[ 631 ] 
Plymouth alfo, where the wave arrived about ninety 
minutes later than at Mountfbay, though the difference 
of their diftance from the firft fource could not, upon 
any fuppofftion, be more than forty or fifty miles. 
Sect. VIII. 
ioo. If we would inquire into the depth, at which 
the caufe lies, that occaffons any particular earth- 
quake, I know of no method of determining it, which 
does not require obfervations not yet to be had j but 
if fuch could be procured, and they were made with 
fufficient accuracy, I think fome kind of guefs might 
be formed concerning it : for, 
i o i . Firft , In thofe inftances, where the vapour 
difcharges itfelf at the mouths of volcanos, (as in the 
cafe of the earthquake at Lima) it might, perhaps, 
be poflible for a careful obferver to trace the * thick- 
nefs of the feveral ftrata from thence to the place 
where the earthquake took its rife, or at leaft as far 
as the fhore, if it took its rife from under the fea. 
If this could be once done in any one inftance,. and 
the velocity of fuch an earthquake nicely determined, 
we might then guefs at the depth of the caufe in 
other earthquakes, where we knew their velocity, by 
taking the d* deptns proportional to thofe velocities, 
which probably would anfwer very nearly. 
102. Secondly , If, in any inftance, it fhould be 
poflible to know how much the motion of any earth- 
quake was retarded by pafiing under the ocean, the 
* This is upon the fuppofition, that the under ftrata, in afcend- 
ing up the hills, come to the day in the manner before defcnbed. 
See art. 43. and Fig. 3. 
f See the note to art. 63. 
depth 
