Earthquakes.—When the explosion of gas within the earth's interior occurs, and no vent for the escape of gas is at hand, the result is a violent heaving of the crust. Immediately above the origin, the earth oscillates up and down, and this point is called the epicentre. In lessening amount round the epicentre the crust is shaken, till at a distance very little effect is experienced. The wave proceeds outwards, however, in ever-broadening circles, and travels round the globe. When the wave approaches the point on the opposite side of the globe to where the earthquake started, the circles will narrow in, and the shaking of the earth becomes appreciable, at least to instruments, called seismographs, which are constructed to record them. The wave which travels over the surface of the earth heaves the crust as a wave in the ocean heaves the ice floes in the Arctic. Before it reaches the antipodal point, another wave that started from the same origin has reached it, travelling directly through the earth's centre. We know the rate at which waves are transmitted through the substances of which the earth's crust is made, and we know the rate at which the wave is transmitted through the earth's centre, therefore we can calculate the rigidity of the earth's centrosphere, which works out about twice that of steel.
Earthquakes may also be caused by the accumulation of strain in the earth's crust. One part may be overloaded, and is tending to sink; the other part is underloaded, and is tending to rise. At last a time comes when the earth cracks between the two portions, a fault is produced, and coincidently the earth is shaken; but in this case the epicentre will be a straight line. The great San Francisco earthquake was of this type, and the
