HYDROSTATICS relate to the resting equilibrium of non-elastic fluids; and to the pressure of solids immersed therein. A fluid is a body whose parts, are infinite ly minute, capable of dislocation in con sequence of the smallest force, invariably (when suffered to rest) resuming a per fect level surface, and presenting an equal resistance throughout every part of the body immersed.
Philosophers consider fluids to be divi tied into two classes, viz. the elastic, such as air, vapour, and gas ; all which may be compressed more or less: and the inelas tic, viz. water, mercury, spirits &c. which cannot be compressed ; though by being heated they distend considerably. It may be proper to observe in this place, that Mr. Canton, in the years 1762 and 1764, published the results of experiments he had made, whereby it was endeavoured to be proved, that all fluids were compres sible, though in so trifling a degree as not to affect their bulks when under exami nation. With the barometer at 29i, and the thermometer at 50, he declares the following compressions were effected.
We leave the reader to judge whether it be probable, by any apparatus of hu man formation, and under human gui dance, to ascertain that the three mil lionth parts, said to have been compress ed, were really so. Indeed, even the sixty-sixth millionth parts, suffered to be compressed in the spirits of wine, must appear extremely doubtful ; though we cannot but conclude that, as air exists in every atom of nature, more or less, with a sufficient force, every fluid were subject to compression into a smaller space than is occupied by it when perfectly at liber ty. Speaking generally, the definitions above given may be considered as appli cable to- all cases with which we are ac quainted; and may, perhaps, be com pletely true.
We shall commence the detail, incident to this subject, with an account of the method of obtaining the specific gravities of bodies: that is, by Stowing the com parative weights of various solids and fluids, as ascertained by the most care ful and skilful chemists. The reader must, however, consider the weights as taken at a medium. See GRAVITY, speci fic, where' is given a table of specific gra vities.
The reader will observe, that the whole of the above are compared with rain-wa ter, which appear at 1.000 parts ; but it
is very remarkable, that the density of that fluid varies greatly according to its temperature ; and that it by no means af fords a regular scale of weight, or of bulk, in proportion to the degrees of heat. This will be seen from the following table, ta ken from the observations of Dr. Blagderi and Mr. Gilpin.
We must suppose the water of the Dead Sea to be highly impregnated; since it appears to weigh nearly a fourth more than common sea water.
The anomalies lay between 32° and 45°, and are accounted for by thb contraction which takes place in water about to freeze, and its sudden expansion after wards; by this we understand the cause of bottles, pitchers, &c. being burst, when the water they contain freezes. The dif ference in bulk between water and li quors, in the winter and in the summer season, averages about three per cent : hence many great dealers have thought it worth their while to buy only in the for mer season, when the liquors have beep most concentratecL The specific gravity of a body, either fluid or solid, is ordinarily found by means of the hydrostatic balance; a most ingenious device for exactly ascertaining the weight, either immersed in the wa ter, or in the air. The construction of this instrument requires peculiar nicety, but it may be appended to any common balance; as will be understood from the following description. Each scale should have a small hook fixed to the centre of its bottom, or lower side ; so these small weights may be attached by means of horsehair, or fine silk, thence to suspend a body in water without wetting the scale. y First weigh the body in the usu al manner in the scales, with great ex actness; immerse it in water, and the equilibrium will be instantly destroyed. To restore it, put into the scale, from which the body immersed in the water is suspended, as much weight as will bring it even with the other scale, in which the opposing weight remains unaltered: The added weight will be equal to that of a quantity of water equalling the immersed body in bulk. Now, if the weight of the body in air be divided by what it weighed in the the quotient will show how much that body is heavier than its bulk of water.