222 PHYSIOLOGY CHAP. 



If these vibrations are recorded on a regularly moving surface, 

 tracings are obtained which vary greatly in form according to 

 the nature of the sounding bodies. The tracings of tones have, 

 whatever their forms, one common characteristic, that they are 

 built up of equal and regularly repeated periods. The tracings 

 of noises, on the contrary, lack periodicity and regularity. We 

 know from the results of physical investigation that simple tones, 

 i.e. the components of compound tones and noises, are pendular 

 or sinusoidal vibrations of elastic bodies, differing only in duration 

 and amplitude. The sensation of pitch depends on the duration 

 of the tone or the number of vibrations per second ; that of 

 intensity on the amplitude of vibration. 



It is easily demonstrated by the graphic method, or by the 

 syren, that tones are higher or lower according as the number of 

 vibrations of the sounding body in the time-unit is greater or less. 

 If a writing-point is fitted to the sounding body and brought in 

 contact with the surface of a drum rotating at constant speed, it 

 is easy to count the vibrations in a second, and thus to prove that 

 the pitch of the note is a result of the number of vibrations. It 

 is still easier with the syren to determine the number of vibra- 

 tions that correspond to a given tone. Seebeck's syren, which is 

 the simplest, consists of a metal disc which has at an equal 

 distance from the centre a given number of equidistant holes, and 

 is fixed to a central axis, on which it can be rotated at uniform 

 speed. By means of a tube communicating with a reservoir of 

 compressed air or a bellows, a blast of air can lie driven on to the 

 disc, and rhythmically set free or interrupted, according as the 

 opening of the tube is opposite to a hole or to a section of the disc. 

 These rhythmical interruptions generate vibrations in the air, 

 and thus produce higher or lower tones according to the greater 

 or lesser speed at which the disc rotates. Experience, moreover, 

 shows that the pitch does not change with the alterations in size 

 of the holes in the disc or the pressure at which the air passes 

 through them. It is thus clear that the height of tone depends 

 solely on the number of vibrations per second. 



It is still easier to show that the intensity of auditory sensa- 

 tions depends on the amplitude of the vibrations of the sounding 

 body. If a monochord or tuning-fork is made to vibrate, the 

 acoustic sensation, which is very strong at first, becomes gradually 

 weaker, as the amplitude of the vibrations, which is visible at the 

 outset, becomes more and more invisible. At each moment of 

 vibration the intensity of the sensation is equal to the kinetic 

 energy with which the vibrating body passes the position of 

 equilibrium, and this is proportional to the square of the velocity 

 or the amplitude of the vibration. 



VIII. The specific capacity of our ear for perceiving in the 

 form of simple and compound tones and noises the regular or 



