Page:Popular Science Monthly Volume 2.djvu/536

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518
THE POPULAR SCIENCE MONTHLY.

Prof. Tyndall observes that, "where the acting force is small and the time great, the result is a slow and almost inappreciable change." Thus, great areas of land may be elevated or depressed. "But where the intensity is great and the time small, sudden convulsion must ensue." Thus, in an instant, mountains may undergo a change of elevation, or be shaken to fragments, or tracks of land sunken and over-flowed. In the delta of the Indus are extensive areas of level ground, over which native villages were scattered, with fortifications and other defences. Of these, the fort at Sindree is shown in Fig. 7 as it was

Fig. 4.
PSM V02 D536 Circular hollow section.jpg
Section of one of the Circular Hollows.

before the disastrous earthquake of 1819. 2,000 square miles of the delta sank from six to 12 feet, and was thus overflowed by the sea. The village of Sindree and its fortifications were upon the sunken area.

Northward, about 5½ miles from Sindree, a range of very low hills was elevated during the earthquake. It was seen over the expanse of waters, and extended about 50 miles, with a breadth in places of 16 miles, and was called by the natives, "Ulla Bund, or the Mound of God."

In 1822, just half a century ago, an earthquake occurred in Chili, of terrific violence, even for that region of convulsions. It was estimated that 100,000 square miles of land were elevated from two to seven feet, the rise being greatest inland, and probably included a portion of the Chilian Andes. The location of the force must have been at great depth, perhaps not less than 20 miles below the base of the Andes; and it is probable that the entire superincumbent mass underwent a change of level of from two to seven feet of perpendicular elevation.

The earthquake at Lisbon, in 1755, has impressed the public mind more than any other in modern times. The shocks, one of which exceeded all the others in violence, continued six minutes. The mountains near were shaken to their foundations, and everywhere split and rent. No part of the city was seriously injured which was built on the limestone or basaltic formations; but the shocks were most violent and