fork, having an elasticity of 440 vibrations per second, and set it a-vibrating. The prong of the fork, in moving from a to a', pushes the layer 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'
to a, causing a vacuum, which is instantly filled up by the return of the air which it had just pushed away. 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 follow 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 reactive 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 between 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,