sound, since otherwise it would stop at the first particle?

3. Whether the great difference we experience in hearing sounds at a distance, when the wind blows towards us from the sonorous body, or towards that from us, can be well accounted for by adding to or subtracting from the swiftness of sound, the degree of swiftness that is in the wind at the time? The latter is so small in proportion, that it seems as if it could scarce produce any sensible effect, and yet the difference is very great. Does not this give some hint, as if there might be a subtile fluid, the conductor of sound, which moves at different times in different directions over the surface of the earth, and whose motion may perhaps be much swifter than that of the air in our strongest winds; and that, in passing through air, it may communicate that motion to the air which we call wind, though a motion in no degree so swift as its own?

4. It is somewhere related, that a pistol, fired on the top of an exceeding high mountain, made a noise like thunder in the valleys below. Perhaps this fact is not exactly related; but, if it is, would not one imagine from it, that the rarer the air, the greater sound might be produced in it from the same cause?

5. Those balls of fire which are sometimes seen passing over a country, computed by philosophers to be often thirty miles high at least, sometimes burst at that height; the air must be exceeding rare there, and yet the explosion produces a sound that is heard at that distance, and for seventy miles round on the surface of the earth, so violent too as to shake buildings, and give an apprehension of an earthquake. Does not this look as if a rare atmosphere, almost a vacuum, was no bad conductor of sound?

I have not made up my own mind on these points, and only mention them for your consideration, knowing that every subject is the better for your handling it.

With the greatest esteem, I am, &c.

B. FRANKLIN.

TO JOHN PRINGLE, IN LONDON.

Relating a curious Instance of the Effect of Oil on

SIR,

Water.

Philadelphia, 1 December, 1762.

During our passage to Madeira, the weather being warm, and the cabin windows constantly open for the benefit of the air, the candles at night flared and run very much, which was an inconvenience. At Madeira, we got oil to burn, and with a common glass tumbler or beaker, slung in wire, and suspended to the ceiling of the cabin, and a little wire hoop for the wick, furnished with corks to float on the oil, I made an Italian lamp, that gave us very good light all over the table. The glass at bottom contained water to about one third of its height; another third was taken up with oil; the rest was left empty that the sides of the glass might protect the flame from the wind. There is nothing remarkable in all this; but what follows is particular. At supper, looking on the lamp, I remarked, that though the surface of the oil was perfectly tranquil, and duly preserved its position and distance with regard to the brim of the glass, the water under the oil was in great commotion, rising and falling in irregular waves, which continued during the whole evening. The lamp was kept burning as a watch-light all night, till the oil was

VOL. VI.

spent, and the water only remained. In the morning I observed, that though the motion of the ship continued the same, the water was now quiet, and its surface as tranquil as that of the oil had been the evening before. At night again, when oil was put upon it, the water resumed its irregular motions, rising in high waves almost to the surface of the oil, but without disturbing the smooth level of that surface. And this was repeated every day during the voyage.

Since my arrival in America, I have repeated the experiment frequently thus. I have put a pack-thread round a tumbler, with strings of the same, from each side, meeting above it in a knot at about a foot distance from the top of the tumbler. Then putting in as much water as would fill about one third part of the tumbler, I lifted it up by the knot, and swung it to and fro in the air; when the water appeared to keep its place in the tumbler as steadily as if it had been ice. But pouring gently in upon the water about as much oil, and then again swinging it in the air as before, the tranquillity before possessed by the water was transferred to the surface of the oil, and the water under it was agitated with the same commotions as at sea.

I have shown this experiment to a number of ingenious persons. Those who are but slightly acquainted with the principles of hydrostatics, &c. are apt to fancy immediately that they understand it, and readily attempt to explain it; but their explanations have been different, and to me not very intelligible. Others, more deeply skilled in those principles, seem to wonder at it, and promise to consider it. And I think it is worth considering; for a new appearance, if it cannot be explained by our old principles, may afford us new ones, of use perhaps in explaining some other obscure parts of natural knowledge. I am, &c.

B. FRANKLIN.

Congelation of Quicksilver. - Cold produced by Evap

oration.*

Perth Amboy, 26 February, 1763.

THE most remarkable discovery that has been made within these three years is, that quicksilver is in reality a melted metal, with this character only, that of all others it requires the least heat to melt it. The Academy of Sciences at Petersburg have found, that by dipping a mercurial thermometer into repeated cooling mixtures, and so taking from the mercury the heat that was in it, they have brought it down some hundred degrees (the exact number I cannot remember) below the freezing point, when the mercury became solid and would sink no longer; and then the glass being broke, it came out in the form of a silver bullet adhering to a wire, which was the slender part that had been in the tube. Upon trial it was found malleable and was hammered out to the bigness of a half-crown, but soon after on receiving a small degree of warmth it returned gradually to its fluid state again. This experiment was repeated by several members of that Academy two winters successively, and an authentic account of it transmitted to our Royal Society.

I suppose you have seen, in the second volume of the new Philosophical Essays of the Edinburgh Society, an account of some experiments to produce cold by evaporation made by Dr. Cullen, who mentions the like having been before made at Petersburg. I think it is but lately, that our European philosophers have known or acknowledged any thing of such a power in

This is a fragment of a letter in the handwriting of Franklin, a part of which only has been found. It is not known to whom it was written. - EDITOR.

nature. But I find it has been long known in the east. Bernier, in the account of his travels into India, written above a hundred years since, mentions the custom of travellers carrying their water in flasks covered with wet wrappers, and hung to the pommels of their saddles, so as that the wind might act upon them, and so cool the water. I have also seen a kind of jar for cooling water, made of potter's earth glazed, and so porous that the water gradually oozed through to the surface, supplying water just sufficient for a constant evaporation. I tried it and found the water within much cooler in a few hours. This jar was brought from Egypt.

FROM JOHN CANTON TO B. FRANKLIN.*

The Melting of Metals by Lightning not a cold Fusion. - Compressibility of Water and other Fluids.

London, 29 June, 1764.

DEAR SIR, Your favor of the 14th of March came to my hands the 15th of May last, and gave me great pleasure. The first experiment of Mr. Kinnersley's, which you mention, is, as you observe, a beautiful one to see; and,

* Mr. Canton was a very ingenious experimental philosopher, and for many years an active and valuable member of the Royal Society. His papers in the Philosophical Transactions, and other publications, contributed much to the improvement of science. His fondness for electricity was the cause of an intimate friendship between him and Dr. Franklin, which continued till the death of Mr. Canton, on the 22d of March, 1772, in the fifty-fourth year of his age. His most important discovery was that of the compressibility of water, in opposition to the received opinion formed on the celebrated Florentine experiment. For this discovery he was honored by the Royal Society with the Copley Medal, in November, 1765. EDITOR.