204 PHILOSOPHY OF ZOOLOGY. 



it. These undulations affect the organs of hearing, and 

 excite the sensations of sound. These undulations vary 

 with the condition of the body which excites them, and 

 when carefully attended to, give us intimations regarding 

 the distance, direction, velocity, connection, and composi- 

 tion of sonorous bodies. 



In judging of the distance of objects by the sense of 

 hearing, we attend chiefly to the strength of the sounds 

 which they emit, considering it as bearing some proportion 

 to their proximity. But as this depends on the quantity 

 of motion excited, and the resistance which is opposed, we 

 may be led to conclude that a body is distant, because the 

 sound emitted is weak, while it is actually near, but the 

 extent of its vibrations limited. 



The position of sonorous bodies is intimated to us J)y the 

 direction in which the vibrations are communicated to the 

 external organs. In many cases, our conjectures are verifi- 

 ed by experience, but in others, we are deceived by the 

 change produced in the direction of the sonorous waves by 

 various obstacles, so that we mistake the reflected for the 

 radiated vibrations, echo for the direct sound. 



In estimating the velocity of moving bodies by means of 

 sound, we attend to the changes which take place in its 

 strength. When the transition from loud to weak, or from 

 weak to loud, is rapid, we infer, that the vibrations have a cor- 

 responding velocity, in proportion to the change of place in 

 the body which produces them. We are more rarely deceiv- 

 ed in this case than in the two former, although it sometimes 

 happens that a change in the direction of the sonorous vibra- 

 tions, may produce corresponding variations in their strength, 

 without any alteration having taken place in their velocity. 



When two bodies are rubbed or struck against each 

 other, we are in many cases able, from the differences of 

 sound emitted, to recognise their composition or structure. 



