in Energy ', tJie Power of Nature 35 



waves in water, or between succeeding maxima of com- 

 pression or of rarefaction in waves of air, is spoken of as the 



wave-length. The am- , 



plitude of a wave is the /^^^ y 

 height from crest to / ^ 



trough (CT), or the V i 



difference in degree of ^"^ 



Compression and dilata- FlG - 6. -Wave-motion CC, crests ; TT, 



troughs. 



tion. 



58. Sound. When a wave of alternate compression and 

 rarefaction of air strikes the ear, it produces the sensation 

 of sound ; the more rapid the vibration and shorter the 

 wave-length the shriller is the sound, but neither very short 

 rapidly vibrating waves of air nor very long slowly vibrat- 

 ing ones affect the ear at all. The greater the ampli- 

 tude of an air -wave, the louder is the sound. Waves 

 of compression and rarefaction pass through the air at 

 the rate of about 1 1 oo feet per second when the tempera- 

 ture is 32 F., and travel 2 feet per second faster for 

 every degree that the air is warmer. Sound-waves pass 

 through water with four times the velocity, and through 

 solids with many times the velocity of their passage through 

 air. Air is set into wave-motion by any substance that 

 is vibrating as a whole, such as a tuning-fork, a stretched 

 string, or a column of air in a pipe. A tuning-fork when 

 made to vibrate sets up air-waves that produce the sensation 

 of a particular musical note in the ear ; if that tuning-fork 

 is at rest, and air-waves of the same kind as those it can 

 set up strike it, they transfer their energy to the fork and 

 start its vibrations. All other air-waves, longer and shorter 

 alike, pass by with but slight and transitory effects, and, 

 stated generally, the law holds that Bodies absorb vibrations 

 of the same period as those which they give out. When 

 certain notes are sung, or struck on a piano, the gas globes 

 in a room absorb the particular waves which they would set 

 up if struck, and ring in response to them. 



59. Molecular Vibrations are the minute movements 

 of the smallest particles of bodies, either as a quivering 

 of the particle itself or as quick oscillations to and fro. 



