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And finally, Daniell says: "A vibrating body, before it can act as a sounding body, must produce alternate compressions and rarefactions in the air, and these must be well marked.
One physicist has declared that the atoms of oxygen and hydrogen are probably not nearer to each other in water than one hundred and fifty men would be if scattered over the surface of England one man for each four hundred square miles. What must the distance be in steam? what the greater distance in the more extreme rarefactions?
Suppose that one of them performs 101 vibrations in the time required by the other to perform 100, and suppose that at starting the condensations and rarefactions of both forks coincide. At the 101st vibration of the quicker fork they will again coincide, that fork at this point having gained one whole vibration, or one whole wavelength, upon the other.
Indeed, such a mind as that of Anaxagoras was sure to investigate all manner of natural phenomena, and almost equally sure to throw new light on any subject that it investigated. Hence it is not surprising to find Anaxagoras credited with explaining the winds as due to the rarefactions of the atmosphere produced by the sun.
Properly agitated, a tuning-fork now sounds in a manner audible to you all, and most of you know that the air through which the sound is passing is parcelled out into spaces in which the air is condensed, followed by other spaces in which the air is rarefied. These condensations and rarefactions constitute what we call waves of sound.
But you can also imagine a state of things where the condensations of the one system fall upon the rarefactions of the other system. Each of them taken singly produces sound; both of them taken together produce no sound. Thus by adding sound to sound we produce silence, as Grimaldi, in his experiment, produced darkness by adding light to light.
A crowding of atoms is called a condensation, and a parting is called a rarefaction, and when we speak of the length of a wave of sound, we mean the distance between two condensations, or between two rarefactions. Although each atom of air moves a very little way forwards and then back, yet, as a long row of atoms may be crowded together before they begin to part, a wave is often very long.
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