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tlie knowledge of Caufes men may employ exceedingly differing 
means to produce the fame EfFefls (as, incur Cafe, Gold,tliac 
Chymifts and Say-mafters are fain to examine by the fire, we 
examine by water) /i;?^ that Philofophical Truths, and particu- 
larly Hydroftatical ones, are not lightly to be defpifed as airy 
and empty Speculations, fince they may be fometimes applied to 
pradical Ufes, to which at firft fight they feem to have no rela- 
tion at alL 
The fecond Seflion, 
Dejcribifig the Conftruliion of thk Injlmment. 
I Proceed now to the ConftruCiion of the Inftrument it felf, ia 
which are to be confidcr'd the Matter and the Form, 
The Matter may be Glafs, Copper, Silver, or almoft any o- 
ther folid Body, that is, or may be made, fit to floatinthc wa- 
t€rj with a Guiny hanging at it, and of a Texture clofe enough 
to keep out the water. For, ifany of^^/r/lhould, by foaking 
or other wife,getin,it may alter theGravity of the Inftrument,and 
render it deceitful. 
My firft Trials were made with bubbles of Glafs , furnifli'd 
with flender ftems. Hermetically fealed at the top ; and thefe , 
when one can procure an Artificer that can blow them wcll^ are 
both thtgeutilejl and the cheapeji, and for fome of the Ufes, that 
may hereafter be mentioned, they are almoft ihtonly ones that 
can be fitly imployed. But, befidesthat 'tis not eafietomeet 
with Artificers that can give Glafs the right bignefs and fliape , 
thofe, as all other Inftruments of glafs, being very frail and fub- 
jefl: to be broken^ thefafeft way and more durable is, to make 
them of fome Metal, efpec/ally either Copper or Silver , (of 
which the former is far more cheap, and the other more gentile, 
but either will ferve well $ ) in regard they are lefs heavy, and, 
being more ftiff, wil! maintain their figure better than Gold or 
Lead, Copper and Silver will alfo fuffer themfelves to be bea- 
ten into plates thin and yet ftrong enough, and are not fo fubjeft 
to ruft as Iron and Steel But in fome cafeS, efpecially in want of 
metallin Inftruments^we may makeufe of well feafon'd wood,laid 
over with fome Chrna Varnifli,or fome other that is very clofe. 
A? 
