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These waves go with a velocity which can be readily 

 determined. That sound waves are much slower than rays 

 of light is proved by the experience of every one who has 

 seen the steam of the whistle of an approaching train 

 several moments before he hears the sound. Or he may 

 have noticed the discharge of a gun or the flash of the 

 lightning before he hears the sounds which these have pro- 

 duced. The velocity of sound vibrations through the air 

 depends upon the density of the air, and as the density of 

 the air depends upon the temperature it is usually said that 

 the velocity of a sound depends upon the temperature of 

 the medium. At the temperature of freezing, the velocity of 

 sound is about 1,092 feet per second. For each additional 

 degree of centigrade the velocity is increased about two 

 feet per second, so that at ordinary mild temperatures the 

 velocity of sound in air is not far from 1,120 feet per 

 second. 



REFLECTION AND REFRACTION Ol SOUND. 



Sound being a vibratory motion it is subject to the same 

 laws of reflection and refraction as light. A sound wave, 

 for instance, striking a high wall or precipice is reflected 

 back and gives rise to what is familiarly known as the echo. 

 An echo in sound is, therefore, like the reflected image in 

 a mirror in the case of light. 



But not only may sound be reflected like an echo, it may 

 be refracted like light through lenses. To do this it is simply 

 necessary to pass the sound wave through a denser medium 

 to converge it, or a rarer medium to diverge it. A large 

 rubber bag shaped like a double convex lens and filled 

 with carbon dioxide, which is denser than air, serves as a 

 condensing lens, somewhat like glass does for light. Very 

 seldom, however, are sound lenses brought into use. 



A familiar result, depending upon the refraction of sound, 



is produced in the carrying of sound by the winds. It is 



apparent that a sound is much plainer when the wind blows 



from that direction. The familiar explanation is, of course, 



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