HYDROSTATICS, from the Greek t;a'av, water, and IG-Tnyd, I stand, is that branch of the science of hydrodynamics which treats of the properties of fluids at rest. It com prehends the pressure and equilibrium of Don elastic fluids, the doctrine of specific gravities, the phenomena of cohe sion and capillary attraction, and the equilibrium of floating bodies.
Definitions and Preliminary Observations.
A • fluid is a collection of very minute material parti• des, (probably of a spherical form,) which cohere so slightly to each other, that they yield to the smallest force, and are easily moved among one another in every direction.
The phenomena exhibited by fluids, whether they are at rest or in motion, afford us no reason to believe, that the particles of which they are composed possess any po larity, or any tendency to arrange themselves in one.parti cular manner mote than another. When a mass of water is in a state of perfect equilibrium, a certain point of one particle is in physical contact with a certain point of another particle ; but if the equilibrium is destroyed by violent agitation, there is no ground even for con jecturing, that the same points of the particles will re turn into contact when the equilibrium is restored. The recent discoveries, however, which have been made in optics, decidedly prove, that in many fluids the particles assume a particular arrangement, analogous to that which is exhibited in some of the crystals of the mineral king dom, and that they may also be made to assume ano ther arrangement, similar to that which is produced in glass, &c. by compression and dilatation. When these fluids are inclosed in a vessel, the particles uniformly al feet a certain arrangement, which is unequivocally indicat ed by their action upon polarised ligl,t. Sec OrTtes and POLARISATION.
Fluids are divided into elastic and inelastic, or cempressf ble and incompressible fluids. The class of elastic and compressible fluids consists of atmospherical air, and the various gaseous or aeritOrm bodies with which chemists have made its acquainted; while the class of inelastic-or incrompressible fluids comprehends water, mercury, alco hol, and the various oils and liquid acids. The first class, in virtue of their elasticity, arc capable of expanding them selves when they are unconfined, so as to fill any given space, or of having their bulk greatly diminished by me chanical compression ;* while the second class possess this property in such a small degree, that the diminu tion of their bulk by mechanical force is scarcely sus ceptible of accurate mensuration. The science of PNEU MATICS considers the mechanical properties of the first class, and that of HYDRODYNAMICS those or the second class.
Till within the last fifty years, it was considered as ar established fact, that the class of incompressible fluids could not be reduced in bulk by the application of the most powerful forces. This conclusion was deduced from 27 experiment by Lord Bacon, r ho filled a leaden globe with water, and attempted to compress it by a great external force. The fluid, however, made its way through the pores of the metal, and stood like clew upon the surface of the globe. The Florentine academicians repeated the same experiment with a silver globe, and, by violent ham mering, they succeeded in altering its form, and expelling the water through the pores of the silver. These trials
seem to have established the doctrine of the incompressi bility of fluids in its most strict acceptation; but Lord Bacon deduced from them the very opposite conclusion, for, after giving an account of the experiment which we have mentioned, he tells us, that he afterwards computed into how much space the water was driven by this violent pressu re.* Although the experiment of the Florentine Academy of Del Cimento was considered as decisive of this point, yet it occurred to Mr Canton, about the year 1761, that it was not hostile to the idea of a small degree of compressibility ; for the academicians were unable to determine whether or not the water forced into the pores, and through the gold, was exactly equal to the diminution of the internal space by pressure. He accordingly set about a series of experi ments on this subject. Having procured a small glass tube about two feet long, and 1-1 inch in diameter, and with a ball at one end of it, he filled the ball and part of the tube with mercury, brought the whole to.the tempera ture of 50°. of Fahrenheit, and observed that the mercury stood at a point exactly 63, inches above the ball. The mercury was then raised by heat to the top of the tube, and the tube was hermetically sealed. The mercury was then brought to the same degree of heat as before, and it now stood in the tube of an inch higher than it did before. By performing the dame experiment with water exhausted of air, instead of mercury, he found that the water stood in the tube of an inch above the mark. Hence it is ob vious, that the weight of the atmosphere, or 73 pounds avoirdupois, pressing on the outside of the bail, and not on the inside, had squeezed it into less, compass, and that, by this compression of the ball, the mercury and the water would be equally raised in the tube. But the water rose of an inch more than the mercury, and conse quently the water must have expanded so much more than the mercury by removing the weight of the atmosphere. In order to determine how much compression was pro duced, either by the weight of the atmosphere, or by a greater weight, he took a glass ball about 1.6 inch in dia meter, joined to a cylindrical tube 4.2 inches long, and T1, of an inch in diameter, and, by weighing the quantity of mercury that exactly filled the ball, and also the quantity that exactly filled the whole length of the tube, he found that the mercury in of an inch of the tube was the 100,000th part of that contained in the ball, and he divided the tube accordingly with the edge of a file. When the ball and part of the tube was filled with water exhausted of air, he placed it in the receiver of an air pump, and also in the receiver of a condensing engine, and he observed the degree of expansion of the water that corresponded with any degree of rarefaction, and the degree of compression that corresponded with any degree of condensation. In this way he found, from repeated trials, that, when the mer cury was at a mean height, and the temperature of the air 50° of Fahrenheit, the water rose four divisions and 6-10ths, or one part in 21740, by removing the weight of the at mosphere ; consequently the compression of water, under twice the weight of the atmosphere, is one part in 10870 of its own bulk.