Page:Popular Science Monthly Volume 27.djvu/846

824 area of a circle of 30°, or nearly 2,000 miles radius, or a diameter of one sixth the circumference of the globe.

The ocean basins are also disturbed, as is proved by the shocks suffered by ships on the deep, without any apparent external cause, and which give an impression as though the vessel were running upon a shoal. The movements of the littoral, also, however slight may be their intensity, are transmitted to the liquid mass. The sea retires from the shore, leaving the bottom dry, sometimes for several miles. Then it returns swiftly upon itself, and, overleaping its normal limit, precipitates itself with fury, and as if in assault, toward the interior of the country, as an enormous wave, which has been frequently known, as in Chili, to reach a height of 100 or 125 feet. Then it retires, carrying out upon the deep whatever it has gathered up in its passage. This terrible oscillation is repeated three or four times with decreasing energy, unless the movements of the ground persist. These invasions of the sea, or tidal-waves, are often more dreaded by the people, who have had experience of them, than the shocks on land. The huge wave* are also propagated in the ocean to a very great distance from the center of disturbance. Twelve hours after an earthquake that destroyed the city of Simoda, Japan, in December, 1854, a formidable wave was precipitated upon the Californian coast, 5,600 miles away. In 1868 a wave of similar origin destroyed Arequipa and Arica, Peru, and ingulfed 30,000 persons. It seemed to have come from Honolulu, in twelve homes, or with a speed of 450 miles an hour. The most striking example is that of the wave that followed the Krakatoa explosion, which traveled over a distance of 11,890 miles, or half-way round the globe, in twenty hours and fifty minutes, or at the rate, according to M. Bouquet de la Grye's estimate, of about 900 feet a second.

Earthquakes may also effect permanent changes in the relief of the land, not only in the shape of crevasses and the overturning of rocks; slight though appreciable elevations have also been observed, as in Chili, in 1822, 1835, and 1837. In the last case, marine shells, still alive and adhering to the rocks on which they had grown, appeared above the level of the sea, and served as indisputable witnesses of the change of level which had been suddenly produced.

Movements of another class are extremely weak, and can not be perceived without the aid of special and delicate instruments. In 1869 M. d'Abbadie, examining the surface of a mercurial bath in his observatory at Abbadia, discovered very slight but frequent variations in the situation of the vertical, from which he inferred that the ground is not always motionless, even when it has all the appearances of being so. The same fact has since been confirmed in many places. Abrupt oscillations that have been frequently perceived in the astronomical glasses at the observatory of Pulkowa, and were observed at Nice on the 27th of November, 1884, are also revelations of disturbances in