Hydrometer

This [the 9 December 1668 mention of “a glass bubble, to try the strength of liquors with” P.G.] seems to refer to the first form of the Hon. Robert Boyle’s hydrometer, which he described in a paper in the “Philosophical Transactions” for June, 1675, under the title of a “New Essay instrument.” In this paper the author refers to a glass instrument exhibited many years before by himself, “consisting of a bubble furnished with a long and slender stem, which was to be put into several liquors to compare and estimate their specific gravity.” Boyle describes this glass bubble in a paper in “Philosophical Transactions,” vol. iv., No. 50, p. 1001, 1669, entitled, “The Weights of Water in Water with ordinary Balances and Weights.”