( 333 ) 
As to the Fcmof the Inftrument, it confifts of three parts § 
the Ball or Globulous part 3 the Seem or Pipe and that whkh 
holds the Coin. 
The Ball or round part confifts of two thin concave Plates of 
Copper, or other Metal, exaftly foder*d together in the middle^ 
and at the diftanteft parts from the Commiffure there ought to 
beleft twooppofite holes, one in each Plate, for the two other 
parts of the Inftrument. This middle part, though for bi^evity 
fake we name it the Ball, fliould not be exaftly round, bur, for 
theConveniency of fwimming, ofanalmoft Elliptical or Oval 
Form,or rather fomevvhat inclining to that of a very deep double 
Convex Glafs ; or it may be of any other fhape that fliall be 
found fitteft to make the Inftrument keep its erefl pofture ftea= 
dily in the water. ThthigHejs of ic muft be fomewhat greater 
orlefs, as the Plateismadethicker or thinner: But the genera! 
Rule for its Capacity is, that it fliould contain as much Air, as 
may ferve to keep the whole Inftrumenr, when furniflic, if need 
be, with its ballaft and clog'd with aGuiny, from finking be- 
neath the top of the Stem, which 5/^7» is the next part to be ta- 
ken notice of. 
Ifthe Inftrument be to have irsBaliaft (if! may focal! it) 
within \isCd.w'iX.y^ it will be convenient , if not neceffary, that 
it fliould be hollow, like a Pipe, exaftly clofed at the upper 
6[id; but where the Ballaft is to be placed mthout ^ the Pipe 
(hc^uld bemade folid, asof a pieceof V/ire, or a little Cylin- 
der of fome lighter matter that will not foak-in water : But, whe- 
ther \t be hollow or no^it ought to be made very flender,that the 
different depreflions of the Inftrument in the water may be the 
more iiotable. And for the fame reafon ic ought not to be too 
ftiortj, efpecially if it be to be applied toother ufes than the ex- 
amining of Guinys. 
The Inftrument, I moft ufe mecrly for Gmnyty hath its Ball 
about the bignefs of a fmall Hen-egg or rather lefs, and the Pipe 
between four and five Inches long^being foder'd-on to the Ball at 
theuppermoft of .the two holes abovementioned ; at the under- 
s^pft of which is inferted and foder'd the undermoft part of the- 
Inftrument, which I call theScrew,or rheStirrup,becaufefome- 
tlrRe5 
