EEMARKABLE ECHOES. 221 



where a thin soil rests upon a continuous stratum 

 of rock, the sound is heard at a great distance, 

 and hence it is the practice among many Eastern 

 tribes to ascertain the approach of an enemy by 

 applying the ear to the ground. 



Many remarkable phenomenon in the natural 

 world are produced by the reflexion and concen- 

 tration of sound. Every person is familiar with 

 the ordinary echo which arises from the reflexion 

 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 reflect- 

 ing 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 reflexion, or the inclination at which the 



