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I VELOCITY OF SOUND. [I. § 3. |
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unmistakable way: a little attention will, however, usually detect a certain amount of movement on the part of the sound-producing apparatus, which is probably capable of being communicated to the surrounding air. Thus, a sounding pianoforte-string can be both seen and felt to be in motion : the movements of a finger-glass stroked on the rim by a wet finger can be recognised by observing the thrills which play on the surface of the water it contains: sand strewed on a horizontal drum head is thrown off when the drum is beaten. These considerations raise a presumption that Sound is invariably associated with agitation of the conveying medium—that it is impossible to produce a sound without at the same time setting the medium in motion. If this should prove to be the case, there would be ground for the further conjecture that motion of a material medium constitutes the mechanical impulse which, falling on the ear, excites within it the sensation we call Sound. Let us try to form an idea of the kind of motion which the conditions of the case require.
3. It will be convenient to begin by determining the rate at which Sound travels. This varies, indeed, with the nature of the conveying medium. It will suffice, however, for our present purpose to ascertain its velocity in air, the medium through which the vast majority of sounds reach our ears. As long as |
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