Page:Popular Science Monthly Volume 29.djvu/322

308 provided with ink-pads, which mark their positions on the dial, thus indicating the hour, minute, and second when the circuit was closed. The second electro-magnet causes a pointer to make a mark on the paper receiving the record of the motion. This mark indicates the part of the earthquake at which the circuit was closed. The duration



of the earthquake is estimated from the length of the record on the smoked paper and the rate of motion of the drum. The nature and period of the different movements are obtained from the curves drawn on the paper.

It may be said, as the result of experiences and observations, that an ordinary earthquake consists of a number of backward-and-forward motions of the ground following each other in quick succession. Sometimes these commence and die out so gradually that those who have endeavored to time the duration of an earthquake have found it difficult to say when the shock began and when it ended. Sometimes the motions gradually increase to a maximum and then die out as gradually; sometimes the maximum comes suddenly; and at other times