270 Conjectures concerning the Cause of Earthquakes. 
S 
subterraneous fires; or where, being subject to the shocks of 
some local earthquake of small extent, they now and then are 
affected by an earthquake, produced by some more distant, but 
much more considerable cause. Of this last case, we seem to 
have had some instances in the earthquake of the Ist of Novem- 
ber 1755, and those local ones, before mentioned, which suc- 
ceeded it. 
63. As we may reasonably infer from many earthquakes com- 
ing to the same place, from the same point of the compass, that 
they are all derived from the same cause, and that a permanent 
one; so we may reasonably infer the same thing also, from their 
being propagated with the same velocity; but this argument will 
still come with the greater force, if it be considered, that the ve- 
locity of any vapour, which insinuates itself between the strata 
of the earth, depends upon the depth of it below the surface ; 
for the deeper it lies, the greater will be its velocity*. We may 
therefore conclude, from the sameness of the velocity of the 
earthquakes of the same place, that the cause of them lies at the 
same depth ; and from the inequality of the velocity of the earth- 
quakes of different places, that their causes lie at different depths. 
Both these are perfectly consistent with the supposition, that 
earthquakes owe their origin to subterraneous fires, since the 
strata in which these subsist, may be easily conceived to lie at 
different depths in different parts of the world. 
SEcTion V,—64. From the same cause, we may easily account 
for those local earthquakes, which succeed the greater and more 
extensive ones. If there are many subterraneous fires subsisting 
in different parts of the world, the vapour coming from one fire 
may very well be supposed, as it passes, to disturb the roof over 
some other fire, and, by that means, oecasion earthquakes by the 
falling in of some part of it: and this may be the case, in some 
measure, even where the vapour passes at some small distance 
over the fire; but it will be most likely to take place, where the 
vapour either passes at some distance under it, or between the 
stratum, in which the fire lies, and that next above or below it. 
[To be continued. ] 
* The velocity of such a vapour, depending entirely upon the elasticity 
of the earth which is over it, will be, ceteris paribus, (if 1am not mistaken) 
in the ratio of the depth below the surface. This seems to follow from a 
~ known law of all elastic bodies, according to which they tend to return to 
their state of rest, when either dilated or compressed, with forces propor- 
tionable to the quantity by which they differ from their natural bounds. 
XLII. On 
