What is meant by affecting a mass of matter “_in all its parts_,” by “displacing and replacing the particles of a _determinate volume_,” we do not precisely understand. That whatever causes motion does it “as far as the motion reaches,” is as unquestionable as any other identical proposition. But that the velocity of the movement of the particles, created by the concussion of shocks, pressure, _upon an unconfined elastic fluid_, depends _solely_ upon its elasticity and density, we dispute. That _pulses_ “are propagated from a _trembling body_ all around in a _spherical_ manner” may be true, if the air is on all sides equally elastic. Such might be the case with those produced by the vibrations of a _bell_, when the surrounding air is undisturbed by other causes, and is uniformly elastic at equal distances from it. It would not be strictly true if the initial pulse were made only in a certain direction. “Every impression made on a fluid is propagated every way throughout the fluid, whatever be the direction wherein it is made;” but it is not true that the impressions are equal at equal distances from the initial pulse, irrespective of its _direction_. This result would presuppose a fluid _perfectly_ elastic; which we never have--and _then_ we might, with equal truth, say that the impressions would be equal at all distances.
Everybody is familiar with the fact that the “transmission of sound,” the pulse which strikes upon the ear to produce the sensation, _is affected_ by currents of air--the direction, force, and velocity of the wind--between the initial pulse and the hearer. _How_? and _how much_? _directly_ or _indirectly_? are questions distinct from the fact itself. The distance through which guns are heard, as well as the loudness of their report, varies with the _direction_, _force_, and _velocity_ of the wind; and, in very still air, with the _aim_ of the gun itself, the _direction of the initial pulse_. For short distances, these differences may be so minute as to escape notice; just as the false proportions of a miniature picture are unobserved until the magnifier displays them. And for longer ranges, they are so small, in contrast with the magnitudes compared, as to seem rather like _accidental_ than _legitimate_ differences. But the difference is not the less real because the reality is less. Words spoken in a faint whisper are clearly heard by a listener immediately _before_ the speaker, when quite inaudible or indistinct to one at an equal distance _behind_ him.