Page:Popular Science Monthly Volume 31.djvu/376

362 noteworthy fact that there is in general no one motion standing out from the rest as greatly larger than those which precede and follow it. The direction of motion varies irregularly during the disturbance—so much so, that in a protracted shock the horizontal movements at a single station occur in all possible azimuths" (that is to say, to all points of the compass). "The duration, that is to say the time, during which the shaking lasts at any one point is rarely less than one minute, often two or three, and in one case in the writer's experience was as much as twelve minutes."

The horizontal path pursued, in an actual earthquake at Tokio, on March 8, 1881, by the part of the recording instrument which was fixed to the ground, is shown in the annexed figure. It is magnified six fold, and the time occupied from the beginning to the end of this part of the vibration was three seconds. This earthquake, although alarming, did no damage except to crack a few walls.

It is obvious that when the motion is so complicated, the impressions of people present go for little as compared with an automatic record. Observers often differ widely among themselves as to what was the direction of the prevailing oscillation, and the magnitude of the displacement of the ground is generally much exaggerated. It is true that in some of the great historic earthquakes the displacements are supposed to have been considerable; for example, according to Mallet, in the Neapolitan shock of 1857 it amounted to a foot, and Abella assigns six feet as the amplitude in the Manila earthquake of 1881. But, without contesting the accuracy of these estimates, it is safe to say that such displacements are very rare, for, as proved by automatic seismographs, when the motion is as much as a quarter of an inch, brick and stone chimneys are generally shattered.

Every railway-traveler knows that it is not the steady speed, but the starting and stopping, which jars him; that is to say, it is change of velocity by which he is shaken. The misconception of an observer in an earthquake arises from the fact that the sensation of being tossed about comes from the change of velocity to which he is subjected, rather than from the extent of his displacement. Now, the greatest change per second of velocity may be considerable in a vibration, while the amplitude is small.

The force of gravity is the most familiar example of a change per second of velocity, for in each second the velocity of a falling body is augmented by a velocity of thirty-two feet a second. Ewing appears