234 PHYSIOLOGY. 



passages, and up the spiral staircase of the cochlea, might 

 come down to the round fenestra, whose elastic membrane 

 would transmit the vibration in the opposite direction. 

 When the sounds, therefore, have done their office, in order 

 that the hearing might not be rendered confused by a pro- 

 longed vibration of the fluid, as an echo is repeated between 

 two mountains, as they strike the above named membrane, 

 they are given off to the air in the tympanum, and there the 

 vibrations end ; for we have seen that it is probably the 

 bones in the tympanum chiefly, and not the air, which trans- 

 mits impressions to the fluids of the labyrinth, and that they 

 receive their impulse from the drum of the ear. ; 



16. Sound. fSound has no distinct existence, but is the 

 result of certain conditions of bodies. We see how, by 

 throwing a pebble into a lake of water at rest, the undula- 

 tions spread in every direction, in the form of a circle, until 

 the impulse is lost in the distance. So in air, frequent vi- 

 brations produce sound ; and hearing has been defined to be 

 that function by which we obtain a knowledge of the vibra- 

 tory motions of bodies. These motions produce waves or 

 undulations in the air, which are propagated in every direc- 

 tion, as the circular waves are produced in the lake, by 

 throwing in the pebble.! 



17. Air is the common vehicle of sound, though all elastic 

 bodies are capable of conveying it. Water conveys sound 

 better than air, and some solid bodies better than either. If 

 two stones are struck together under water, a person whose 

 head is under the surface may hear the sound at a great dis- 

 tance. A blow struck with a hammer by a workman in a 

 diving bell, far below the surface of the water, is heard dis- 

 tinctly many fathoms above. So also if the ear be applied 

 to the end of a long stick of timber a slight scratch made at 

 the other extremity will be distinctly heard. The report of 

 a cannon is heard to a much greater distance over the frozen 

 surface of snow ; and, under such circumstances, firing has 

 been heard from one to two hundred miles. Savages are 



