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THE POPULAR SCIENCE MONTHLY. 



fork, having an elasticity of 440 vibrations per second, and set it a-vibrat- 

 ing. The prong of the fork, in moving from a to a', pushes the la}'er of 

 air in front of it, which, in its endeavor to recover from this huddling, 

 pushes against the next layer, which is thus in its turn compressed, the 

 compression or push passing in this way, from layer to layer, through the 

 air the wider the swing of the prong the greater the compression in the 

 air, and the louder the sound ; meanwhile the prong moves back from a' 



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Fig. 3. 



to , causing a vacuum, which is instantly filled up by the return of the 

 air which it had just pushed awaj^. But the fork now swings back to 

 a", causing the layer of air not only to return to its ordinary density 

 at a, but causing it to expand, in order to fill up the vacuum from a 

 a'\ thus producing a rarefaction, or stretch, in the air, which draws 

 back on every other layer, causing a pulse of rarefaction to follow every 

 pulse of compression; in other words, causing a stretch-gap to fol- 

 low every push. A clear idea of this may be had by again using our 

 illustration of a crowd : the place where some are just falling back 

 on those behind them illustrates the wave of compression, while the gap 

 between those falling back and those who have just recovered their 

 balance illustrates the wave of rarefaction which follows it. An air- 

 wave is made up of a compression and a rarefaction a push and a 

 stretch the two being produced in one vibration of the prong, the 

 compression by the motion from a to a', and the rarefaction by the re- 

 active motion from a to a". On its way back to a, the prong lets up 

 on the stretch, and goes on to a' with another push, and so on as long 

 as it vibrates. These compressions and rarefactions, represented in the 

 figure by its shadows and lights, correspond to the crests and hollows 

 of water-waves. 



In water we measure the length of waves (that is, the distance be- 

 tween them) from swell to swell. Sound-waves are measured from 

 huddle to huddle. Now, how are we going to measure this ? Let us 

 take the case of water. If we knew that in 100 yards of water there 

 were 100 equal waves, we would know that each wave was one yard 

 in length that is, that the wave-swells were thus far apart ; or, if 

 there were 50, each wave would stretch two yards. We would find 

 the length of wave by dividing the distance covered by the number 

 of waves stretched over it. The length of sound-waves is measured in 

 the same way. We will measure the length, or distance apart, of the 

 waves of our A-fork experiment. Sound travels, in round numbers, 



