NATURE OF SOUND. 277 



only one direction. In this way, sound may be trans- 

 mitted to great distances without considerable loss of 

 intensity. 



434. Pitch. The second element of a musical sound 

 is pitch, by which we mean the quality that constitutes 

 the difference between a low or grave tone and a high 

 tone. All persons are more or less able to recognize 

 differences in pitch. A person who is able to judge 

 accurately of the pitch of sounds is said to have a " good 

 ear for music." The pitch of a sound depends upon 

 the rapidity of vibration of the sounding body, 

 or, in other words, upon the rate at which sound pulses 

 follow each other. The more rapid the vibrations, the 

 higher the tone. 



435. Experimental Proof of the Cause of 

 Pitch. That pitch depends upon 



rapidity of vibration, may be roughly 



shown by drawing the finger nail across 



the teeth of a comb, slowly the first time 



and rapidly the second time. It may be 



shown more satisfactorily by means of 



Savart's wheel, shown in Fig. 218. This 



consists of a heavy brass ratchet-wheel, 



supported on an iron frame and pedestal. The wheel may 



be set in rapid revolution by a cord wound around the axis. 



By holding a card against the teeth, when in rapid motion, 



a shrill tone will be produced, gradually falling in pitch as 



the speed is lessened. 



(a.) If the sounding body and the listening ear approach each 

 other, the sound waves will beat upon the ear with greater rapidity. 

 This is equivalent to increasing the rapidity of vibration of the 



