424 PHYSIOLOGICAL PHYSICS. [Chap. xxxn. 



wheel revolved each tooth struck against the edge of 

 a card, each stroke producing a vibration. "With a 

 certain rapidity a musical note was produced, rising 

 in pitch as the rapidity was increased, and falling as it 

 was diminished. 



The siren shows the dependence of pitch upon 

 rapidity still more accurately. The simple siren 

 consists of a zinc disc, which is perforated with holes 

 arranged in rings, one ring having the holes verv near 



O O ' v 



to one another, and others having them at greater 

 distances. By multiplying wheels the disc can be 

 made to revolve with great rapidity. A tube with 

 a fine opening at one end is brought up near the 

 surface of the disc, and air may be driven through the 

 tube by a bellows, or by the mouth. Now when air 

 is blown through the tube it will pass through a hole 

 in the zinc plate, and communicate an impulse to the 

 air on the opposite side, should the open end of the 

 tube happen to be opposite a hole in the disc. If the 

 disc be slowly revolved, at one moment a hole in the 

 plate will be opposite the tube and air will pass 

 through, at the next moment an unperforated part of 

 the plate will be opposite the tube, and the air will 

 not puss through. With a slight further turn another 

 hole will be opposite the tube, and the air will receive 

 a second impulse. As the plate is revolved with 

 greater rapidity, an increasing number of impulses 

 will be given to the air. At first, then, on revolving the 

 plate nothing is heard but the separate pufts as one 

 hole after another comes opposite the tube through 

 which the air is being blown. Gradually, however, 

 with increased speed, the putt's follow one another with 

 greater rapidity, till they become fused, and a low 

 musical note is heard, which rises higher and higher 

 as the plate revolves faster and faster, till the pitch 

 becomes very high with the utmost speed that can be 

 communicated to the plate. 



