There are three physical states or conditions of matter, which are the
_solid_, _liquid_ and _gaseous_, which in this connection apply to Ice,
Water and Steam. _A solid_ offers resistance both to change of shape
and to change of bulk.
_A Fluid_ offers no resistance to change of shape. Again fluids can be
divided into _liquids_ and vapors or gases. Water is the most familiar
example of a liquid. A liquid can be poured out in drops while a gas or
vapor flows in a stream or streams.
_Gas_ is a term at first used as meaning the same as the name _air_,
but is now restricted to fluids supposed to be permanently elastic, as
oxygen, hydrogen, etc., in distinction from vapor such as steam which
become liquid upon a reduction of temperature.
It is important to note that experiment proves that every vapor becomes
a gas at a sufficiently high temperature or low pressure, while, on
the other hand, every gas becomes a vapor at sufficiently low and
high pressures. In present popular usage the term gas applies to any
substance in the aeriform elastic condition.
_Hydraulics_ is that branch of science or of engineering which treats
of the motion of liquids, especially of water and of the laws by which
it is regulated.
As a science, hydraulics includes _hydrodynamics_ or the principles of
mechanics applicable to the motion of water.
As a branch of engineering, hydraulics consists in the practical
application of the mechanics of fluids, to the control and management
of water, with reference to the wants of man, including water works,
hydraulic machines, pumps, water wheels, etc.
The term hydraulics, so familiar in daily use, is formed from two Greek
words meaning: 1, water; 2, a pipe; hence, it will be observed with
interest how close the original meaning follows the development of the
science in its practical adaptation; there is always the “pipe” or
holding vessel and the “water” or its equivalent.
From the same elementary word meaning water, in the Greek language,
has been formed very many other words in common use, for example
hydrophobia, hydrogen, hydrant, hygrometer, etc., as well as the
following:
_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.