74 EARTHQUAKES. 
uncertain, and the dreaded evil has arrived in all 
kinds of weather. 
Under the tropics the regularity of the horary 
variations 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 opened 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. 
