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, a, strikes against the air, its particles are driven together or condensed in front of it, and, as the prong retreats, it leaves a partial vacuum behind. Each vibration thus generates
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 circles 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 "Mysteries 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 actually 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.