296 THE SENSORIAL FUNCTIONS. 



miles in a minute: it is greater in dense, and smaller in rare- 

 fied air; being, in the same medium, exactly proportioned to 

 the elasticity of that medium. 



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 distances, in water than in air, 

 on account of the greater elasticity of the constituent parti- 

 cles of water, within the minute distance required for their 

 action in propagating sound. Stones, struck together 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. According to Chladni, the velocity of 

 sound in water is about 4900 feet in a second, or between 

 four and five times as great as it is in air. 



Solid bodies, especially such as are hard and elastic, and 

 of uniform substance, are also excellent conductors of sound. 

 Of this we may easily convince ourselves by applying the 

 ear to the end of a log of wood, or a long iron rod, in which 

 situation we shall hear very distinctly the smallest scratch 

 made with a pin at the other end; a sound, which, had it 

 passed through the air only, would not have been heard at 

 all. In like manner, a poker suspended by two strings, the 

 ends of which are applied to the two ears, communicates to 

 the organ, when struck, vibrations which would never have 

 been heard under ordinary circumstances. It is said that 

 the hunters in North America, when desirous of hearing the 

 sounds of distant footsteps, which would be quite inaudible 

 in any other way, apply their ears close to the earth, and 

 then readily distinguish them. Ice is known to convey 

 sounds, even better than water: for if cannon be fired from 

 a distant fort, where a frozen river intervenes, each flash of 

 light is followed by two distinct reports, the first being con- 

 veyed by the ice, and the second by the air. In like man- 

 ner, if the upper part of the wall of a high building be struck 

 with a hammer, a person standing close to it on the ground, 



