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sufficient to knock a stout man down; and I believe a stroke from two or three, in the head, would kill him. Has Dr. Colden's new book reached you in Boston? If not, I will send it to you.
With great respect, I am, Sir,
Your most humble servant,
P. S. The glass-maker being from home, I cannot now get the account. The tin is laid on with common paste, made of flour and water boiled together, and the pieces may lap over each other a little.
TO PETER COLLINSON.
Hypothesis of the Sea being the grand Source of Lightning retracted. — Positive, and sometimes negative, Electricity of the Clouds discovered. New Experiments and Conjectures in Support of this Discovery. - Observations recommended for ascertaining the Direction of the Electric Fluid.—Size of Rods for Conductors to Buildings.
Appearance of a Thun
Philadelphia, September, 1753.
In my former paper on this subject, written first in 1747, enlarged and sent to England in 1749, I considered the sea as the grand source of lightning, imagining its luminous appearance to be owing to electric fire, produced by friction between the particles of water and those of salt. Living far from the sea, I had then no opportunity of making experiments on the sea water, and so embraced this opinion too hastily.
For, in 1750 and 1751, being occasionally on the
seacoast, I found, by experiments, that sea water in a bottle, though at first it would by agitation appear luminous, yet in a few hours it lost that virtue; hence and from this, that I could not by agitating a solution of sea salt in water produce any light, I first began to doubt of my former hypothesis, and to suspect, that the luminous appearance in sea water must be owing to some other principles.
I then considered whether it were not possible, that the particles of air, being electrics per se, might, in hard gales of wind, by their friction against trees, hills, buildings, &c., as so many minute electric globes, rubbing against non-electric cushions, draw the electric fire from the earth, and that the rising vapors might receive that fire from the air, and by such means the clouds become electrified.
If this were so, I imagined that by forcing a constant violent stream of air against my prime conductor, by bellows, I should eléctrify it negatively; the rubbing particles of air drawing from it part of its natural quantity of the electric fluid. I accordingly made the experiment, but it did not succeed.
In September, 1752, I erected an iron rod to draw the lightning down into my house, in order to make some experiments on it, with two bells to give notice when the rod should be electrified; a contrivance obvious to every electrician.
I found the bells rang sometimes when there was no lightning or thunder, but only a dark cloud over the rod; that sometimes, after a flash of lightning, they would suddenly stop; and at other times, when they had not rung before, they would, after a flash, suddenly begin to ring; that the electricity was sometimes very faint, so that, when a small spark was obtained, another could not be got for some time after; at other
times the sparks would follow extremely quick, and once I had a continual stream from bell to bell, the size of a crow-quill; even during the same gust there were considerable variations.
In the winter following I conceived an experiment, to try whether the clouds were electrified positively or negatively; but my pointed rod, with its apparatus, becoming out of order, I did not refit it till towards the spring, when I expected the warm weather would bring on more frequent thunder-clouds.
The experiment was this; to take two phials; charge one of them with lightning from the iron rod, and give the other an equal charge by the electric glass globe, through the prime conductor; when charged, to place them on a table within three or four inches of each other, a small cork ball being suspended by a fine silk thread from the ceiling, so as it might play between the wires. If both bottles then were electrified positively, the ball, being attracted and repelled by one, must be also repelled by the other. If the one positively, and the other negatively, then the ball would be attracted and repelled alternately by each, and continue to play between them as long as any considerable charge remained.
Being very intent on making this experiment, it was no small mortification to me, that I happened to be abroad during two of the greatest thunder-storms we had early in the spring; and, though I had given orders in the family, that, if the bells rang when I was from home, they should catch some of the lightning for me in electrical phials, and they did so, yet it was mostly dissipated before my return; and, in some of the other gusts, the quantity of lightning I was able to obtain was so small, and the charge so weak, that I could not satisfy myself; yet I sometimes saw what heightened my suspicions, and inflamed my curiosity.
At last, on the 12th of April, 1753, there being a smart gust of some continuance, I charged one phial pretty well with lightning, and the other equally, as near as I could judge, with electricity from my glass globe; and, having placed them properly, I beheld, with great surprise and pleasure, the cork ball play briskly between them, and was convinced, that one bottle was electrized negatively.
I repeated this experiment several times during the gust, and in eight succeeding gusts, always with the same success; and being of opinion (for reasons I formerly gave in my letter to Mr. Kinnersley, since printed in London), that the glass globe electrizes positively, I concluded, that the clouds are always electrized negatively, or have always in them less than their natural quantity of the electric fluid.
Yet, notwithstanding so many experiments, it seems I concluded too soon; for at last, June the 6th, in a gust which continued from five o'clock, P. M., to seven, I met with one cloud that was electrized positively, though several that passed over my rod before, during the same gust, were in the negative state. This was thus discovered.
I had another concurring experiment, which I often repeated, to prove the negative state of the clouds, viz., while the bells were ringing, I took the phial, charged from the glass globe, and applied its wire to the erected rod, considering, that if the clouds were electrized positively, the rod, which received its electricity from them, must be so too; and then the additional positive electricity of the phial would make the bells ring faster; but, if the clouds were in a negative state, they must exhaust the electric fluid from my rod, and bring that into the same negative state with themselves, and then the wire of a positively charged phial,
supplying the rod with what it wanted (which it was obliged otherwise to draw from the earth by means of the pendulous brass ball playing between the two bells), the ringing would cease till the bottle was discharged.
In this manner I quite discharged into the rod several phials, that were charged from the glass globe, the electric fluid streaming from the wire to the rod, till the wire would receive no spark from the finger; and, during this supply to the rod from the phial, the bells stopped ringing; but, by continuing the application of the phial wire to the rod, I exhausted the natural quantity from the inside surface of the same phials, or, as I call it, charged them negatively.
At length, while I was charging a phial by my glass globe, to repeat this experiment, my bells of themselves stopped ringing, and, after some pause, began to ring again. But now, when I approached the wire of the charged phial to the rod, instead of the usual stream, that I expected from the wire to the rod, there was no spark; not even when I brought the wire and the rod to touch; yet the bells continued ringing vigorously, which proved to me, that the rod was then positively electrified, as well as the wire of the phial, and equally so; and, consequently, that the particular cloud then over the rod was in the same positive state. This was near the end of the gust.
But this was a single experiment, which, however, destroys my first too general conclusion, and reduces me to this; That the clouds of a thunder-gust are most commonly in a negative state of electricity, but sometimes in a positive state.
The latter I believe is rare; for, though I, soon after the last experiment, set out on a journey to Boston, and was from home most part of the summer, which prevented my making farther trials and observations;