4 CONNECTION OF SOUND WITII MOTION. [I. § 2. explosion of a powder-magazine is capable of shatter¬ ing the windows of houses at several miles’ distance. Sounds of moderate loudness, such as the rattle of carriage-wheels, the stamping of feet, the clapping of hands, are produced by movements of solid bodies which cannot take place without setting up a very perceptible agitation of the air. In the case of weaker sounds, the accompanying air-motion cannot, it is true, be ordinarily thus recognized ; but, even here, a little attention will 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 recognized 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 convey¬ ing 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 mo¬ tion of a material medium constitutes the mechanical impulse which, falling on the ear, excites within it