HYDROSTATICS, that branch of engi neering science which deals with the mechanics of liquids at rest, and the action of forces upon static liquids; that is, the theories of pressure and equilibrium as concerning liquids, especially water.
The mechanics of confined liquids are based largely on the following conclusion: any in crease of pressure at one point of a confined liquid is transmitted almost instantly to every other point. This proposition discards the minute modifications of capillarity, skin friction and surface tension, which are practically negli gible; it also dismisses the almost infinitesimal compressibility of a liquid as inconsequential. A liquid has size but not shape; if poured out on a level table it covers the entire surface, and will fill all depressions, but its upper surface will be uniform and level. In a large body of water it is seen that this level conforms to the curvature of the earth. It is a common pro cedure to explain this level by referring it to the constant pressure of the atmosphere 14.7 pounds per square inch,. and this is not mislead ing if it is borne in mind that the pressure of the atmosphere is due to the attraction of gravi tation — to which the water level is actually attributable. A liquid offers no permanent re sistance to forces tending to change its shape. The particles of a fluid are mobile; and while in the case of liquids very considerable forces of cohesion are exhibited, yet the particles show great freedom to alter their relative positions, and to pass from place to place within the gen eral mass. A very important property that i follows from the nature of fluids is that of the equable transmission of pressure. Suppose a liquid inclosed in a vessel A, which is fitted with a piston P. If pressure is applied to P it will be transmitted in all directions through cylinder. The collar consists of a circle of solid leather, or cup-leather AAA. When water pres sure is applied to move the piston, some of it leaks into the ring-space AA, and the leather ring curls and swells, filling the space water tight, so that there is no leaking. In modern practice this primitive device has been suc the liquid. If other openings are made, and if they arc fitted with pistons, it is evident that to keep each of these pistons in its place, pressure inward must be applied. The pressure that
must be applied to any piston equal in area to the area of P is equal to the pressure on P; and if the area of one of the other pistons is greater or less than the area of P, the pressure required to keep it in its place is proportionately greater than or less than the pressure that is applied tO P.
It is thus apparent that there is a normality of pressure, and in calculating such pressure it is only necessary to consider one direction of pressure. The law is: Any addi tional pressure applied to a confined liquid at any point will be transmitted equally to any other point of contract. This principle finds application in the hydraulic press. It is to be noted, however, that water under static pressure does not produce work until it begins to move, and moving water comes under the head of hydraulics. These two divisions of science are therefore closely related, and need to be studied together. (See HYU1LAULICS). In the apparatus known as the °hydrostatic bel lows," a true hydrostatic equilibrium is ob tained. This experimental contrivance consists of a pair of circular boards connected by a band of pliable leather fastened watertight around the circumferences of the boards (like a pair of bellows). A small tube inserted into an opening near the bottom connects a tall tube rising perpendicularly. Heavy weights may be put on the upper circular board, and if water be then poured into the upright tube they will be raised up by the pressure of the water from below. For the pressure to which the under side of the circular board is exposed is equal to the weight of a column of water whose section to the surface. The upward pressure on the solid is greater than the downward by an amount equivalent to the weight of the liquid displaced by the solid; for if, instead of the solid, the quantity of liquid displaced by it were present, its weight would be upborne by the pressure on every side. These pressures now act on the solid and whether or not the solid floats tinder their influence, as much of the weight of the solid as corresponds to this pres sure is supported by the surrounding fluid. Experiments that are made for the purpose of determining the specific gravity of bodies heav ier than water also depend on this principle. Sec SPECIFIC GRAVITY.