( 34^ ) 
the Water, whether the Coin be right or counterfeit. I fiiall 
add, that, if the piece, to be examined, be noc much heavy., 
er than a Guiny, it may be convenient to pafs a very fmall per- 
forated plate of Copper or Lead over the upper ftem (or pipe, ) 
fo as to make it reH upon the ball before the Inftrument is ad- 
jufted* For, by this means nothing need be altered beneath 
the Ball \ and fuch pieces of metal (of which feveral differingly 
heavy may beeafily provided) being thin and light, will nor, (as 
trial has (hewn) make the Inftrumenc top-heavy, though one of 
them be placed above the Center of Gravity, and may be very 
readily taken off, and (if need be) fcrap'd or fil'd to lighten the 
Inftrument, when an extraordinarily heavy Guiny, or a Coin - 
fome what more weighty than a Guiny , is to be examined. 
But to return to what I was faying about adding a weight of 
Gold to a piece of propos'd Coin 5 in order to this ufe it will be 
neceffary, that thcSlit or Aperture at the bottom of the Inftru- 
' raent, which is to be fhut and open'd by the lateral Screw, be 
made (as it eafily may without Inconvenience) wide enough to 
receive double the thicknefs of a Guiny, that fo different Coins, 
disEpigliJb^FremhySpa/jiJh^Scc. and the Grain-weights, neceflfary 
to bring them to the weight required (in the Air,) may be fecure- 
]y faften'd to the Inftrument by the Screw. 
If the Ball be large, and the Pipe well proportioned to it, 
Coins,that do not much exceed the weight of a Guiny,may be ex- 
amined without much altering the weight of our Inftrument, 
provided it be at firft adjufted fo, as that a Guiny will not de- 
prefs it fo far as not to leave a confiderable part of the pipe a- 
bove water, that the Coin heavier than a Guiny may not be able 
to draw it quite under water. 
According to the Method above defcribed, may half Guhys 
beexamin'd. For, if the Inftrument be good, itwillfliewa 
manifeft difference, if, inftead of an intire Guiny, you faften in 
the fcrevv a half Guiny that you know to be true, and that which 
is 
