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XXXVIII. Account of a new Hygrometer. By M.J.A. De Luc, Citizen of Geneva, F. R. S. and Correspond. Member of the Academies of Paris and J Montpellier.

Read June 10, IN laying before the Royal Society an 1773 account of my attempts to find out a method for measuring the moistureof the air, I think myself obliged to relate the gradual steps of my mind, the obstacles I met with, the means by which I en­ deavoured to overcome them , the degree at which I flatter myself to have arrived, the hopes that may be entertained of farther advances, and the uses which may be derived from my first experiments. Attempts to inventan HYGROMETER. I. In order to proceed regularly in this investiga­ tion, I began by examining the essential requisites in a machine intended to measure humidity, which I found to be the three following: 1st, The settling of of a fixed point, from which every measure of the same kind should be taken, such, for instance, as that of boiling water in a ther­ mometer, when the barometer is at a certai n height. 2d, Degrees equally determined, or comparable, in different hygrometers, sueh as are in the thermo-

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