904 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 cumposi- 
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 bearmg 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 mtimated to us by 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. Weare 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. 
