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THE POPULAR SCIENCE MONTHLY. 



ways. As they seem to be connected with volcanic action, the study 

 of them may help to throw light on that, and vice versa. As an 

 earthquake-wave travels along from strata to strata, the study of its 

 reflections and changes in transit may lead to the discovery of pecul- 

 iarities in rocky structure, of which we should otherwise have no accu- 

 rate knowledge. It may teach us something about the transmission 

 of disturbances in elastic media, about the earth's magnetism, the elec- 

 tric currents of the earth, and other kindred problems. It is of in- 

 terest to the meteorologist to know the connections which probably 

 exist between earthquakes and the fluctuations of the barometer, the 

 changes of the thermometer, and the quantity of rainfall. In a prac- 

 tical point, we may ask ourselves what are the effects of earthquakes 

 upon buildings, and how, in earthquake-shaken countries, the build- 

 ings are to be made to withstand them. 



A typical earthquake consists of a series of small tremors succeeded 

 by a shock, or of a series of shocks separated by more or less irregular 

 both in period and in amplitude vibrations of the ground. Man 

 can take but little account of these movements, for they come upon 

 him by surprise, and, by the time he is ready to begin to observe, they 

 are over. Hence we must have recourse to instruments. It is easy 

 enough to construct an instrument that shall move at the time of an 



earthquake, and leave a record 

 of its motion a seismoscope ; 

 but an instrument that shall 

 record the period, extent, and 

 direction of each of the vibra- 

 tions constituting the earth- 

 quake a seismometer or seis- 

 mograph is a more compli- 

 cated affair. 



The earliest seismoscope of 

 which we find any historical 

 record is that of the Chinese 

 Choko, which was invented in 

 a. d. 136. According to the 

 historical account given of it, 

 it consisted of a spherically 

 formed copper vessel (Fig. 1), 

 eight feet in diameter. " Its 

 outer part," the account says, 

 "is ornamented by the figures of different kinds of birds and ani- 

 mals, and old, peculiar -looking letters. In the inner part of this 

 instrument is a column so suspended that it can move in eight di- 

 rections. Also, in the inside of the bottle, there is an arrangement 

 by which some record of an earthquake is made according to the 

 movement of the pillar. On the outside of the bottle there are eight 



Fig. 1. 



