HEARING. 123 



have been illustrated by comparing them to the little circular 

 waves or undulations which are produced when a stone is 

 thrown into stagnant water. Similar motions are probably 

 produced in the air by the vibrations of sonorous bodies. 

 The celerity with which sounds, or undulations of air, move, 

 has been computed. All sounds, whether acute or grave, 

 strong or weak, move at the average rate of 1142 feet in a 

 second of time. Hence, whenever lightning, or the fire of 

 artillery, is seen, its distance may be ascertained by determin- 

 ing the interval which elapses before the thunder or the re- 

 port is heard.* 



' The force or intensity of sound is augmented by reflection 

 from surrounding bodies. It is from this cause that the hu- 

 man voice, or any other noise, is always weaker and less dis- 

 tinctly heard in the open air than in a house. A musket 

 discharged upon a wide and extensive plain, sounds but little 

 louder than the burning of a squib, whilst, among buildings 

 or hills, where there are a thousand reverberations, its report 

 is very loud.' 



The modifications of sound are not less various than those 

 of tastes or odors. The ear is capable of distinguishing some 

 hundred tones in sound, and probably as many degrees of 

 strength in the same tones. By combining these, many thou- 

 sand simple sounds, which differ either in tone or in strength, 

 are perceived and distinguished by the ear. A violin, a flute, 

 a French-horn, may each of them give the same tone; but 

 the ear easily makes the distinction. The immense variety 

 of sensations, arising from the organs of smelling, of tasting, 

 and of hearing, enables animals to judge concerning the na- 

 ture and situation of external objects. By habit we learn to 

 know the bodies from which particular species of sound pro- 

 ceed. Previous to all experience, we could not distinguish 

 whether a sound came from the right or the left, from above 

 or below, from a greater or a smaller distance, or whether 

 it was the sound of a coach, of a drum, of a bell, or of an 

 animal. 



Hearing enables us to perceive all the agreeable sensations 

 conveyed to our minds by the melody and harmony of sounds. 

 This, to man at least, is a great source of pleasure and of 

 innocent amusement. But some men are almost totally des- 

 titute of the faculty of distinguishing musical sounds, and of 



* ' It has been lately ascertained that the velocity of sound is considerably affect- 

 ed by different states of the atmosphere and of the weather, and by the wind. The 

 lowest rate of its velocity is 1099, the highest 1164 feet per second ' 



