[ 6 32 ] 
depth of the ocean being known, the depth at which 
the vapour pafled would be known alfo ; for the ve- 
locity under the water would be to the velocity, if 
there had been no water, in the fubduplicate ratio of 
the weight in the latter cafe to the weight in the 
former ; hence allowing earth to be about two and 
halt times the weight of water, the depth will be 
•readily found. 
103. ‘ Thirdly , Let us conceive the earth to be 
termed according to the idea before given of it, and 
that tiie fame ffrata are at a medium of the fame 
thicknefs for a very great extent, as well in thofe 
places, where feveral of the upper ones are wanting, 
as where they are not. Upon this fuppofition, we 
iAdK Ciifcover the depth, at which the vapour paffes, 
by coinparing the feveral velocities of the fame earth- 
quake in places, where the * thicknefles of the fuper- 
incumbent mafs are different. It mult be acknow- 
legcd, indeed, that fuch obfervations with regard to 
time, as would enable us to determine thefe veloci- 
ties, are in general much too nice to be expeded : 
the matter, however, is not altogether defperate, as 
we may colled: them, in fome meafure perhaps, from 
other circumffances, fuch, for indance, as the degree 
of f agitation in different waters, the proportional 
+ fuddennefs, with which the earth is lifted in dif- 
ferent places, &c. 
104. As 
* In order to know this difference, it will be neceflary to trace 
the thicknefs of thofe ffrata, which are found in fome of the places 
but are wanting in others. 1 * 
t See art. 71 and 72. 
t This may be known from the diftance to which the mercury 
fubliues in the barometer, upon the firft raifing of the earth by the 
vapour. 
