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Hydrostatics.

The Hydrostatic-Balance is alsoan Instrument invented on the same Prin-ciple.

a Brass Ball below to screw on with a very nice Shoul-der ; then every Part will be durable, and very securefrom any Alteration of its Weight.

4. When this Instrument is swimming in the Li-quor, the Part of the Fluid displaced by it will be equalin Bulk to the Part of the Instrument under Water,and equal in Weight to the Weight of the wholeInstrument. Suppose the Weight of the Whole were4000 Grains, then ’tis evident we can by this Meanscompare together the different Bulks of 4000 Grains ofvarious Sorts of Fluids. For if the Weight F be suchas shall cause the Hydrometer to sink in Rain-Water,till its Surface comes to the middle Point of the StemG ; and if after this, it be immersed in common SpringWater, and the Surface observed to stand T * e of an Inchbelow the middle Point G, 'tis evident that the sameWeight of each Water differs in Bulk only by the Mag-nitude of one Tenth of an Inch in the Stem.

5. Now suppose the Stem were 10 Inches long, andweigh’d 100 Grains ; then every 10th of an Inch wouldbe 1 Grain Weight, and since the Stem is of Brass,and Brass is about 8 Times heavier than Water, the fameBulk of Water will be equal to f of a Grain; and con-sequently to the £ of g/Js-e Part, that is a 32ooodthPart of the whole Bulk, which is a Degree of Exact-ness as great as can be desired. Yet the Instrument iscapable of still greater, by making the Stem or Neckto consist of a flat thin Slip of Brass, instead of one thatis round or cylindrical : By this Means we increase theSurface, which is the most requisite Thing ; and dimi-nish the Solidity, by which the Instrument is render’dmore exact.

6. In order to adapt this Instrument to all Sorts ofUses, there ought to be two different Stems to screw onand oft in a small Hole at A. One Stem should besuch a nice thin Slip of Brass, or rather of Steel, likea Watch-Spring set strait, as I have just mentioned, onone Side of which ought to be the several Marks, orDivisions, to which it will sink in various Sorts of Wa-ter, as Rain-Water, River-Water, Spring-Water ,