64 EARTHQUAKES. 



of New- Andalusia the people become uneasy when, 

 in excessively hot weather and after long drought, 

 the breeze suddenly ceases, and the sky, clear at the 

 zenith, presents the appearance of a reddish vapour 

 near the horizon. But these prognostics are very 

 uncertain, and the dreaded evil has arrived in all 

 kinds of weather. 



Under the tropics the regularity of the horary va- 

 riations of the barometer is not disturbed on the days 

 when violent shocks occur. In like manner, in the 

 temperate zone the aurora borealis does not always 

 modify the variations of the needle, or the intensity 

 of the" magnetic forces. 



When the earth is open and agitated, gaseous 

 emanations occasionally escape in places consider- 

 ably remote from unextinguished volcanoes. At 

 Cumana, flames and sulphureous vapours spring 

 from the arid soil, while in other parts of the same 

 province it throws out water and petroleum. At 

 Riobamba, a muddy inflammable mass called moya 

 issues from crevices which close again, and forms 

 elevated heaps. Flames and smoke were also seen 

 to proceed from the rocks of Alvidras, near Lisbon, 

 during the earthquake of 1755, by which that city 

 was ravaged. But in the greater number of earth- 

 quakes it is probable that no elastic fluids escape 

 from the ground, and when gases are evolved, they 

 more frequently accompany or follow than precede 

 the shocks. 



The subterranean noise which so frequently at- 

 tends earthquakes, is generally not proportionate to 

 the strength of the shocks. At Cumana it always 

 precedes them ; while at Quito, and for some time 

 past at Caraccas and in the West India islands, a 

 noise like the discharge of a battery was heard long 

 after the agitation had ceased. The rolling of thun- 

 der in the bowels of the earth, which continues for 

 months, without being accompanied by the least 

 shaking, is a very remarkable phenomenon 



