280 LETTERS ON NATURAL MAGIC. 



arises from the reflection of sound from an even surface, 

 such as the face of a wall, of a house, of a rock, of a hill, 

 or of a cloud. As sound moves at the rate of 1090 feet 

 in a second, and as the sound which returns to the person 

 who emits it has travelled over a space equal to twice his 

 distance from the reflecting surface, the distance in feet 

 of the body which occasions the echo may be readily found 

 by multiplying 545 by the number of seconds which 

 elapse between the emission of the sound and its return 

 in the form of an echo. This kind of echo, where the 

 same person is the speaker and the hearer, never takes 

 place unless when the observer is immediately in front of 

 the reflecting surface, or when a line drawn from his 

 mouth to the flat surface is nearly perpendicular to it, 

 because in this case alone the wave of sound is reflected 

 in the very same direction from the wall in which it 

 reaches it. If the speaker places himself on one side of 

 this line, then the echo will be heard most distinctly by 

 another person as far on the other side of it, because the 

 waves of sound are reflected like light, so that the angle 

 of incidence or the inclination at which the sound falls 

 upon the reflected surface is equal to the angle of reflec- 

 tion, or the inclination at which the 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 inter- 

 vene 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 reflection, 

 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 reflecting surfaces, as is 



