In double acting pumps the motion of the piston in one direction causes an inflow of water, and a discharge at the same time, in the other; and on the return stroke this action is renewed as the discharge end alternately becomes the suction end; the pump is thus double acting. Finally pumps may be classified with reference to particular uses to which they are specifically adapted by their form and the materials they are required to handle. Pumps now raise, convey and deliver beer, molasses, acids, oils, and melted asphalt. They also handle such gases as air, ammonia, lighting gas and even oxygen. In the orderly progress of the contents of the volume, it will be seen that the main subject, occupying many pages with illustrations, is that relating to “Steam pumps;” those having a _steam-end_ and a _water-end_ and which consist of pump and steam-motor combined. An interesting class under Vacuum-pumps will be found, 1, the combined vacuum and feed; 2, the combined vacuum and circulating and, 3, the combined vacuum and refrigerating pumps. Under Pumping Engines and the Steam Fire Engine will be found a description of the most brilliant and fascinating of modern scientific and mechanical achievements; these two sections relate to hydraulic engineering in its highest development. HAND PUMPS. The _theoretical action of_ a pump has already been described and illustrated;—the _practical operation_ is described in the note below. The subject is important enough to justify the space it takes to present these two descriptions of the action of a pump. [Illustration: FIG. 156.] The parts of which a pump is composed are: 1, _the barrel or cylinder_; 2, _the plunger or piston_; 3, _the valves_; 4, _the pipes_. The barrel of a modern pump is a tube of metal having a water tight plunger or piston which moves freely up and down at the pleasure of the operator. This plunger is in its simplest form made of cast iron in two parts. The upper part consists of an arched part having a hole in its center to receive a bolt which passes through it and a jaw on the lower end of the pitman or connecting rod. The upper end of this pitman is attached to the pump handle by means of a bolt. The inside of the upper part of the plunger is threaded to receive the lower part with a cup leather packing and contains a valve of metal having a conical seat. Fig. 156 shows a design of a pump in common use in the 14th century.