_Hydromechanics_ is that branch of natural philosophy which treats of the mechanics of liquid bodies, or in other words, of their laws of equilibrium and motion. Hydromechanics comprises properly those phenomena of liquids by which these bodies differ from solids or from bodies at large; hence, its foundation is laid in the properties that distinguish the liquid from other states of bodies, viz.: the presence of cohesion, with great mobility of parts, and perfect elasticity. _Hydrostatics_ is that branch of science which relates to the pressure and equilibrium of non-elastic fluids, as water, mercury, etc.; thus, the hydrostatic press is a machine in which great force with slow motion is action communicated to a large plunger by means of water forced into the cylinder in which it moves, by a forcing pump. _Statics_ treats of forces that keep bodies at rest or in equilibrium, the water through which the force operates in the hydrostatic press always remaining at rest serves as a good illustration. _Pneumatics_ is that branch of science, which relates to air, or gases in general or their properties; also of employing (compressed) air or other gas as a motive power. The use of pneumatic pumping machinery is constantly increasing, especially of the direct pressure types; under the section of this work relating to Air Pumps additional data will be presented. _Hydropneumatics_ is defined as involving the combined action of water and air, or gas, as shown, for example, in the hydropneumatic accumulator. The word is a compound formed of the Greek words meaning water and air. _Semi-liquids._ All the results stated in reference to water are further modified in those semi-liquids which have greater or less viscidity, as pitch, syrup, fixed oils, etc. Viscosity may be defined as the quality of flowing slowly, thus the viscosity of such liquids as have been named is very great as compared with that of a mobile liquid like alcohol. HYDRODYNAMICS. _Water, considered from a chemical standpoint_, is a compound substance consisting of hydrogen and oxygen, in the proportion of two _volumes_ of the former gas to one volume of the latter; or _by weight_ it is composed of two parts of hydrogen united with sixteen parts of oxygen. It should be noted that the union of these two gases is effected by _chemical action_ and not by _mechanical mixture_. Pure water is transparent, inodorous and tasteless. Under ordinary conditions water passes the liquid form only at temperatures lying between 32° F. and 212° F.; it assumes a solid form, that of ice or snow at 32° F., and it takes the form of vapor or steam at 212° F. _There are four notable temperatures for water_, namely: 32° F., or 0° C. = the freezing point under one atmosphere. 39°·1 or 4° = the point of maximum density. 62° or 16°·66 = the standard temperature. 212° or 100° = the boiling point, under one atmosphere.