A complete view of Acoustical Science & its bearings on music, for musicians & music students.

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6                       HAND-BOOK OF ACOUSTICS.
Fig. 4.
lifted slightly by each cog as it passes, but is almost immediately released, and falls back against the succeeding one; that is, it vibrates once for every cog that passes it. As long as the wheel is revolving slowly, the card may thus be heard striking against each cog separately. If, however, the speed be increased, the taps will succeed one another so rapidly as to coalesce, and then a continuous sound will be heard.
The well-known Trevelyan's rocker is intended to illustrate the same thing. It consists of a rectangular-shaped piece of copper about 6 inches long, 2J inches broad, and 1 inch thick. The lower side is bevelled, and has a longitudinal groove running down the middle, as shown in fig. 5. Attached to one end is a somewhat
Fig. 5.
slender steel rod, terminating in a brass ball. If we place the rocker, with its bevelled face resting against a block of lead, and with the ball at the other end resting on the smooth surface of a table, it will rock from side to side on being slightly displaced, but not quickly enough to produce a musical sound. If, however, we hold the rocker in the flame of a Bunsen burner, or heat it over a fire, for a few minutes, and then place it as before against the leaden block, we shall find it giving forth a clear and continuous sound. This phenomenon may be explained thus :—When any