HYDROMECHANICS. An important property of a fluid is that the shape of any isolated mass depends largely upon the form of the containing vessel or confining walls. If water is poured from a jug into a glass the form of the water in the glass is determined largely by the shape of the glass, and not at all by the shape of the jug which previously contained it. The shape, but not the extent, of the water-glass surface is determined by the form of the glass. The shape of the water-air surface depends partly upon the form of the glass and partly upon the physical properties of water, air and glass. Generally this surface is very nearly a horizontal plane. This is particularly noticeable in the part which is not close to the glass, and the natural inference is that this levelling of the water-air surface is due to some physical properties of the two fluids. The water-air surface appears to be well defined, but if the glass contained chlorine, a greenish-yellow gas heavier than air, the boundary between the air and chlorine would not be so well defined and would become difficult to detect in a short time. The air and chlorine, in fact, diffuse into one another, and in a few hours the chlorine would have practically all disappeared. Water will also do this to some extent on account of evaporation, but this process is slow in comparison with the diffu sion of gases.
The diffusivity of a gas and the slow evaporation of a liquid can only be adequately explained by a kinetic theory in which a fluid is supposed to be composed of discrete atoms or molecules. In hydromechanics a fluid is treated as a continuous substance and the phenomena of diffusion are largely ignored. The science may be expected, then, to describe the mechanics of liquids more per fectly than the mechanics of gases, and this is indicated by the choice of the prefix "hydro" which is derived from the Greek word ii& ip meaning water.
Hydromechanics is concerned chiefly with the visible motion of a fluid and with certain phenomena which manifest themselves when the fluid appears. to be at rest. These phenomena are gen erally grouped under the heading hydrostatics. By means of the theory of hydrostatic pressure a reason is found for the fact that the free surface of a liquid at rest is very nearly a horizontal plane, while, with the aid of the theory of surface tension, it can be understood why the free surface rises or falls in the im mediate neighbourhood of the wall of the containing vessel. The phenomena of capillarity or surface tension (q.v.) may be ex pected to enter when two or more different substances are in contact. In a preliminary treatment of hydromechanics these phenomena are ignored, attention being paid chiefly to the main body of the fluid which is generally treated as nearly or actually homogeneous. Except at a boundary between two different fluids, the nature of a fluid generally changes so slowly from point to point that capillary phenomena associated with such changes can certainly be ignored. In many hydrodynamical problems the density of the fluid is assumed to be constant, and this assumption is a good approximation to the truth, but when large masses of fluid are involved or when the fluid is moving very rapidly, the simplifying assumption of constant density may not be advisable.