C 62 5 ] 
% 
there may be a quantity of earth fufficient to reprefs 
the vapour of w 7 ater, and keep it within its original 
limits, though ever fo much heated. Now, when- 
ever this is the cafe, it is manifeft, that it can pro- 
duce no effeCt : or, it may happen, that though the 
quantity of earth may not be fufficient abfolutely to 
reprefs the vapour, yet it may be fo great, as to fuffer 
it to expand but very little : in this cafe, an earth- 
quake arifing from it would be but of fmall extent ; 
the wave-like motion would be little or nonej the 
vibratory motion would be felt every-where ; and 
the propagation of the motion would be very quick. 
Th is laft circumftance being aim oft the only one, 
by wffiich thefe earthquakes can be known from thofe 
which owe their origin to {hallo wer fires, it muft be 
very difficult to dillinguifh them with certainty, as 
it is almoft impoffible to dillinguifh the difference of 
the time of their happening in different places, when 
the whole, perhaps, is comprehended within the fpace 
of two or three minutes ; poffibiy, however, f'ome of 
the earthquakes, which we have had in England, may 
have been of this clafs. 
Sect. VII. 
If we would inquire into the place of the ori- 
gin of any particular earthquake, we have the fol- 
lowing grounds to go upon. 
pi. Firjl , The different directions, in which it 
arrives at feveral diffant places : if lines be drawm in 
thefe directions, the place of their common inter- 
feCtion muft be nearly the place fought : but this is 
liable to great difficulties ; for there muft neceffarily 
be great uncertainty in obfervations, which cannot, at 
