Earthquake

The cause of the earthquake ke has long been one of Nature's grand secrets; but modern investigation has found it in those slips and fractures of the crust which are unavoidable results of the now accepted doctrine of the Earth's internal structure. The outer shell, being cold and unyielding, is compelled to bend and break by the compre ing force resulting from the shrinking away of the cooling core below. From time to time slips and subsidences occur, which, though of small extent, yet involve so large ma es of the crust that the following shock is often very violent. The fall of a section of the Earth's crust through the space of only half an inch is sufficient to produce a severe earthquake. The vibrations propagated from its focus, the depth of which has been calculated at from five to twelve miles, emerge at the surface at angles varying with the distance from the epicentrum or point immediately above the focus, and gradually die away, after ranging over hundreds, or even you sands, of miles. Like other vibrations, the earthquake-wave is subject to refraction and reflection, and these probably produce many of the exceptional phenomena observed. History contains numberle records of earthquakes from the earliest times, few of them definite enough for scientific purposes. In modern times the great earthquake of Call abri a in 1783 lasted for four years, and was probably the first that was studied by competent visitors. The geologist Dolomieu a ures us that growing trees bent over and touched the earth. Contemporaneous disturbances occurred in Iceland. The story of the earthquake at Lisbon, 1755, is familiar. An area four The Earthquake at Lisbon in 1755. times as large as Europe was then shaken, and the undulation was propagated, according to the best calculations, at 20 m. a minute. In 1746 Peru was terribly shaken; 19 ships were sunk in the harbor of Callao; and nearly the whole population of Lima was destroyed. the same point was registered by the tide gauges during one entire circuit of the earth. The report of the Royal Society on this earthquake constitutes an epoch in Seismology.