340 Conjectures concerning the Cause of Earthquakes, &c. 
the latter case to the weight in the former: hence aliowing earth 
to be about two and half times the weight of water, the depth 
will be readily found. 
103. Thirdly, Let us conceive the earth to be formed accord- 
ing to the idea before given of it, and that the same strata are at 
a medium of the same thickness for a very great extent, as well 
in those places, where several of the upper ones are wanting, as 
where they are not. Upon this supposition, we may discover the 
depth, at which the vapour passes, by comparing the several ve- 
locities of the same earthquake in places where the thicknesses* 
of the superincumbent mass are different. It must be acknow- 
ledged, indeed, that such observations with regard to time, as 
would enable us to determine these velocities, are in general much 
too nice to be expected: the matter, however, is not altogether 
desperate, as we may collect them, in some measure perhaps, 
from other circumstances; such, for instance, as the degree of 
agitation in different waters}, the proportional suddenness} with 
which the earth is lifted in different places, &c. 
104. As the observations relating to the earthquake of the Ist 
of November 1755 are too gross, it would be in vain to attempt, 
by any of the foregoing methods, to determine with any cer- 
tainty the depth at which the cause of it lay; but, if I might be 
allowed to form a random guess about it, I should suppose, (upon 
a comparison of all circumstances,) that it could not be much less 
than a mile, or a mile and half, and [ think it is probable, it did 
not exceed three miles. 
Conelusion.—105. Thus have I endeavoured to show how the 
principal phenomena of earthquakes may be produced, by a cause 
with which none, that I have seen, appear to me to be incom- 
patible. As I have not knowingly misrepresented any fact, so 
neither have I designedly omitted any that appeared to affect the 
main question; but, that I might not unnecessarily swell what 
had already much exceeded the limits at first intended for it, I 
have omitted, 
106. First, Those minuter appearances, which almost every 
reader would easily account for, from what has been said already, 
and which did not seem to lead to any thing further: such, for 
* In order to know this difference, it will be necessary to trace the thick- 
ness of those strata, which are found in some of the places, but are wanting 
in others. + See art. 71 and 72. 
{ This may be known from the distance to which the mercury subsides in 
the barometer, upon the first raising of the earth by the vapour. I don’t 
find that this phenomenon, which is a common attendant on earthquakes, 
was observed any-where, at the time of the earthquake of the Ist of Novem- 
ber 1755, except at Amsterdam, where the mercury subsided more than an 
inch. See Hist. and Philos. of Earthq. p.309. 
instance, 
