70 ON THE PHYSIOLOGICAL CAUSES OF 



That the sensation thus excited is a sensation of musical 

 tone, does not depend in any way upon the peculiar 

 manner in which the air is agitated, but solely on the 

 peculiar powers of sensation possessed by our ears and 

 auditory nerves. I remarked, a little while ago, that 

 when the tones are loud the agitation of the air is per- 

 ceptible to the skin. In this way deaf mutes can perceive 

 the motion of the air, which we call sound. But they do 

 not hear, that is, they have no sensation of tone in the 

 ear. They feel the motion by the nerves of the skin, 

 producing that peculiar description of sensation called 

 whirring. The limits of the rapidity of vibration within 

 winch the ear feels an agitation of the air to be sound, 

 depend also wholly upon the peculiar constitution of the 

 ear. 



When the siren is turned slowly, and hence the puffs of 

 air succeed each other slowly, you hear no musical sound. 

 By the continually increasing rapidity of its revolution, 

 no essential change is produced in the kind of vibration 

 of the air. Nothing new happens externally to the ear. 

 The only new result is the sensation experienced by the 

 ear, which then for the first time begins to be affected by 

 the agitation of the air. Hence the more rapid vibrations 

 receive a new name, and are called Sound If you admire 

 paradoxes, you may say that aerial vibrations do not be- 

 come sound until they fall upon a hearing ear. 



I must now describe the propagation of sound through 

 the atmosphere. The motion of a mass of air through 

 which a tone passes, belongs to the so-called wave motions 

 • — a class of motions of great importance in physics. 

 Light, as well as sound, is one of these motions. 



The name is derived from the analogy of waves on the 

 surface of water, and these will best illustrate the pecu- 

 liarity of this description of motion. 



AYhen a point in a surface of still water is agitated — as 



