[ 6o 3 ] 
confidered, that the velocity of any vapour, which 
i.nfinuates itfelf between the firata of the earth, de- 
pends upon the depth of it below the furface 5 for 
the deeper it lies, the greater will be its * velocity. 
We may therefore conclude, from the famenefs of the 
velocity of the earthquakes of the fame place, that 
the caufe of them lies at the fame depth ; and from 
the inequality of the velocity of the earthquakes of 
different places, that their caufes lie at different 
depths. Both thefe are perfectly confident with the 
fuppofition, that earthquakes owe their origin to fub- 
terraneous fires, fince the firata in which thefe fuhfift, 
may be eafily conceived to lie at different depths in 
different parts of the world. 
Sect. V. 
64. From the fame caufe, we may eafily account 
for thofe local earthquakes, which fucceed the greater 
and more ex ten five one's. If there are many fubter- 
raneous fires fubfifting in different parts of the world, 
the vapour coming from one fire may very well be 
fuppofed, as it paffes, to difturb the roof over fome 
other fire, and, by that means, occafion earthquakes 
by the falling in of fome part of it : and this may be 
the cafe, in fome meafure, even where the vapour 
paffes at fome fmall difiance over the fire ; but it will 
be mofi likely to take place, where the vapour either 
* The velocity of fuch a vapour, depending intirely upon the 
elafticity of the earth which is over it, will be, cateris paribus , (if I 
am not miftaken) in the ratio of the depth below the furface. This 
feems to follow from a known law of all elaftic bodies, according 
to which they tend to return to their ftate of reft, when either di- 
lated or comprefied, with forces proportionable to the quantity by 
which they differ from their natural bounds. 
Vol.LI. , 4 1 paffes 
