HYDROMETER. The principle of the hydrometer is this :—When a body is im mersed in a fluid it loses as much of its weight as is equal to the weight of that par Lion of the fluid which it displaces. Hence, if the same body be immersed successively in two different fluids, the portions of weight which it will thereby lose will be directly pro portional to the specific gravities of those fluids. The body in this case is supposed to be specifically heavier than the fluid. If it be lighter it will float upon the surface, so that its tendency to descend, or its weight, will then be entirely counteracted by the fluid ; from which it appears that, when a body floats upon the surface of a fluid, the weight of the portion of fluid displaced is equal to the entire weight of the body. Now, since the weight of the fluid displaced by a floating body is con stant (being always equal to the weight of the body), whatever may be the density of that fluid, it is obvious that if we, can determine how ranch of the body is immersed we may immediately deduce the specific gravity of the fluid; because, when the weight is constant, the specific gravity varies inversely as the oulk.
Upon this principle is constructed the well known instrument called Sykes's Hydrometer, which is now employed in the collection of the spirit revenue of Great Britain. It con sists of a thin brass stem about six inches in length, passing through and soldered to a hol low ball of the same material, and about one inch and a half in diameter. To the inferior extremity of the stem, from which the hollow ball is about one inch distant, a permanent pear-shaped weight is attached ; so that, when the instrument is placed in a fluid, the other extremity may float perpendicularly to the surface. There are also ten weights of dif ferent magnitudes, nine of which are circular, and one in the form of a parallelopiped. By the successive application of these weights the instrument may be sunk so as to obtain the complete range of specific gravity, from that of pure alcohol to that of distilled water.
See further under ALCOHOMETER and SPE CIFIC GRAVITY.