44 THE POPULAR SCIENCE MONTHLY. 



these being in some degree flexible, enabled the windows to yield to 

 pressure without much fracture of the glass. Every window in the 

 church, front and back, was bent inward. In fact, as the sound-wave 

 reached the church, it separated right and left, and for a moment the 

 edifice was clasped by a girdle of intensely-compressed air, which forced 

 all its windows inward." 



Now, was this " sound-wave " of compressed air, that struck the 

 church, a wind-storm from the place of explosion ? If not, whence all 

 this force ? That there was no wind is plain from the fact no dust was 

 raised, nor a leaf stirred from its place. We must look for another 

 explanation. 



Suppose that, in the middle of a closely-packed crowd, " room " were 

 suddenly made by pushing back the by-standers. These, thus suddenly 

 losing their balance, would fall back on those behind them, and these 

 in turn on others, and so on to the outsiders. It is easy to see that 

 each one would recover his own balance by pushing against the one 

 behind him, and so the fall-back movement would be seen to pass like a 

 wave through the crowd, each one passing it on as it reached him. In 

 like manner, the push of the expanding gases, at the explosion, was 

 transmitted to the church, the intervening air only passing the push 

 along. If the windows of the church had been elastic, they would have 

 swayed with the air ; as it was, they were pushed in, but had no 

 back-spring. 



The impulse which struck the church struck many ears in the same 

 way, but their drums taking up the air-push and its back-snap, sent it 

 to the brain, where it was put down as a tremendous sound. Sound, 

 then, is only the beating of air-waves in the ear. 



Now, a sound is either a noise or a musical tone. We take a noise 

 to be the blow of a single wave, or an irregular succession of waves 

 striking the ear, while a tone is the sound made by the beating of the 

 same kind of a wave, at regular intervals, in such rapid succession as to 

 form a sound-blend in the ear akin to the spoke-blend presented to the 

 eye by the spokes of a fast-turning wheel. 



We have divided sound into noise and musical tones, and have 

 spoken of a tone, distinguished from noise, as being a sound-blend 



Fig. 1. 



made in the ear by the beating of the same kind of a wave, at regular 

 intervals, in rapid succession. Let us prove this. We will strike mid- 

 dle C on a piano. We get a musical tone from its string, which is set 

 a-vibrating, as shown in Fig. 1. But how shall we determine the num- 



