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TEE POPULAR SCIENCE MONTHLY. 



at the rate before stated, of 400 miles an hour, or about 6 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 draught has continually preceded it, and has withdrawn the water 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 bur- 

 nished 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. 



FOET OF SlNDBEE, ON THE EASTERN BEANCn OF THE INDUS, BEFOBE IT WAS SUBMERGED BY THB 



Earthquake of 1S19. 



which took place in the morning of the 1st of November, was pre- 

 ceded 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 



