_Pressure or Hydraulic Motors_ form an interesting variety of hydraulic
devices; they consist of working cylinders with valves and pistons,
and resemble forcing pumps in their construction, but differ from
them in their operation; the pistons not being moved by any external
force applied to them through cranks, levers, etc., but by _the weight
or pressure of a column of water acting directly upon or against the
pistons_. Pressure engines or motors are applicable to locations—such
as afford a suitable supply of water for the motive column; but
where-ever refuse, impure, salt or other water can be obtained from a
sufficient elevation, such may be used to raise a quantity of fresh
water by these machines.
_The stress considered in hydro-mechanics is always a pressure_, as
liquids are in general capable of sustaining only a slight tension
without disruption: _the intensity of the pressure is measured by the
number of units of force per unit of area_. Thus we say, one thousand
pounds of pressure per square inch of piston—the pounds and the square
inches are the units used in these calculations.
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