9. The fpecific gravity of a folid that is lighter than the fluid in which it is immerfed, is found by the following procefs. To the lighter body, whofe fpecific gravity is required, annex another body, that is much heavier than the fluid, so that the compound mafs may fink in the fluid. Weigh the heavier body, and the compound mafs, feparately, both in water and out of it; then find how much each lofes in water, by fubtracting its weight in water from its weight in air; and subtract the less of these remainders from the greater; then fay, as this laft remainder is to the weight of the lighter body in air, fo is the specific gravity of the fluid to the fpecific gravity of that body.

10. The specific gravities of bodies of equal weights are reciprocally proportional to the quantities of weight loft in the fame fluid. Hence is found the ratio of the specific gravities of folids, by weighing in the fame fluid, maffes of them that weigh equally in air, and noting the weight lost by each.

11. A body defcends in a fluid that is fpecifically lighter, but afcends in a fluid that is fpecifically heavier, with a force equal to the difference between its weight and the weight of an equal bulk of the fluid.

12. A body finks in a fluid that is specifically heavier, so far, as that the weight of the body is equal to the weight of a quantity of the fluid of the fame bulk as the part of the body which is immerfed it. the fluid. Therefore, as the specific gravity of the fluid is to that of the body, fo is the whole magnitude of the body to the magnitude of the part immerfed. By this theorem is found the abfolute weight of any body immersed in a fluid: for the weight of that quantity of the fluid which is displaced by the folid body is always equal to the whole weight of the folid; thus, if a boat on the water be fo loaded, that it displace three cubical feet of water, its whole weight will be 3000 ounces or 1874 pounds, that being the weight of three cubical feet of water.

13. In equal folids the specific gravities are as their parts immerfed in the fame fluid. The

The foregoing theorems have been fufficiently demonftrated by various authors, from the principles of mechanics: they are also exactly conformable to experience; as hath been fufficiently proved from several courses of philofophical experiments.

Various tables have been given, by different authors, of the specific gravities of many kinds of bodies. It will be sufficient in this place to give the specific gravities of some of the most useful bodies that have been determined with greater certainty and accuracy. The numbers in this table exprefs the number of avoirdupois ounces contained in a cubical foot of each body; that of common water being juft 1000 ounces, or 62 pounds.

As these numbers are the weights of a cubic foot, or 1728 cubic inches of each of the foregoing bodies in avoirdupois ounces, the quantity, in any other weight, or the weight of any other quantity, may be found by proportion.

For

For example, required the content of an irregular block of common ftone, weighing one hundred weight, or 1792 ounces: here, as 2500, the ounces in a cubic foot of common ftone, is to 1792, so is 1728, the inches in a cubical foot, to 12384 cubical inches, the contents.

Again, what is the weight of a block of granite, the length whereof is 63 feet, and the breadth and thickness each 12 feet, being the dimensions of one of the ftones of granite in the walls of Balbeck? Here the folid content of this tone is 9072 feet; therefore, as 1 is to 9072, fo is 3500 ounces to 31,752,000 ounces, or 885 tons 18 cwt. 3 qrs. the weight of the ftone.

SECT. II.

THE CONSTRUCTION And use of THE HYDROSTATIC BALANCE, HYDROMETER, AND HYDRO

STATIC Bellows.

The Hydroftatic Balance.

THE hydrostatic balance is the most convenient inftrument of any hitherto invented, for discovering the specific gravity of all fubftances, whether fluid or folid. It is conftructed in various forms; but that which is moft generally retained is the following:

VCG (fig. 1, plate 23) is the frand or pillar of the inftrument, which is to be fixed in a table. From the top A, by two filken strings, hangs the horizontal bar B B, from which

VOL. II.

is

is fufpended by a ring i, the fine beam of a balance b, which is prevented from defcending too low on each fide by the gently fpringing piece 1, x, y, z, fixed on the fupport M. The harness is annulated at o, to fhow exactly the perpendicular pofition of the examen, by. the fmall-pointed index fixed above it. On each fide of the piece A is a pulley, over which paffes a ftring, which goes down to the bottom on the other fide, and hangs over the hook at V, which hook is moveable about an inch and a quarter, backward and forward, by means of the fcrew P, fo that the balance may be raised or depressed fo much. But when a greater elevation or depreffion is required, the fliding piece S, which carries the fcrew-pin, is readily removed to any part of the fquare brass rod V K, and fixed by means of the screw.

By these means the motion of the balance is adjusted; the other parts of the apparatus are as follow: D is a piece to fupport the small board H H, fixed under the two scales & and e, and is moveable up and down by a long flit in the pillar above C, in which D flides, having a fcrew in the back part to fasten it when neceffary. From the bottom of the middle of each scale d and e, hangs a brass wire a d and a c, by a fine hook; these wires pafs through two holes, m m, in the table. To the wire ad is fufpended a curious cylindrical wire r perforated at each end for that purpose. This wire r s is covered with paper, graduated by equal divifions, and is about five inches long.

In one corner of the board, at E, is a fixed tube of brass, on which a round wire his fo adapted as to move fomewhat freely, by its flat head I. Upon the lower part of this moves another tube Q, which has fufficient friction to make it remain in any pofition required; to this is fixed an index T, which moves horizontally when the wire his turned round; and therefore may be easily fet to the graduated wire rs. To the lower end of the wires hangs a weight L, which has a wire, with a fmall brass ball g at the end, about a quarter of an inch in diameter. On the other wire ас, from

the