It is a law of acoustics that a low pitch is resounded in a comparatively large resonant chamber; a high pitch in a comparatively small one. A simple and instructive experiment in illustration of this principle is this: Take a large bottle, strike a C tuning fork, hold it over the empty bottle, and no sound will be heard. The bottle does not respond, because the cavity is too large for the pitch of the fork. If water is poured into the bottle, the air column inside thereby being shortened until the proper sized chamber is formed, by then holding the high-pitched tuning fork over it, the sound of the fork will be resounded by the resonant chamber and the tone will burst forth quite loudly. Use any number of tuning forks, each on a different key, and a resonant chamber can thus be made which will resound each fork.
I once tried an experiment with two tuning forks which were fastened to sounding boxes and which had been tuned to exactly the same pitch. I struck one fork and stopping its vibration, the sound of the other, vibrating responsively, was distinctly heard. The same result was achieved when one of the tuning forks was placed in a remote part of the room. I also placed the fork upon the piano, struck it, and the string of the same pitch, in connection with its overtones, responded. In order that any resonant cavity may resound, the pitch that belongs to that cavity must be struck. Every room in a house, in consequence of its size, its form, and the material of which it is constructed, resounds to a certain pitch. Sometimes in the course of conversation the globe of a chandelier in the room resounds. This is because the pitch which is agreeable to its size, form, and substance is struck.
Overtones are tones above in pitch, but harmonic with the fundamental tone. They are caused by the vibration of the aliquot parts of a string as distinct from that of its whole length. These parts being shorter vibrate with greater rapidity, thereby giving a higher pitch than the fundamental note, though in perfect harmony with it. An overtone can be discovered by holding near one of the vibrating aliquot parts a chamber of the right size, form, and substance to reinforce the tone of one of these parts. This resonance would be loud enough to be distinguished from the fundamental tone.