( 335 ) 
or by fcraping or grating offa litcle of the ballaft-plate above- 
mentioned ; or, if you have put any weight into the Cavity to 
poifeit, by taidngout fomeof that, till you have made it lighc 
enough: But if, w^henyou leave the rnflrum^ntto it felf, ite- 
merge, you muft then add a little weight to it, either by putting' 
into the ftem, if ic be hollovv/ome duft-fhot, filings of Leadj or 
fome other minute and heavy Body, orelfe by putting on tha 
(hort ftem abovementioned, that comes our beneath the ball, a 
flat,round,and perforated piece of Lead of weight, fufficient to 
enable the Guiny to deprefs the weight as low as 'tis defired : 
Whichbeingdone, amarkistobemade juft at the place where 
thefurfaceof the water touches the ftem, and then taking out 
your Inftrumenr, fubftitute in the place of your Guiny a litfle 
round plate of Brafs, of the fame weight, or a grain or two hea- 
vier, in the Air ; and putting the Inftrument into the water as be- 
fore, fufFer it to fettle, and make another mark at the interfefti- 
on of the ftem and the Horizontal furface of the Water. 
Aboutthis way ofadjuftingour Inftrum.ent^the following Par- 
ticulars may be noted : 
If a Screw be employed to fuftain the Guiny, the Coin ougbt 
to be fo placed, that one half, according to the eftiuiate of the 
Eye,may beontheright hand, and the other on the lefc handof 
thc lirew ; that the Inftrument being depreffed may continue in 
an ereited pofture,and not fwerve to an inclined. 
Though when the ftem is hollow,and the Inftrument too light, 
it may feem the better to add Quickfilver than any other weight, 
becaufeof itsfluidnefs, and great fpecifick gravity ; yer.unlefs 
the Inftrument be of glafs, nis not fafe to imploy Mercury, be- 
caufe 'tis apt to diffolve the foder. 
If the Marks be made of a white Colour^ they will be fo much 
the more confpicuous : And thefe marks may bemade,if the pipe 
be hollow, by making round Impreffions with afmallfile, and 
incompaffing them with littleGircles of fine wire of Silver, Gold, 
&c. And, if the ftem be folid, it may then be either quite perfo- 
rated at the requifite places, and have the holes filled with chaw- 
ed Maftic, or fome fuch white fubftance that diflblves not in wa- 
ter,or elfe have little holes, that pierce not quite through, fleck 
