132 ECHOES. SECT. XVI. 



rocally the surface by its reaction will communicate its 

 undulations back again into the air. Such reflections 

 produce echoes, and as a series of them may take place 

 between two or more obstacles, each will cause an echo 

 of the original sound, growing fainter and fainter till it 

 dies away ; because sound, like light, is weakened by 

 reflection. Should the reflecting surface be concave 

 toward a person, the sound will converge toward him 

 with increased intensity, which will be greater still if 

 the surface be spherical and concentric with him. Un- 

 dulations of sound diverging from one focus of an ellip- 

 tical shell (N. 175) converge in the other after reflec- 

 tion. Consequently a sound from the one will be heard 

 in the other as if it were close to the ear. The rolling 

 noise of thunder has been attributed to reverberation 

 between different clouds, which may possibly be the 

 case to a certain extent. Sir John Herschel is of opin- 

 ion, that an intensely prolonged peal is probably owing 

 to a combination of sounds because the velocity of elec- 

 tricity being incomparably greater than that of sound, 

 the thunder may be regarded as originating in every 

 point of a flash of lightning at the same instant. The 

 sound from the nearest point will arrive first, and if the 

 flash run in a direct line from a person, the noise will 

 come later and later from the remote points of its path 

 in a continued roar. Should the direction of the flash 

 be inclined, the succession of sounds will be more rapid 

 and intense, and if the lightning describe a circular curve 

 round a person, the sound will arrive from every point 

 at the same instant with a stunning crash. In like 

 manner the subterranean noises heard during earth- 

 quakes like distant thunder, may arise from the conse- 

 cutive arrival at the ear of undulations propagated at the 

 same instant from nearer and more remote points ; or 

 if they originate in the same point, the sound may 

 come by different routes through strata of different den- 

 sities. 



Sounds under water are heard very distinctly in the 

 air immediately above ; but the intensity decays with 

 great rapidity as the observer goes farther off, and is 

 altogether inaudible at the distance of two or three 

 hundred yards. So that waves of sound, like those of 



