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THE POPULAR SCIENCE MONTHLY. 



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 A, 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 



Fia. 12. 



v \\v Nv : 



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 thank- 

 ful 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. Fig. 12 will give an idea of the "hope" 

 waves going through the air, end-foremost, of course, as they were spo- 

 ken. And so words follow each other in sets of waves like the above, 

 with rests between the sets made by the pauses between words. 



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 be- 

 come. 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 spok- 

 en, 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 



Fig. 13. 



