418 THE SENSORIAL FUNCTIONS. 



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 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 hammer, a person standing close 

 to it on the ground, will, hear two sounds after 

 each blow, the first descenliing through the wall, 

 and the second through the air. 



As sounds are weakened by diffusion over a 

 larger sphere of particles, so they are capable of 

 having their intensity increased by concentra- 

 tion into a smaller space ; an effect which may 

 be produced by their being reflected from the 

 solid walls of cavities, shaped so as to bring the 

 undulations to unite into a focus ; it is on this 

 principle that the ear-trumpet, for assisting per- 

 sons dull of hearing, is constructed: and the 

 same effect sometimes takes place in echoes, 

 which occasionally reflect a sound of greater 

 loudness than the original sound which was 

 directed towards them. 



If the impulses given to the nerves of the ear 

 be repeated at equal intervals of time, provided 

 these intervals be very small, the impressions 



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