HYDROPHYTES. 383 HYDROSTATICS. contact with the material than is true with the more compact air leaves. Thus the absorption capacitj' of the leiif is increased. Finely divided leaves arc also doubtless more able to escape the dangers coming from currents of water than leaves which are r.ioie compact. The hydrophytie plant societies are essentially all edapliic, that is. they are conditioned by local causes. In this respect there is a wide contrast as compared with the xerophytic plant societies. Perhaps some of the ocean formations may be conditioned by climatic causes to some extent, but the ordinarj' hydrophytie societies of swamps and ponds are due to essentially local conditions. Perhaps no plants have such a wide distribution as certain of the hydrophytes. This is particu- larly true of ocean plants, where it can easily be accounted for by the almost universal distribu- tion of the oceans themselves. It is true, to a striking degree, as well of the pond and swamp plants. Such plants as the pond weeds, cattails, and bulrushes, are found almost throughout the world where the habitats are favorable. Perhaps the chief reason for the wide distribution of hydrophytie species is the great ease of dispersal by means of the water itself, but a reason, almost if not quite as important, is furnished by the wide degree of uniformity of hydrophj-tic con- ditions. Since water is colder in summer and warmer in winter than adjoining portions of the land, it is obvious that water plants can. for reasons of temperature, have a much wider dis- tribution than land plants. The hydrophytie plant societies may be rough- ly subdivided into those associated with salt water and those with fresh water. The former are treated under the heads Plankton; Ben- thos: JI.NGROVE SwAiiP: and H.lopiiytes; the latter under the head of Sw.Mps, where it will be convenient, not only to treat the swamps proper, but also, to some extent, the development of the swamps from ponds and lakes. See Dis- tribution OF Plants. HYDROSTATIC PRESS. See Htdbaulic Press. HYDROSTATICS (from Gk. Hup, hydor, water + a-ariKog, statikos, causing to stand, from Icravai, histunai, to stand). That branch of mechanics which treats of the properties of liquids in equilibrimn. and of solids either totally or in part immersed in liquids. Many of the laws and phenomena of hydrostatics apply equal- ly well to bqth 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 .mother: 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 blouing, i.e. no currents, no wind. In the lase of the open vessel, there is a force pressing down on the free surface due to the weight of the atmosphere, and, since the liquid presses against the solid walls, Ihcj' have a force of reaction against the liquid: thus it is exactly as if the liquid were contained in a vessel and a tight- litting piston were pressing down on its top sur- face. If a gas is contained in a balloon or in a room, 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 comes closer to the earth. Fluid Pressure. .s a result of the reaction of the containing walls on a liquid or a gas there is always a 'pressure' at each point throughout the fluid, i.e. there is a force acting over any surface immersed in the fluid; the numerical value of the pressure over a;iy area is by defini- tion the force per square centimeter, and the 'pressure at a point' is the limiting value of the force acting on any surface at that point divided by the area of the surface, as the area is sup- posed to be taken smaller and smaller until it becomes practically a point. This pressure, due to the reaction of the walls, is the same for all points in the fluid. There is also an additional pressure at each point of a fluid on the surface of the earth owing to the fact that any horizontal plane passing through that point has to support the iceight of the column of fluid vertically above it. If the area of this plane is A; the vertical height above it to the top of the fluid, h; the average density of the fluid, p; the accel- eration due to gravity of a falling body, g; the upward force will be phg.l, and, therefore, the pressure is pgh. These two pressures are the only ones which afl'ect our senses or produce mechanical efi'ects in general; but there is also, of course, at any point in a liquid what may be called 'cohesion,' or pressure due to the action of the molecules on each otlier. Some idea of the magnitude of this may be obtained by separating the molecules, e.g. by boiling the liquid. It is greatly afl"ected by dissolving substances in the liquid. The pressure against any surface im- mersed in a fluid at rest is always at right angles to it. otherwise there would be produced a flow- ing owing to the component of the pressure along the surface. Further, the pressure at any point in a fluid at rest is the same in all directions, because, if it were greater in one direction than in another, the fluid would flow. Therefore the pressure at any point in a fluid at rest is the sum of the pressure due to the reaction of the walls. P. and that due to gravity, pgh. As noted above, the former is the same for all points in the fluid. As a consequence, if a fluid is in- closed in a cylinder into which fit two pistons of
