ON THE PITCH OF MUSICAL SOUNDS. 31 

One of the earliest instruments constructed for this purpose was Savart's toothed wheel (fig. 4) which has just been referred to. A registering apparatus, H, which can be instantly connected or disconnected with the toothed wheel by touching a spring, records the number of revolutions made by it. In order to discover the vibration number of any given sound, the velocity of the wheel must be gradually increased until the sound it produces is in unison with the given sound. The registering apparatus is now thrown into action, while the same speed of rotation is maintained for a certain time, say a minute. The number of revolutions the wheel makes during this time, multiplied by the number of teeth on the wheel, gives the number of vibrations performed in one minute. This number divided by 60 gives the number of vibrations per second required to produce the given sound, that is, its vibration number.
Cagniard de Latour's Syren is an instrument constructed on the same principle as the Wheel Syren, but different in detail. Two 







Fig. 20. Fig. 21.
brass discs, rather more than an inch in diameter, are each similarly pierced with the same number of holes arranged in a circle (figs. 20 & 21). The upper disc revolves with the least possible 
