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THE POPULAR SCIENCE MONTHLY. 



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 



Fig. 2. 



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. Some- 

 times these commence and die out so gradually that those who have 

 endeavored to time the duration of an earthquake have found it diffi- 

 cult to say when the shock began and when it ended. Sometimes the 

 motions gradually increase to a maximum and then die out as gradu- 

 ally ; sometimes the maximum comes suddenly ; and at other times 



