, an instrument for measuring the properties and effects of water, as its density, gravity, force, velocity, &c.

That with which the specifie gravity of water is determined, is often called an aerometer, or water-poise.

The general principle on which the construction and use of the Hydrometer depends, has been illustrated under the article Spccific Gravity; where it is shewn that a body specifically lighter than several fluids, will serve to sind out their specific gravities; because it will sink deepest in the fluids whose specific gravity is the least. So if AB (fig. 5, pl. 12) be a small even glass tube, hermetically sealed, having a scale of equal divisions marked upon it, with a hollow ball of about an inch in diameter at bottom, and a smaller ball C under it, communicating with the first; into the little ball is put mercury or small shot, before the tube is sealed, so that it may sink in water below the ball, and float or stand upright, the divisions on the stem skewing how far it. sinks.—If this instrument be dipped in common water, and sink to D, it will sink only to some lower point E in salt water; but in port wine it will sink to some higher point F, and in brandy perhaps to B.

It is evident that an Hydrometer of this kind will only shew that one liquid is specifically heavier than another; but the true specific weight of any liquid cannot be determined without a calculation for this particular instrument, the tube of which should be truly cylindrical. Besides, these instruments will not serve for fluids whose densities are much different.

Mr. Clarke constructed a new Hydrometer, shewing whether any spirits be proof, or above or below proof, and in what degree. This instrument was made of a ball of copper (because ivory imbibes spirituous liquors, and glass is apt to break), to which is soldered a brass wire about a quarter of an inch thick; upon this wire is marked the point to which it exactly sinks in proo<*> spirits; as also two other marks, one above and one below the former, exactly answering to one-tenth above proof and one-tenth below proof. There are also a number of small weights made to add to it, so as to answer to the other degrees of strength besides those above, and for determining the specific gravities of different fluids. Philos. Trans. Abr. vol. vi, p. 326.

Dr. Desaguliers contrived an Hydrometer for determining the specific gravities of different waters, to such a degree of nicety, that it would shew when one kind of water was but the 40,000th part heavier than another. It consists of a hollow glass ball of about 3 inches in diameter, charged with shot to a proper degree, and having fixed in it a long and very slender wire, of only the 40th part of an inch in diameter, and divided into tenths of inches, each tenth answering to the 40,000th part, as above. See his Exper. Philos. vol. 2, p. 234.

Mr. Quin and other persons have also constructed Hydrometers, with other and various contrivances, and with different degrees of accuracy; but all nearly on the same general principles.

But there is one circumstance which deserves particular attention in the construction and graduation of Hydrometers, for determining the precise strength of different brandies, and other spirituous liquors. Mr. Reaumur discovered, in making his spirit thermometers, that when rectified spirit and water, or phlegm,| the other constituent part of brandy, are mixed together, there appears to be a mutual penetration of the two liquors, and not merely juxtaposition of parts; so that a part of the one fluid seems to be received into the pores of the other; by which it happens, that if a pint of rectified spirit be added to a pint of water, the mixture will be sensibly less than a quart. The variations hence produced in the bulk of the mixed fluid render the Hydrometer, when graduated in the usual way by equal divisions, an erroneous measure of its strength; because the specific gravity of the compound is found not to correspond to the mean gravity of the two ingredients. M. Montigny constructed a scale for this instrument in the manner before suggested by Dr. Lewis, on actual observation of the sinking or rising of the Hydrometer in various mixtures of alcohol and water, made in certain known proportions. Hist. de l'Acad. Roy. des Sci. 1768; also Neumann's Chem. by Lewis, p. 450, note r.

M. De Luc has lately published a scheme for the construction of a comparable Hydrometer, so that a workman, after having constructed one upon his principles, may make all others similar to each other, and capable of indicating the same degree on the scale, when immersed in the same liquor of the same temperature. This instrument is proposed to be constructed of a ball of flint glass, communicating with a small hollow cylinder, containing such a quantity of quicksilver for a ballast, that the instrument may sink nearly to the top, in the most spirituous liquor, made as hot as possible; to which is also attached a thin silvered tube, for a scale, &c. The whole description may be seen at large in the Philos. Trans. vol. 68, p. 500.

M. Le Roi also published a proposal for constructing comparable Hydrometers. See Hist. de l'Acad. des Scien. for 1770, Mem. 7.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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