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58 MEASURE OF PITCH. [II
set up in, the external air, during any given interval of time. If, when a note of given pitch has been attained by the Syren, we check any further increase of velocity and cause the disc to rotate uniformly at the rate which it has just reached, no further alteration of pitch will occur, and the note will be steadily held by the instrument so long as the uniform rotation of its disc is kept up. Hence the number of aerial vibrations executed in a given time determines the pitch of the sound heard.
28. The Syren, besides teaching us this most important fact, gives us the means of determining the number of vibrations corresponding to any given note. If we know the number of rotations which the disc has performed in a given time, we have only to multiply this number by the number of holes on the disc in order to ascertain how many tube-discharges have occurred, and therefore how many corresponding aerial vibrations have been performed, in the period in question. The Syren is provided with a counting-apparatus which registers the number of times its disc rotates per second.
In order, therefore, to obtain the number of vibrations in a second which correspond to an assigned note, we have only to proceed as follows. Let the note be steadily sounded by some instrument of sustained power, e.g. organ or harmonium, and then