- preferable to one composed of links or parts vent a stroke of lightning. (See pages 289, hooked together. 296.) But to have a better chance of obtain (No part of the chimney was damaged ; ing this end, the points should not be too near because the lightning passed in the rod. And to the top of the chimney or highest parts of this instance agrees with others in showing, the building to which they are affixed, but that the second and principal intention of the should be extended five or six feet above it; rods is obtainable, viz. that of conducting the otherwise their operation in silently drawing lightning. In all the instances yet known of off the fire (from such fragments of cloud as the lightning's falling on any house guarded float in the air between the great body of cloud by rods, it has pitched down upon the point of and the earth) will be prevented. For the the rod, and has not fallen upon any other experiment with the lock of cotton hanging part of the house. Had the lightning fallen below the electrified prime conductor shows, on this chimney, unfurnished with a rod, it that a finger under it, being a blunt body, ex- would probably have rent it from top to bottends the cotton, drawing its lower part down- tom, as we see, by the effects of the lightning wards; when a needle, with its point present on the points and rod, that its quantity was ed to the cotton, makes it fly up again to the very great ; and we know that many chimprime conductor; and that this effect is strong. neys have been so demolished. But no pas. est when as much of the needle as possible of this was damaged, only (F) (8) (h) at the appears above the end of the finger; grows foundation, where it was shattered and se VOL II. ... 2 R


veral bricks torn out. Here we learn the every room in it through the windows; and principal defect in fixing this rod. The lower this I suppose to have been the case at Mr. joint being sunk but three feet into the earth, Maine's; and that, except in and near the did not it seems go low enough to come at hearth, from the causes above-mentioned, it water, or a large body of earth so moist as to was not in any other part of the house; the receive readily from its end the quantity it flash meaning no more than the light of the conducted. The electric fluid, therefore, lightning. It is for want of considering this thus accumulated near the lower end of the difference, that people suppose there is a kind rod, quitted it at the surface of the earth, di- of lightning not attended with thunder. In viding in search of other passages. Part of fact there is probably a loud explosion acit tore up the surface in furrows, and made companying every flash of lightning, and at holes in it: part entered the bricks of the the same instant;-but as sound travels slower foundation, which being near the earth are than light, we often hear the sound some segenerally moist, and, in exploding that moist-conds of time after having seen the light; ure, shattered them. (See page 311.) Part and as sound does not travel so far as light, went through or under the foundation, and we sometimes see the light at a distance too got under the hearth, blowing up great part great to hear the sound. of the bricks (m) (s), and producing the other (n) The breaking some pieces of china in effects (o) (p) (r). The iron dogs, log- the bæufet, may nevertheless seem to indicate gerhead, and iron pot were not hurt, being that the lightning was there: but as there of sufficient substance, and they probably pro- is no mention of its having hurt any part of tected the cat. The copper tea-kettle being the bæufet, or of the walls of the house, I thin suffered some damage. Perhaps, though should rather ascribe that effect to the confound on a sound part of the hearth, it might cussion of the air, or shake of the house by at the time of the stroke have stood on the the explosion. part blown up, which will account both for

Thus, to me it appears, that the house and the bruising and melting.

its inhabitants were saved by the rod, though That it ran down the inside of the chimney the rod itself was unjointed by the stroke; (k) I apprehend must be a mistake. Had it and that, if it had been made of one piece, done so, I imagine it would have brought and sunk deeper in the earth, or had entered something more than soot with it; it would the earth at a greater distance from the founprobably have ripped off the pargetting, and dation, the mentioned small damages (except brought down fragments of plaster and bricks. the melting of the points) would not have The shake, from the explosion on the rod, was happened. sufficient to shake down a good deal of loose soot. Lightning does not usually enter houses by the doors, windows, or chimneys, as

Dr. Heberden, London. open passages, in the manner that air enters On the Electricity of the Tourmalin. them: its nature is, to be attracted by sub

CRAVEX-STREET, June 7, 1759. stances, that are conductors of electricity ; it I now return the smallest of your tourmapenetrates and passes in them, and, if they lins, with hearty thanks for the kind present are not good conductors as are neither wood, of the other, which though I value highly for brick, stone nor plaster, it is apt to rend them its rare and wonderful properties, I shall ever in its passage. It would not easily, pass esteem it more for the friendship I am honourthrough the air from a cloud to a building ed with by the giver. were it not for the aid afforded it in its pas I hear that the negative electricity of one sage by intervening fragments of clouds be- side of the tourmalin, when heated, is absolow the main body, or by the falling rain. lutely denied (and all that has been related of

