cannot explore the nerve-waves, but, projected in the air, they become a picture that we can study. First there is the rough breathing or tremor h, then the mouth tunes itself for the musical tone o. Suppose the o to be made in a man's voice at a pitch A, below middle C. The o-making overtone is its octave overtone or second, which in this case will be A above middle C, the pitch to which the mouth will resound. Besides this prominent overtone, o has some feeble third and fourth overtones, and for the personal peculiarity say a little fifth. What is
Fig. 12.
this o, then? A tone-vibration of 220 per second, frilled with overtone vibrations of 440, 660, 880, and 1,100 per second. In the air, on its way to an ear, this o is a matter of air-waves 5 feet in length, filled in with waves of 30, 20, 15, and 10 inches in length, and—let us be thankful that we do not have to understand o before we can exclaim it. Following this, the mouth suddenly shuts up and pushes off the vowel-ripples with a noisy billowy p. Fig. 12 will give an idea of the "hope" waves going through the air, end-foremost, of course, as they were spoken. And so words follow each other in sets of waves like the above, with rests between the sets made by the pauses between words.
Fig. 13.
Now, how far will these waves be loud enough to be heard—that is, how long will they keep strong enough to beat the drum-head of an ear? The farther they go the more they spread, and the weaker they become. A strong voice may be heard at an eighth of a mile, or about 700 feet. As sound travels 1,100 feet per second, it follows that, in less than a second after being spoken, the waves become too weak to make words. Let us be quick, then, to find what they are saying. Sun-waves, spreading from a focus, may be brought again to a focus by condensing them with a lens. So the
