FISHES AND FISHING. " 89 



by aid of the gills (the lungs and medium of the cir- 

 culation of the blood in aqueous animals). "When 

 the air is set in motion by the sounds I have men- 

 tioned, or any other sound, the result is, that from the 

 point where the noise or sound is created, a succes- 

 sion of circular waves of air expand on every side in 

 the form of a globe ; if these waves meet with any 

 obstruction to their expansion, and the obstruction be 

 of a concave character, the waves of air are collected 

 into a focus, [and are reflected back, forming what is 

 called an echo ; and if there be several of these concave 

 ebstructing substances, there may be several echoes 

 one after another, louder or fainter, and more or less 

 immediate, according to their several distances. 



A very plain exemplification of this may be shown 

 by throwing a stone, or any substance, into a pool of 

 still water ; circular undulations, or waves, will im- 

 mediately diverge from the spot ; these waves will be 

 higher, but smaller in circumference, immediately 

 around the place where the substance struck the 

 water, and as they recede from the centre they be- 

 come less in height, and larger in circumference, until 

 they are no longer to be seen. Now these undulations 

 extend below the surface, forming a half-sphere ; and 

 like as waves or undulations of air affect the auditory 

 organs of man, and cause vibration which is commu- 

 nicated to the brain, through the auditory nerves, so 



