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in their solid state, and are not rarefied into vapor, as the water is in clouds. Perhaps some future experiments on vaporized water may set this matter in a clearer light.
One seemingly material objection arises to the new hypothesis, and it is this; if water, in its rarefied state, as a cloud, requires and will absorb more of the electric fluid than when in its dense state as water, why does it not acquire from the earth all its wants at the instant of its leaving the surface, while it is yet near, and but just rising in vapor? To this difficulty I own I cannot at present give a solution satisfactory to myself. I thought, however, that I ought to state it in its full force, as I have done, and submit the whole to examination.
And I would beg leave to recommend it to the curious in this branch of natural philosophy, to repeat with care and accurate observation the experiments. I have reported in this and former papers relating to positive and negative electricity, with such other relative ones as shall occur to them, that it may be certainly known whether the electricity communicated by a glass globe be really positive. And also I would request all, who may have an opportunity of observing the recent effects of lightning on buildings, trees, &c., that they would consider them particularly with a view to discover the direction. But in these examinations, this one thing is always to be understood, viz., that, a stream of the electric fluid passing through wood, brick, metal, &c., while such fluid passes in small quantity, the mutually repulsive power of its parts is confined and overcome by the cohesion of the parts of the body it passes through, so as to prevent an explosion; but, when the fluid comes in a quantity too great to be confined by such cohesion, it explodes, and rends or fuses
If it be wood,
the body that endeavoured to confine it. brick, stone, or the like, the splinters will fly off on that side where there is least resistance. And thus, when a hole is struck through pasteboard by the electrified jar, if the surfaces of the pasteboard are not confined or compressed, there will be a bur raised all round the hole on both sides the pasteboard; but if one side be confined, so that the bur cannot be raised on that side, it will be all raised on the other, which way soever the fluid was directed. For the bur round the outside of the hole is the effect of the explosion every way from the centre of the stream, and not an effect of the direction.
In every stroke of lightning, I am of opinion that the stream of the electric fluid, moving to restore the equilibrium between the cloud and the earth, does always previously find its passage, and mark out, as I may say, its own course, taking in its way all the conductors it can find, such as metals, damp walls, moist wood, &c., and will go considerably out of a direct course, for the sake of the assistance of good conductors; and that, in this course, it is actually moving, though silently and imperceptibly, before the explosion, in and among the conductors; which explosion happens only when the conductors cannot discharge it as fast as they receive it, by reason of their being incomplete, disunited, too small, or not of the best materials for conducting. Metalline rods, therefore, of sufficient thickness, and extending from the highest part of an edifice to the ground, being of the best materials and complete conductors, will, I think, secure the building from damage, either by restoring the equilibrium so fast as to prevent a stroke, or by conducting it in the substance of the rod as far as the rod goes, so that there shall be no explosion but what is above its point, between that and the clouds.
If it be asked, What thickness of a metalline rod may be supposed sufficient? in answer, I would remark, that five large glass jars, such as I have described in my former papers, discharge a very great quantity of electricity, which nevertheless will be all conducted round the corner of a book, by the fine filleting of gold on the cover, it following the gold the farthest way about, rather than take the shorter course through the cover, that not being so good a conductor. Now, in this line of gold, the metal is so extremely thin as to be little more than the color of gold, and on an octavo book is not in the whole an inch square, and therefore not the thirty-sixth part of a grain, according to M. Réaumur; yet it is sufficient to conduct the charge of five large jars, and how many more I know not. Now, I suppose a wire of a quarter of an inch diameter to contain about five thousand times as much metal as there is in that gold line; and, if so, it will conduct the charge of twenty-five thousand such glass jars, which is a quantity, I imagine, far beyond what was ever contained in any one stroke of natural lightning. But a rod of half an inch diameter would conduct four times as much as one of a quarter.
And with regard to conducting, though a certain thickness of metal be required to conduct a great quantity of electricity, and, at the same time, keep its own substance firm and unseparated; and a less quantity, as a very small wire, for instance, will be destroyed by the explosion; yet such small wire will have answered the end of conducting that stroke, though it become incapable of conducting another. And, considering the extreme rapidity with which the electric fluid moves without exploding, when it has a free passage, or complete metal communication, I should think a vast quantity would be conducted in a short time, either to or
from a cloud, to restore its equilibrium with the earth, by means of a very small wire; and therefore thick rods should seem not so necessary. However, as the quantity of lightning discharged in one stroke cannot well be measured, and in different strokes is certainly very various, in some much greater than in others; and as iron (the best metal for the purpose, being least apt to fuse) is cheap, it may be well enough to provide a larger canal to guide that impetuous blast than we imagine necessary; for, though one middling wire may be sufficient, two or three can do no harm. And time, with careful observations well compared, will at length point out the proper size to greater certainty.
Pointed rods erected on edifices may likewise often prevent a stroke, in the following manner. An eye so situated as to view horizontally the under side of a thunder-cloud, will see it very ragged, with a number of separate fragments, or petty clouds, one under another, the lowest sometimes not far from the earth. These, as so many stepping-stones, assist in conducting a stroke between the cloud and a building. To represent these by an experiment, take two or three locks of fine, loose cotton; connect one of them with the prime conductor by a fine thread of two inches (which may be spun out of the same lock by the fingers), another to that, and the third to the second, by like threads. Turn the globe, and you will see these locks extend themselves towards the table (as the lower small clouds do towards the earth), being attracted by it; but, on presenting a sharp point erect under the lowest, it will shrink up to the second, the second to the first, and all together to the prime conductor, where they will continue as long as the point continues under them. May not, in like manner, the small electrized clouds, whose equilibrium with the earth is soon restored
by the point, rise up to the main body, and by that means occasion so large a vacancy, as that the grand cloud cannot strike in that place?
These thoughts, my dear friend, are many of them crude and hasty; and, if I were merely ambitious of acquiring some reputation in philosophy, I ought to keep them by me, till corrected and improved by time and farther experience. But since even short hints and imperfect experiments in any new branch of science, being communicated, have oftentimes a good effect, in exciting the attention of the ingenious to the subject, and so become the occasion of more exact disquisition, and more complete discoveries, you are at liberty to communicate this paper to whom you please; it being of more importance that knowledge should increase, than that your friend should be thought an accurate philosopher.
TO PETER COLLINSON.
Notice of another Packet of Letters.
Philadelphia, 23 November, 1753.
DEAR FRIEND, In my last, via Virginia, I promised to send you per next ship, a small philosophical packet; but now, having got the materials (old letters and rough drafts) before me, I fear you will find it a great one. Nevertheless, as I am like to have a few days leisure before this ship sails, which I may not have again in a long time, I shall transcribe the whole, and send it; for you will be under no necessity of reading it all at once, but may take it a little at a time, now and then of a winter