Chap, xxxi.] REFLECTION OF SOUND. 421 



density, but to the high elasticity in relation to the 

 density. 



MeHectioii of soimd. Sound may be reflected 

 just as light. Tims, suppose one takes a concave 

 mirror, and from a lamp at a distance throws upon it 

 a beam of light, and finds the position in front of the 

 mirror to which the light is reflected (the conjugate 

 focus), then removes the lamp, in its place puts a 

 watch, and holds the ear in the position of the conju- 

 gate focus, the ticking of the watch will be heard 

 with great distinctness, though in any other position 

 the person might not hear it at all. Or let the watch 

 be placed in the focus near the mirror, a person with 

 his ear at the place of distant focus will hear it dis- 

 tinctly. The laws for the reflection of sound are the 

 same as those for light. An echo is a reflected sound. 



Refraction of soimd may also be shown expe- 

 rimentally. For a converging lens thin indiarubber 

 balloons filled with a denser gas than air, e.g. car- 

 bonic acid gas, may be used. They will converge the 

 sound waves of a sounding body, placed behind, to a 

 focus in front, just as a lens would do with light. 



Transmission of sound toy tubes, etc, 

 In free air the intensitv of a sound diminishes as the 



V 



distance through which it is transmitted increases ; 

 it varies inversely as the square of the distance. 

 From the centre of disturbance the waves of sound 

 expand in all directions, and the more they travel the 

 more widely are they diffused, so that their intensity 

 diminishes as they advance. By the use of tubes 

 (a speaking trumpet, for example) the expansion on 

 all sides is prevented and the waves are concentrated 

 in one direction, so that the intensity is maintained. 

 This is one reason why a sound of no great loudness 

 may be transmitted with little enfeeblement through 

 narrow tubes of considerable length, if the inner 

 walls be smooth. Biot, in Paris, was able to hold a 



