426 PHYSIOLOGICAL PHYSICS. [Chap. xxxn. 



air again escapes, and the disc gets an increased 

 rotatory impulse. In a single revolution of the disc, 

 fifteen separate shocks will have been given to the air. 

 Now if the bellows be worked slowly, the plate D will 

 revolve slowly, and only the sounds of the separate 

 puffs of air will be heard. But if the bellows be 

 driven vigorously the disc will revolve faster and 

 faster, and an increasing number of shocks will be 

 given. With a certain velocity a low musical note 

 will begin to be heard, and as the speed increases, the 

 pitch will increase till the limit of velocity, when the 

 pitch will be very high. At the upper part of the 

 instrument are two wheels c and c, which register on 

 dial plates in front the number of revolutions of the 

 disc. At the side is a button, by which the wheels 

 may be put into or out of action. Suppose it is 

 desired to learn the number of vibrations producing 

 a sound of a given pitch, the siren is worked till 

 the proper pitch is obtained, then by pressing the 

 button the wheels are thrown into action, the time 

 being carefully noted. At the end of one minute, 

 during which, by proper working of the bellows, the 

 same pitch has been maintained, the wheels are 

 stopped by pulling out the button, and the number of 

 revolutions made by the disc in one minute are read 

 off. With each revolution fifteen shocks are given 

 to the air, therefore the number of vibrations pro- 

 ducing the given pitch is obtained by multiplying 

 15 by the number of revolutions per minute. 

 This gives the number of vibrations per minute, and 

 dividing by 60, one obtains the number per second. 

 Suppose the vibrations to be 100 per second, 

 then the period of each vibration will be T^tli f a 

 second. The length of the wave can also be obtained 

 if the number of vibrations be given. At freezing 

 temperature sound travels 1090 feet per second, and 

 in that time we have one hundred wave movements 



