Dear SIR, July 20, 1762.
I have perused your paper on sound, and would freely mention to you, as you desire it, every thing that appeared to me to need correction: — But nothing of that kind occurs to me, unless it be, where you speak of the air as “the best medium for conveying sound.” Perhaps this is speaking rather too positively, if there be, as I think there are, some other mediums that will convey it farther and more readily. — It is a well-known experiment, that the scratching of a pin at one end of a long piece of timber, may be heard by an ear applied near the other end, though it could not be heard at the same distance through the air. — And two stones being struck smartly together under water, the stroke may be heard at a greater distance by an ear also placed under water in the same river, than it can be heard through the air. I think I have heard it near a mile; how much farther it may be heard, I know not; but suppose a great deal farther, because the sound did not seem faint, as if at a distance, like distant sounds through air, but smart and strong, and as if present just at the ear. — I wish you would repeat these experiments now you are upon the subject, and add your own observations. — And if you were to repeat, with your naturally exact attention and observation, the common experiment of the bell in the exhausted receiver, possibly something new may occur to you, in considering,
1. Whether the experiment is not ambiguous; i. e. whether the gradual exhausting of the air, as it creates an increasing difference of pressure on the outside, may not occasion in the glass a difficulty of vibrating, that renders it less fit to communicate to the air without, the vibrations that strike it from within; and the diminution of the sound arise from this cause, rather than from the diminution of the air?
2. Whether as the particles of air themselves are at a distance from each other, there must not be some medium between them, proper for conveying 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 substracting 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 30 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 70 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.;