HEARING. 373 



with greater precision, to be equal to 4708 feet in 

 a second, when: the water is at a temperature of 

 46" of Fahrenheit. 



SoHd bodies, especially such as are hard and 

 elastic, and of uniform substance, are also excellent 

 conductors of sound. Of this we may easily con- 

 vince 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 sus- 

 pended by two strings, the ends of which are ap- 

 plied 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 de- 

 sirous 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 lired from 

 a distant fort, where a frozen river intervenes, each 

 flash of light is followed by two distinct reports ; 

 the first being conveyed by the ice, and the second 

 by the air. In like manner, if the upper part of 

 the wall of a high building be struck with a ham- 

 mer, a person standing close to it on the ground, 

 will hear two sounds after each blow, the first de- 

 scending through the wall, and the second through 

 the air. Cast iron, at the temperature of 51" Fahr. 

 was found by Biot to convey sounds with a velocity 

 of 11,090 feet in a second ; which is about ten times 

 greater than that in air. 



