an instrument to measure the gravity, density, velocity, force, &c. of water and other fluids. See HYDROSTATICS, no. 13.

Though this instrument is incapable of determining the specific gravity of liquids with perfect accuracy, yet in the way of public business it has undoubtedly the advantage of every other, on account of the ease and expedition with which it can be used; and for this reason it has been adopted by government, in or- der to determine the strength of spirituous liquors.

Phil. Trans./Dr Blagden, who was lately employed to make ex- periments on this subject, is of opinion, that glass is the most proper material for the construction of an hy- drometer. Its sensibility depends on the size of its stem. In the old arcometers the stem was made so large, that the volume of water displaced between its leaves and greatest immersions was equal to the whole difference of specific gravity between water and alco- hol, or perhaps more; whence its scale of divisions must be very small, and could not give the specific gravity with much accuracy. On this account weights were introduced, by means of which the stem could be made smaller; each weight affording a new commence- ment of its scale; so that the size of the divisions on a given length was doubled, tripled, &c. as one or more weights were employed, the diameter of the stem being increased in the subduplicate proportion of the increased length of the divisions. This method, however, in our author's opinion, has been carried to excess; and the following is recommended as a proper mean be- twixt these extremes, to determine the specific gravi- ty of spirituous liquors to three places of decimals.

In this method the weight of water is supposed to be unity, or 1 with any number of cyphers annexed: the whole compass of numbers, therefore, from rec- tified spirit to water, at 60 degrees of heat, would be the difference between 825, the weight of rectified spirit, and 1000 the weight of water, which is 175. To make allowance for the lightest spirit and heaviest water, however, at all the common temperatures, the difference may be supposed 220. The stem might show every twenty of these divisions, and thus ten weights would be sufficient for the whole. Hence the inconvenience of shifting the weights, which has always been complained of, would in a great measure be a- voided: as people versed in that business would cer- tainly err so far as to the whole amount of the difference previous to making any trial. Hence also the stem may be made small enough, and the scale graduated so nicely as to make the instrument sufficiently accu- rate.

According to this arrangement, it would be proper to have the weights adapted to the hydrometer marked with the different specific gravities which they are intended to indicate; Zero on the top of the stem without a weight being supposed to mean 800, and 20 at the bottom to signify 820, which number the first weight would carry; the successive weights being marked, 840, 860, &c.; and the division on the stem cut by the fluid under trial, would be a number always to be added to that on the weight; the sum of the two showing the true specific gravity. The weights should undoubtedly be made to apply on the top of the stem, so as never to come in contact with the liquor; and in using the hydrometer, its stem should always be pressed down lower than the point at which it will ul- timately rest, that by being wetted it may occasion no reflaintance to the fluid. The instrument itself should be of as regular a shape and with as few inequalities as possible, that all impediments to its motions may be avoided.