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 rarefaction also spreads all around the fork. But, as the condensations produced at c and c and the rarefactions at r and r spread with 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 14-circle lines around the fork in Fig. 39 represent the middle of the condensed shells of air, while the broken 14-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.
Experiment 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
