SECT, xvi.] HCHOES. 153 



tions are retrograde, while in the other half they are direct ; 

 consequently, at the edges where these two portions meet, 

 the motions of the air will neither be retrograde nor direct, 

 and therefore it must be at rest. 



It appears, from theory as well as daily experience, that 

 sound is capable of reflection from surfaces (N. 174) accord- 

 ing to the same laws as light. Indeed, any one who has ob- 

 served the reflection of the waves from a wall on the side of 

 a river, after the passage of a steam-boat, will have a perfect 

 idea of the reflection of sound and of light. As every sub- 

 stance in nature is more or less elastic, it may be agitated 

 according to its own law by the impulse of a mass of undu- 

 lating air ; and reciprocally the surface by its re-action 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 towards a person, 

 the sound will converge towards him with increased intensity, 

 which will be greater still if the surface be spherical and 

 concentric with him. Undulations of sound diverging from 

 one focus of an elliptical shell (N. 175) converge in the other 

 after reflection. 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 opinion, that an intensely 

 prolonged peal is probably owing to a combination of sounds, 

 because, the velocity of electricity being incomparably greater 

 than that of sound, the thunder may be regarded as origin- 

 ating 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 con- 

 tinued roar. Should the direction of the flash be inclined. 



