Page:Popular Science Monthly Volume 2.djvu/538

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at the rate before stated, of 400 miles an hour, or about 6½ miles in a minute, in the deep ocean. It is described by Mallet as "a low, broad swell of the sea. It might pass beneath the vessel unobserved." Approaching the shore, the front becomes elevated. The under-draught has continually preceded it, and has withdrawn the water from the shore, so that vessels at anchor are frequently grounded, and the wave seems to stand upon the bottom like a gigantic wall. At Arica it was unbroken by a ripple, and "shone in the sun like burnished silver."

A notion prevails that earthquakes are always preceded by unusual conditions of the atmosphere, but careful observations have shown that they occur during all kinds of weather. The Lisbon earthquake,

Fig. 7.
PSM V02 D538 Fort of sindree.jpg
Fort of Sindree, on the Eastern Branch of the Indus, before it was submerged by the earthquake of 1819.

which took place in the morning of the 1st of November, was preceded by a "period of clear autumnal weather," but the morning was calm, foggy, and warm. At Arica, as we have learned, the sky was serene and the atmosphere tranquil. Some of the greatest convulsions have been preceded by a close, hazy sky. Sir Charles Lyell observes that "irregularities in the seasons frequently precede and follow shocks. Sudden gusts of wind interrupted by dead calms, violent rains at unusual seasons, or in countries where they seldom occur, are phenomena often attending earthquakes."

The number of important earthquakes up to the year 1881, of which we have a reliable account, is, according to Prof. Ansted, 7,000. So meagre are early records that only 787 of these are spoken of previous to the year 1500. There is a catalogue of 3,340 which occurred from