The _actual velocities_ of wind and sound differ so widely that the small fraction by which their _relative velocity_ is denoted is held as proof that the propagation of sound--_the pulse_--through distances of a few yards or feet, is not affected by currents of air: that there are no _differences in the “velocity of sound.”_ Yet the ear detects them as one of the small differences between discord and harmony in music; distinctness and confusion of speech. In music these differences may be blended by the prolonged intonation of _vowel sounds_; but in speech, whose distinct significance is due to _consonants_, “which cannot be sounded without the aid of a vowel,” these differences are fatally evident. The sharp edges of the vocal pulses, which give shape and meaning to vowel sounds, are destroyed alike by a husky voice and a puff of air. What remains is _vox et præterea nihil_.
It seems to us that some of the many failures in practical acoustics come from considering the air--the material involved--as perfectly elastic. From this it is inferred that sound is not affected by the direction of the initial _pulse_: that the direction and velocity of the _effective_ pulse are not varied by currents and blasts of air. In short, that the slight inaccuracy of these assumptions will be the actual measurement of resultant error.
Were the purpose only to ascertain the _acoustic_ properties of unadulterated air, varied experiments might eliminate the errors of anomalous results. But when the process is reversed, and we deduce _effects_ from _only one_ among concurrent and conflicting causes, theory is confounded by discordant facts. Theories of _sound in purely elastic air_ might give results approximately realized in practice, if the actual pulses with which we are concerned were given by a flail; but are pregnant of error when the atmosphere is mixed with vicious vapors, and the _pulse_ is a breath of air. Then, the assumption that “_pulses of sound_” proceed _equally_ in all directions from the initial point, is simply false; and theories based upon it can only complicate the problems to be solved.
Water, as well as air, is a highly elastic fluid, and, if confined and subjected to pressure, the force applied is exerted on all sides of the confined volume. But the _effect_ of a _pulse_ or blow upon a surface of large extent varies with the _direction_ of the force as well as with its _power_ and _velocity_. We have seen fish swimming near the surface killed or paralyzed by a blow upon the water immediately over them. And we have seen the blow fail of its intended effect _solely_ because it was _misdirected_. Perhaps the water in the latter case was _not perfectly elastic_! Neither is the air of churches and public halls, when their atmosphere has yielded a portion of its _oxygen_, and, in return, is charged with _carbonic acid_ and moist vapors from the breath of crowded assemblies. _Carbonic acid gas_ is heavier _by one-half_ than atmospheric air. It does not, then, always rise toward the ceiling or roof, but remains in solution with impure exhalations; or else, condensed by contact with the colder walls, descends to poison the lower air and impair its elastic force--its power of transmitting the “_pulse of sound_” to the ear.