Page:Popular Science Monthly Volume 29.djvu/323

Rh during an earthquake the observer's feelings distinctly tell him that there are several maxima. The chief results which investigators have aimed at have been the measurement of the amplitude, period, direction, and duration of the motions; and attention has been given to the velocity with which the disturbance is propagated.

If we were to ask the inhabitants of a town which had been shaken by an earthquake the direction of the motion they had experienced, it is not unlikely that their replies would include all the points of the compass. Many, in consequence of their alarm, have not been able to make accurate observations. Others have been deceived by the motion of the building in which they were situated. Some tell us that the motion was north and south, while others say that it was east and west. A certain number have recognized several motions, and among the rest there will be a few who have felt a wriggling or twisting. Leaving out exceptional cases, the general result obtained from personal observation as to the direction of an earthquake of moderate intensity is extremely indefinite, and the only satisfactory information to be got is that derived from instruments or from the effects of the earthquake as exhibited in shattered buildings and bodies which had been overturned or projected. By the use of seismographs it has been shown that during an earthquake the ground may move in one, two, or several directions, and it is only when a decided shock is experienced that we can determine with any confidence the direction in which the motion has been propagated. The apparently twisting or wriggling motions are supposed to be the result of combinations of linear movements in different directions. It is often difficult, when reading accounts of earthquakes, to determine the length of time a shaking was continuous. Disturbances which succeed one another with sufficient rapidity to cause an almost continual trembling of the ground may be regarded as collectively forming one great seismic effort, which may last a minute, an hour, a day, a week, or even several years. Strictly speaking, they are a series of separate earthquakes, the vibrations of which more or less overlap. Whenever a large earthquake occurs, it is generally succeeded by a considerable number of smaller shocks. Disturbances of this character are compared by Mallet to "an occasional cannonade during a continuous but irregular rattle of musketry." Continuous motions perceptible to our senses without the aid of instruments usually last from thirty seconds to about two or three minutes. The principal vibrations or shocks of the disturbance occur at unequal intervals; and in the periods of vibration there are irregularities in any given earthquake, and different earthquakes differ from one another. The extent of the movement is much less than the feelings of one experiencing a shock would lead him to estimate it. It is usually within the fraction of an inch in either direction. According to Dr. Wagener, the earth's horizontal motion at the time of a small earthquake is usually only the fraction of a millimetre, and it seldom exceeds three or