222 LETTERS ON NATURAL MAGIC. 



sound is returned from the wall. If two persons, 

 therefore, are placed before the reflecting wall, 

 the one will hear the echo of the sound emitted 

 by the other, and obstacles may intervene between 

 these two persons, so that neither of them hears 

 the direct sound emitted by the other; in the 

 same manner as the same persons similarly placed 

 before a looking-glass would see each other 

 distinctly by reflexion, though objects might 

 obstruct their direct view of each other. 



Hitherto we have supposed that there is only 

 one reflecting surface, in which case there will 

 be only one echo ; but if there are several reflect- 

 ing surfaces, as in the case in an amphitheatre of 

 mountains, or during a thunder-storm, where 

 there are several strata or masses of clouds ; or 

 if there are two parallel or inclined surfaces 

 between which the sound can be repeatedly re- 

 flected, or if the surface is curved, so that the 

 sound reflected from one part falls upon another 

 part, like the sides of a polygon inscribed in a 

 circle, in all these cases there will be numerous 

 echoes, which produce a very singular effect. 

 Nothing can be more grand and sublime than the 

 primary and secondary echoes of a piece of 

 ordnance discharged in an amphitheatre of pre- 

 cipitous mountains. The direct or primary echoes 

 from each reflecting surface reach the ear in 

 succession, according to their different distances, 

 and these are either blended with or succeeded 

 by the secondary echoes, which terminate in a 

 prolonged growl, ending in absolute silence. Of 

 the same character are the reverberated claps of 

 a thunder-bolt reflected from the surrounding 

 clouds, and dying away in the distance. The 



