« PreviousContinue »
or shades of colours. I laid them all out upon the snow in a bright sunshiny morning. In a few hours (I cannot now be exact as to the time) the black, being warmed most by the sun, was sunk so low as to be below the stroke of the sun's rays; the dark blue almost as low, the lighter blue not quite so much as the dark, the other colours less as they were lighter; and the quite white reinairded on the surface of the snow, not having entered it at all.
What signifies philosophy that does not apply to some use? — May we not learn from hence, that black clothes are not so fit to wear in a hot sunny climate or season, as white ones; because in such clothes the body is more heated by the sun when we walk abroad, and are at the same time heated by the exercise, which double heat is apt to bring on putrid dangerous fevers? That soldiers and seamen,
who must march and labour in the sun, should in the East or West Indies have an uniform of white ? That summer hats, for men or women, should be white, as repelling that heat which gives head-achs to many, and to some the fatal stroke that the French call the coup de soleil? That the ladies summer hats, however, should be lined with black, as not reverberating on their faces those rays which are reflected upwards from the earth or water? That the putting a white cap
paper or linen within the crown of a black hat, as some do, will not keep out the heat, though it would if placed without. That fruit-walls being blacked may receive so much heat from the sum in the day-time, as to continue warm in some degree through the night, and thereby preserve the fruit from frosts, or forward its growth? with sundry other particulars of less or greater importance, that will occur from time to time to attentive minds? -I am,
TO PETER COLLINSON, ESQ. F. R. S.
Philadelphia, Oct. 19, 1752. SIR, As frequent mention is made in public papers from Europe of the success of the Philadelphia experiment for drawing the electric fire from clouds by means of pointed rods of iron erected on high buildings, etc. it may be agreeable to the curious to be informed that the same ex
#periment has succeeded in Philadelphia, though made in a different and more easy mayner, which is as follows:
Make a small cross of two light strips of cedar, the arms so long as to reach to the four corners of a large thin silk hanilkerchief when extended; tie the corners of the handkerchief to the extremities of the cross, so you have the body of a kite; which being properly accommodated with a tail, loop, and string, will rise in the air, like those made of paper; but this being of silk is fitter
to bear the wet and wind of a thunder-gust without tearsing. To the top of the upright stick of the cross is to 3 be fixed a very sharp pointed wire, rising a foot or more
above the wood. To the end of the twine, next the hand, is to be tied a silk ribbon, and where the silk and twine join, a key may be fastened. This kite is to be raised when a thunder-gust appears to be coming on, and the person who holds the string must stand within a door or window, or under some cover, so that the silk ribbon may not be wet; and care must be taken that the twine does not touch the frame of the door or window. As soon as any of the thunder clouds come over the kite, the pointed wire will draw the electric fire from them, and the kite, with all the twine, will be electrified, and the loose filaments of the twine will stand out every way, and be attracted by an approaching finger. And when the rain has wetted the kite and twine, so that it can conduct tlie electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle.
At this key the phial may be charged; and from electric fire thus obtained, spirits may be kindled, and all the other electric experiments be performied, which are usually done by the help of a rubbed glass globe or tube, and thereby the sameness of the electric matter with that of lightniug completely demonstrated.
TO THE SAME.
Philadelphia. Sept. 1753.
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 Inminous tappearance 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 60 embraced this opinion too hastily,
For in 1750, and 1751, being occasionally on the sea. coast, 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 bard gales of wind, by their friction against trees, hills, build. ings, etc. as so many minute electric globes, rubbing against non-electric cushions, draw the electric fire from the earth, and that the rising vapours 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 bel. lows, I should electrify 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, 1 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 rang 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 ne. gatively; but iny 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 and 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 cieling, 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 negra tively, 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 niortification to me, that I happened to be abroad during two of the greatest thunderstorms we had early in the spring; and though I had given orders in my 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 surprize and pleasure, the cork ball play briskly between them; and was convinced that one bottle was electrised 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 electrises positively, concluded that the clouds are always electrised negatively, or have always in them less than their natural quantity of the electric fluid.
Yet, notwithstanding so many experiments, it seems ! 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 electrised positively, though several that passed over ny 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 electrised positivelg , 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 Huid 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, 1 exhausted the natural quantity froin 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 thé 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 coinmonly 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; yet Mr. Kinnersley, returning from the Islands just as I left home, pursued the experiments during my absence, and informs me that he always found the clouds in the negative state.
So that, for the most part, in thunder-strokes, it is the earth that strikes into the clouds, and not the clouds that strike into the earth.
Those who are versed in electric experiments, will easily conceive, that the effects and appearances must be nearly the same in either case; the same explosion, and the same flash between one cloud and another, and be. tween the clouds and mountains, etc. the same rending of trees, walls, etc. which the electric fluid meets with in its passage, and the same fatal shock to animal bodies; and that pointed rods fixed on buildings, or masts of ships, and communicating with the earth or sea, must be of the same service in restoring the equilibrium silently between the earth and clouds, or in conducting a flash or stroke, if one should be, so as to save harmless the house or vessel: for points have equal power to throw off, as to draw on, the electric fire, and rods will conduct up as well as down.
But though the light gained from these experiments makes no alteration in the practice, it makes a consider