R VII. 



\ 



SOUND. 



ECTION I. 



NATURE, REFRACTION AND REFLECTION 

 OF SOUND. 



415. Definition of Sound. Sound is that mode 

 of motion which is capable of affecting the auditory 

 nerve. 



(a.) The word sound is used in two different senses. It IK often 

 used to designate a sensation caused by waves of air beating upon 

 the organ of hearing ; it is also used to designate these aerial waves 

 themselves. The former meaning refers to a physiological or 

 psychological process ; the latter to a physical phenomenon. If 

 every living creature were deaf there could be no sound in the 

 former sense, while in the latter sense the sound would exist but 

 would be unheard. The definition above considers sound in the 

 physical sense only. 



416. Undulations. In beginning the study of 

 acoustics, it is very important to acquire a clear idea of 

 the nature of undulatory motion. When a person sees 

 waves approaching the shore of a lake or ocean, there 

 arises the idea of an onward movement of great masses of 

 water. But if the observer give his attention to a piece of 

 wood floating upon the water, he will notice that it merely 



