HYDROSTATICS (from (;k. iiocdp, hyd6r, water + ararts6c, statikos, causing to stand, from iararat, histanai, to stand). That branch of mechanics which treats of the properties of liquids in equilibrium, and of solids either totally or in part immersed in liquids. :Many of the laws and phenomena of hydrostatics apply equal ly well to both liquids and gases. i.e. to fluids. A fluid may be defined to be such a form of matter that it yields to any force, however small, which acts to make one layer of the substance move over another; thus the shape of a liquid or gas depends entirely on the forces acting on it. how ever small, and not on the body itself, as in the case of a solid. A portion of liquid left to it self—as a falling drop—assumes a spherical shape owing to the contraction of the surface layer. (See CAPILLARITY.) Under the action of gravity a liquid contained in an open vessel takes the shape of the vessel so far as all the surface is concerned, except that portion in contact with the air and the portions near the edges of this 'free surface.' This portion is hori zontal, being perpendicular to the vertical force of gravity if the liquid is at rest: because, if it were inclined to this. there would be a component of gravity tending to make the higher portion of the liquid slide down. When a fluid is said to be at rest, it is not implied that there is no motion of the molecules. but simply that there
is no blowing, i.e. no currents, no wind. In the ase of the open vessel, there is a force pressing down on the free surface due to the weight of tne atmosphere, and. since the liquid presses against the solid walls, they have a force of reaction against the liquid; thus it is exactly as if the liquid were contained in a vessel and a tight fitting piston were pressing down on its top sur face. if a is contained in a balloon or in a mom. it expands and is uniformly distributed throughout the space open to it; it presses against the containing walls, and they have an equal reaction on the gas. if a small quantity of a certain liquid is poured into a tall cylindri cal vessel, then another liquid with which the first does not mix is poured carefully on top of this, etc.; the equilibrium—if there is any—will be stable only if the density of any one liquid is less than that of the liquid below it and greater than that of the one above it. For, if the density of any layer is greater than that of the one below it. the potential energy of the two will be decreased if the heavier liquid gets to the bottom, and so conies closer to the earth.