EXPERIMENTS IN SOUND. 



71 



other in their vibration. Then the action of the faces c and c on the 

 air is to condense it, and this condensation tends to spread all around 

 the fork. But, by the same movement, the space r r between the 

 prongs is enlarged, and hence a rarefaction is made there. This rare- 

 faction also spreads all ai'ound the fork. But, as the condensations 

 produced at c and c and the rarefactions at r dnd r spread Avith the 



Fig. 39. 



same velocity, it follows that they must meet along the dotted lines q, 

 q, q, q, drawn from the edges of the fork outward. The full ^-circle 

 lines around the fork in Fig. 39 represent the middle of the condensed 

 shells of air, while the broken ^-circle lines stand for the middle of the 

 rarefied shells of air. 



Now what must happen along these dotted lines, or, rather, surfaces ? 

 Evidently there is a struggle here between the condensations and the 

 rarefactions. The former tend to make the molecules of air go nearer 

 together, the latter try to separate them; but, as these actions are 

 equal, and as the air is pulled in opposite directions at the same time, it 

 remains at rest — does not vibrate. Therefore, along the surfaces q, q, 

 q, q, there is silence. When the prongs vibrate toward each other they 

 make the reverse actions on the air; that is, rarefactions are now sent 

 out from c and c, while condensations are sent from r and r, but the 

 same effect of silence along q, q, q, q, is produced. 



Experimeyit 62. — That this is so, is readily proved by the following 

 simple experiment : Vibrate the fork and hold it upright near the ear. 

 Now slowly turn it round. During one revolution of the fork on its 

 foot, you will perceive that the sound goes through four changes. 

 Four times it was loud, and four times it was almost if not quite gone. 

 Twirl the fork before the ear of a companion : he will tell you when it 



