372 THE SENSORIAL FUNCTIONS. 



surface of water around any body which by its 

 motion ruffles that surface ; only that instead of 

 merely extending in a horizontal plane, as waves 

 do, the sonorous undtdations spread out in all 

 directions, forming, not circles in one plane, but 

 spherical shells ; and, whatever be the intensity of 

 the sounds, the velocity with which the undulations 

 advance is uniform, as long as they continue in a 

 medium of uniform elasticity. In air, this velocity 

 is, on an average, about 1100 feet in a second, or 

 twelve and a half miles in a minute : it is greater in 

 dense, and smaller in rarefied air; being, in the 

 same medium, exactly proportional to the elasticity 

 of that medium. All other aeriform elastic fluids, 

 such as steam, and the vapour of alcohol or ether, 

 are capable of propagating sounds with the same 

 facility as air.* 



Water is the medium of sound to aquatic animals, 

 as the air is to terrestrial animals. Sounds are, 

 indeed, conveyed more quickly, and to greater dis- 

 tances, in water than in air, on account of the 

 greater elasticity of the constituent particles of 

 water, within the minute distance required for their 

 action in propagating sound. Stones, struck to- 

 gether under water, are heard at great distances by 

 a person whose head is under water. Franklin 

 found, by experiment, that sound, after travelling 

 above a mile through water, loses but little of its 

 intensity. Chladni estimated the velocity of sound, 

 when conveyed by water, to be about 4900 feet in 

 a second, or between four and five times greater 

 than it is in air. M. Colladon has determined it, 



* Biot, Traite de Physique, ii, 4. 



