6 THE POPULAR SCIENCE MONTHLY. 



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through the row. In a similar way the action of a vibrating body 

 upon the air is to produce a series of condensations and rarefactions 

 which are sent successively forward through the atmosphere, and each 

 condensation, with its associated rarefaction, constitutes a sonorous 

 wave. This is illustrated in Fig. 6, where A B represents a tuning- 

 fork in vibration. As the prong, , strikes against the air, its particles 

 are driven together or condensed in front of it, and, as the prong re- 

 ' treats, it leaves a partial vacuum behind. Each vibration thus gen- 



Fig. 5. 



Propagation of Impulses through Balls. 



erates a wave. The oscillations of the air-particles are communicated 

 to the adjacent particles, and the impulse is sent forward. In Fig. 

 6, b c d represent the condensations, and b' c d' the accompanying 

 rarefactions in the propagation of impulses through the air. 



If, now, we imagine these dark and light spaces prolonged in cir- 

 cles round the tuning-fork, we shall have an idea of the way sound 

 moves in all directions. We are to conceive of air- waves as bubbles 

 or spheres, which rapidly expand from the point of vibration, and 

 chase each other outward with the speed of musket-balls. 



"We have said that the waves of sound take place in an invisible 

 realm, yet it is in the power of science to bring them into view. This 

 triumph of experiment is due to a German named Toepler. Prof. 

 Rood has given an account of it in his admirable lecture on the " Mys- 

 teries of the Voice and Ear." It depends upon the principle that, 

 " when light which is travelling through the atmosphere meets with a 

 denser or rarer layer, it is usually turned a little out of its straight 

 path a very little but enough, sometimes, to render the layer actu- 

 ally visible, if proper optical means are employed." But, how is a 

 wave to be made visible, if it moves with the speed of a cannon-ball, 

 " which goes so fast we cannot see it ? " It is by getting a glimpse 

 of it so quickly that it has no time to move, and appears as if at rest. 

 Those who have seen a railway-train at high speed illuminated by a 

 flash of lightning, will remember that it appeared as if standing still. 

 So, if a cannon-ball were passing through a darkened room, and could 

 be illuminated by an electric flash, it would seem to be at rest in mid- 

 air. By suitable arrangements, and the use of the electric spark, Prof. 



