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 peculiarities in rocky structure, of which we should otherwise have no accurate knowledge. It may teach us something about the transmission of disturbances in elastic media, about the earth's magnetism, the electric currents of the earth, and other kindred problems. It is of interest 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 practical point, we may ask ourselves what are the effects of earthquakes upon buildings, and how, in earthquake-shaken countries, the buildings 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 
Fig. 1. 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 vibrations constituting the earthquake a seismometer or seismograph is a more complicated affair.
The earliest seismoscope of which we find any historical record is that of the Chinese Chôko, 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 animals, and old, peculiar-looking letters. In the inner part of this instrument is a column so suspended that it can move in eight directions. 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
