74 NATURAL HISTORY. 



But there is another kind of earthquake, very di* 

 ferent in its effects, and perhaps in its causes. There 

 are earthquakes too which are felt at great distances, 

 and which shake a long course of ground, without any 

 new volcano or eruption appearing. 



To understand rightly what may be the causes of 

 this kind of earthquake, it must be remembered, that all 

 inflammable matters capable of explosion, produce like 

 gun-powder, by inflammation, a great quantity of air ; 

 that this air produced by fire is in a state of very great 

 rarefaction ; and that, by a state of compression in 

 which it is found in the bowels of the earth, it must pro- 

 duce very violent effects. Let us therefore suppose, 

 that, at a very considerable depth, as at about one or 

 two hundred fathoms, pyrites, and other sulpherous 

 matters are to be met with ; and that, by the fermen- 

 tation produced by the filtration of the water, or other 

 causes, they inflame. At first, these matters are not dis- 

 posed regularly by horizontal strata, as the more ancient 

 matters are, which have been formed by the sediment 

 of the waters. On the contrary, they are formed in 

 perpendicular strata, in caverns at the foot of these 

 clefts, and in other parts where the water can act and 

 penetrate. These matters inflaming, will produce a 

 great quantity of air or vapour, the spring of which, 

 compressed in a small space, like that of a cavern, will 

 not shake the earth immediately above, but will search 

 for passages, in order to make its escape. It will there- 

 fore naturally force its way through those parts where 

 it meets least obstruction, and will therefore proceed 

 through the interstices between the different strata, or 

 through any channel or caverns through which it can 

 find a passage. This subterraneous air or vapour will 

 therefore produce in its passage a noise and motion pro- 



