SIRENE

In Acoustics, an instrument for determining the velocity of aerial vibration, corresponding to the diflerent pitches of musical sounds. " In this elegant instrument the wind of a bellows is emitted through a small aperture, before which revolves a circular disc pierced with a certain number of holes, arranged in a circle concentric with the axis of rotation, exactly equidistant from each other, and of the same size, . The orifice through which the air pa es is so situated that each of these holes, during the rotation of the disc, shall pa over it, and let through the air; but the disc is made to revolve so near the orifice that in the intervals between the holes it shall act as a cover, and intercept the air. If the holes be placed obliquely, the action of the current of air alone will set the disc in motion; if perpendicular to the surface, the disc must be moved by wheel-work, by means of which its velocity of rotation is easily regulated, and the number of impulses may be exactly counted. The sound produced is clear and sweet,'like the human voice. If, instead of a single aper  [s. 1133]