( 33^ ) 
mo ic, and chefe may likewife be filled with the fame fubftance, 
which, if further diftinGion be defired, may have fome parts of 
it differingly coloured before they be cmploy'd. 
Twill be requificcjto imploy inadjufting thelnftrument one of 
the heavieft Guinys you canger,todeprefs the Inftrument as low 
as 'fislike to be by any piece of that Coin, lead otherwife meet- 
ing with one confiderably heavier than that you made ufe of,the 
Jnflrument may be thereby made to fink to/he very bottom of 
the water. 
The Reafon why 'tis above prefcribed, that the Inftrument be ' 
immcrfed almjly notqiiite, to the /Ipex of theftem, is, becaufe 
I have found, t ha tG»/;yi are not all precifely cf the fame weight, 
nor all waters neither ; and therefore 'cis fafeft, to leave a fmali 
parcof theftem, as an eighth, or, in longer I nftruments, a quar- 
ter of an Inch, extant above the water, that we may fecure the 
Inftrument from being by a heavier Guiny made quite to 
fink. 
1 forefee, it may be hence objefied, that thefe Contingencies 
may make our Inftrument ufelefs : To which it is not difficult to 
anfwer, that,though fome Guwjs weigh a grain or two more than 
others, it is not thatwiU fruftrate the ufe of our Inftrument, 
and lefs will the difference of our waters do it, fince ( as I have 
obferved in another Paper, v;here I mention fome Trials of this 
kind) having examined and compared together the fpecifick 
gravities of (commonj Pump-water, Thames-water, and Rain- 
water, I found the difference far more inconfiderable than one 
would have thought, and confequently unable to keep Hydro- 
ftatical Trials of Metals from being accurate enough fox pra6ice, 
and more exadi than thofe troublefome and chargeable ones that 
are commonly relied on. 
Thefe Anfwers to th« recited Objeflions, will be made good 
by this. That 'tis not a doubtful or inconfiderable difference, 
that appears upon the differing deprcfUons of the Inftrument, 
that are made by a true Gi]iny,and by a piece of Brafs or of Cop- 
per,of the fame weight w ith it in the Air. For, in the Inftru- 
ment lately defcrib'd, though fmaller than moft that I haveim- 
ployed, the diftance betwixt the mark to which the Gold, and 
ihit 
