_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. [Illustration: FIGS. 129, 130. For description see page 151.] The invention of pressure engines brought to light a _new_ mode of employing water as a motive agent: and also the means of applying it in locations where it could not otherwise be used; with pressure engines the motive agent may be taken to the machine itself. In valleys or lowlands, having no natural fall of water, but where that liquid can be conveyed in tubes from a sufficient elevation (no matter how distant the source may be), such water, by these machines, may be made to propel others; unlike the steam engine, a pressure engine is inexpensive, and simple in construction—it requires neither chimneys, furnaces nor fuel; neither firemen nor engineers, nor is there any danger of explosions. It may be placed in the comer of a room, or be concealed under a counter or a table. It may be set in operation in a moment, by opening a cock, and the instant the work is done, it may be stopped by shutting the same, and thus prevent waste of power. Pressure engines afford an illustration of the variety of purposes to which a _piston and cylinder_ may be applied. These were probably first used in piston bellows; next in the syringe; subsequently in pumps of every variety; and then in water-pressure and steam engines. _The moving piston is the nucleus or elemental part that gives efficiency to them all_; and the apparatus that surround it in some of them, are but its parts. The history of machines composed of pistons and cylinders also illustrates the process by which some simple inventions have become applied to purposes, foreign to those for which they were originally designed—each application opening the way for a different one.