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56 CONTINUITY OF PITCH.
whenever a hole passes across the orifice b of the tube ab. During the intervals of time which elapse between the passage of adjacent holes across b, no air can pass through the disc. Hence, if the disc be rotating uniformly, a series of such discharges will succeed each other at precisely equal intervals of time. The air on the other side of the disc will necessarily be agitated by the process. Every time that air is driven through one of the holes, an increase of pressure occurs close to it, and accordingly a pulse of condensation is formed there. The elastic force of the air will give rise to a pulse of rarefaction during each interval between successive discharges. Hence the Syren supplies us with a regular series of the alternate condensations and rarefactions which, as we have seen, constitute waves of Sound.
27. While air is being blown steadily into the tube, let the disc be made to rotate slowly, and then with gradually increasing rapidity. At first nothing will be audible but a series of faint intermittent throbs, due to the impact of the air driven through the tube against the successive portions of the disc which separate its holes. This sound may be exactly reproduced by moving the fore-finger to and fro rapidly before the lips, while blowing through them. It contributes nothing to the proper musical sound of the instrument, and is only audible in its imme-