Page:Popular Science Monthly Volume 31.djvu/377

Rh to have been the first to think of comparing the greatest change per second of velocity in an earthquake with gravity. Thus at Tokio, on March 11, 1882, walls were cracked and chimneys knocked over, and in this shock the greatest change per second of velocity may be expressed by the phrase one thirty-fifth of gravity; in other words, the greatest change per second of velocity was of a foot per second. This conclusion enables us also to illustrate the mechanical consequences of the shock in another way; for if a wall thirty-five feet high leans over, so that the top brick projects a foot beyond the bottom brick, the forces tending to upset the wall are the same as those which occurred in this earthquake.

No great shock has ever yet been recorded by automatic instruments, and it is not unlikely that in these great disasters the instruments would be thrown out of gear, and no record would be obtained. Thus, earthquakes which only work a moderate amount of destruction are the most favorable for scientific operation.

Since the oscillations at any one spot are usually in all sorts of directions, it is impossible, from observation at a single place, to form a sound opinion as to position of the origin of the disturbance. Much information useful for the study of vibrations and of the laws of their decrease with increasing distance, has resulted from a laborious series of experiments made by Milne at Tokio. Artificial earthquakes were produced by the explosion of gun-cotton in holes in the ground, and by the fall of heavy weights, and the records at various distances from the origin were obtained.

From theoretical considerations, confirmed by these experiments, it is established that earthquake-waves consist of oscillations of two kinds, namely, waves or vibrations of compression, and of distortion. In the first kind the motion of each particle of the ground is to and fro in the direction in which the wave is traveling; and in the second kind the excursions are at right angles to the direction of wave-propagation. As the former vibrations travel more rapidly than the latter, all the compressional waves may have passed a given station before the arrival of the distortional waves, and thus the shock may be apparently duplicated. Or, nearer to the origin, the two series will overlap, and a complex movement ensues, such as that exhibited in the figure above. The phenomena are further complicated by frequent reflections and refractions, as the wave passes from one geological stratum to another. The rate at which these waves travel depends on the nature of the rock through which the movement passes; velocities ranging from a mile per second to five miles per second are usual.

The destructive effects of earthquakes on buildings are notorious, and it is unnecessary to describe them here. By an examination of ruined buildings a competent observer is often able to obtain a good deal of information as to the nature of the shock. Thus Mallet visited the towns destroyed by the Neapolitan earthquake of 1857, and, by a