It is said that the house was filled with its it ascribed to prejudice in favour of a system) flash (7). Expressions like this are common by some ingenious gentlemen abroad, who in accounts of the effects of lightning, from profess to have made the experiments on the which we are apt to understand that the light- stone with care and exactness. The experining filled the house. Our language indeed ments have succeeded differently with me; seems to want a word to express the light of yet I would not call the accuracy of these lightning as distinct from the lightning itself. gentlemen in question. Possibly the tourWhen a tree on a hill is struck by it, the malins they have tried were not properly cut; lightning of that stroke exists only in a nar so that the positive and negative powers were row vein between the cloud and tree, but its obliquely placed, or in some manner whereby light fills a vast space many miles round; their effects were confused, or the negative and people at the greatest distance from it are parts more easily supplied by the positive.apt to say, “The lightning came into our Perhaps the lapidaries who have hitherto cut rooms through our windows.” As it is in it- these stones, had no regard to the situation of self extremely bright, it cannot, when so near the two powers, but chose to make the faces of as to strike a house, fail illuminating highly the stone where they could obtain the great

est breadth, or some other advantage in the I heated the large one in boiling water. form. If any of these stones, in their natural Then I brought the large stone

near to the state, can be procured here, I think it would suspended small one, be right to endeavour finding, before they are Which immediately turned its flat side to cut, the two sides that contain the opposite the side B of the large stone, and would cling powers, and make the faces there. Possibly to it. in that case, the effects might be stronger, I turned the ring, so as to present the side and more distinct; for though both these A of the large stone, to the flat side of the stones that I have examined have evidently small one. the two properties, yet, without the full heat The flat side was repelled, and the small given by boiling water, they are somewhat stone, turning quick, applied its high side to confused; the virtue seems strongest towards the side A of the large one. one end of the face; and in the middle, or This was precisely what ought to happen, Dear the other end, scarce discernible; and on the supposition that the flat side of the the negative, I think, always weaker than the small stone, when heated in water, is positive, positive

and the high side negative; the side A of I have had the large one new cut, so as to the large stone positive, and the side B negamake both sides alike, and find the change of tive. form has made no change of power, but the The effect was apparently the sameas would properties of each side remain the same as I have been produced, if one magnet had been found them before. It is now set in a ring in suspended by a thread, and the different such a manner as to turn on an axis, that I poles of another brought alternately near it. may conveniently, in making experiments, I find that the face A, of the large stone, come at both sides of the stone. The little rim being coated with leaf-gold (attached by the of gold it is set in, has made no alteration in white of an egg, which will bear dipping in its effects. The warmth of my finger, when hot water) becomes quicker and stronger in I wear it, is sufficient to give it some degree its effect on the cork ball, repelling it the inof electricity, so that it is always ready to at- stant it comes in contact; which I suppose to tract light bodies.

be occasioned by the united force of the differThe following experiments have satisfied ent parts of the face, collected and acting to me that M. Æpinus's account of the positive gether through the metal. and negative states of the opposite sides of

B. FRANKLIN. the heated tourmalin is well founded.

I heated the large stone in boiling water.

As soon as it was dry, I brought it near a Professor Winthrop to B. Franklin. very small cork ball, that was suspended by a New Observation relating to Electricity in the silk thread.

Atmosphere. Cambridge, (Massachusetts,) The ball was attracted by one face of the Sept. 29, 1762. stone, which I call A, and then repelled. THERE is an observation relating to elec

The ball in that state was also repelled by tricity in the atmosphere, which seemed new the positively charged wire of a phial, and at- to me, though perhaps it will not to you: howtracted by the other side of the stone, B. ever, I will venture to mention it. I have

The stone being heated afresh, and the side some points on the top of my house, and the B brought near the ball, it was first attracted wire where it passes within-side the house is and presently after repelled by that side. furnished with bells, according to your me

In this second state it was repelled by the thod, to give notice of the passage of the elecnegatively charged wire of a phial.

tric fluid. In summer, these bells, generally Therefore, if the principles now general- ring at the approach of a thunder-cloud; but ly received, relating to positive and negative cease soon after it begins to rain. In winter, electricity, are true, the side A of the large they sometimes, though not very often, ring stone, when the stone is heated in water, is while it is snowing; but never, that I rememin a positive state of electricity; and the side ber, when it rains. But what was unexpectB, in a negative state.

ed to me was, that, though the bells had not The same experiments being made with rung while it was snowing, yet, the next day, the small stone stuck by one edge on the end after it had done snowing, and the weather of a small glass tube, with sealing-wax, the, was cleared up, while the snow was driven same effects are produced. The flat side of about by a high wind at W.or N. W. the bells the small stone gives the signs of positive rung for several hours (though with little inelectricity; the high side gives the signs of termissions) as briskly as ever I knew them, negative eleetricity.

and I drew considerable sparks from the wire. Again : I suspended the small stone by a silk The phenomenon I never observed but twice, thread.

viz. on the 31st of January, 1760, and the I heated it as it hung, in boiling water. 3d of March, 1762.-I am, sir, &c.

« AnteriorContinuar »