[ 6 26 ] 
beft, be made with any great precifion, and which arc 
generally made by minds too little at eafe to be nice 
obfervers of what paffes j moreover, the directions 
themlelves may be l'omewhat varied, by the inequali- 
ties in the weight of the fuperincumbent matter, un- 
der which the vapour paffes, as well as by other 
caufes. 
92. Secondly , We may form fome judgment con- 
cerning the place of the origin of a particular earth- 
quake, from the time of its arrival at different places ; 
but this alfo is liable to great difficulties. In both 
thefe methods, however, we may come to a much 
greater degree of exaCtnefs, by taking a medium 
amongft a variety of accounts, as they are related by 
different obfervers. But, 
93. Thirdly , We may come to the greatefl degree 
of exaCtnefs in thofe cafes, where earthquakes have 
their fource from under the ocean ; for, in thefe in- 
fiances, the proportional diftance of different places 
from that fource may be very nearly afcertained, by 
the interval between the earthquake and the fucceed- 
ing wave: and this is the more to be depended on, 
as people are much lefs likely to be miftaken in de- 
termining the time between two events, which fol- 
low one another at a fmall interval, than in obferving 
the precife time of the happening of fome fingle 
event. 
94. Let us now, by way of example, endeavour 
to inquire into the fituation of the caufe, that gave 
rife to the earthquake of the ift of November 17 ff, 
the place of which feems to have been under the 
ocean, fomewhere between the latitudes of Lifbon 
and Oporto, (though probably fomewhat nearer to 
