PUMPS AND FIRE ENGINES.
FIG. 1.
FIG. 2.
Pumps are used for lifting fluids above their level into some higher situation, such as from the hold of a ship or from a well. [Fig. 1] shows the different parts of a common “lifting pump;” a a is a cylinder, b a piston rod or “plunge,” c the sucker made of leather to fit nicely the cylinder, d a valve in the sucker to open upwards, e a valve fixed to the cylinder, also to open upwards, f a box with a spout.
The piston being raised by lowering the handle of the pump, a partial vacuum is formed below the upper valve, which shuts down directly the piston is raised by the pressure of the air; this vacuum causes the external air to force the water some way up the tube g. On the piston descending, the lower valve is forced down and the upper one opened, this keeps the water where it is and allows the piston to descend without forcing the water down again, and on its being raised a second time the upper valve shuts and the lower one opens, the water being drawn up still higher, and this takes place till the box at the top is full to the spout, when it runs out. The air-pump is on the same principle, and is generally made with two cylinders worked by means of a “rack” and wheel ([fig. 2]); this is only to save time, instead of pumping water it pumps out air from any vessel called a “receiver,” because it receives any object to be placed in a “vacuum,” that is to say a partial vacuum, for the air-pump cannot produce a vacuum, as the air is only partly removed by each stroke of the piston, leaving the air more rarefied inside; and although each stroke of the piston increases the rarefaction, yet it cannot get all, as it merely takes part, and always leaves part.
FIG. 3.
Fire and garden engines are only applications of the pump to different purposes. The fire-engine has generally two cylinders and pistons, and has moreover an air-chamber for the purpose of making the stream of water continuous. It acts in this way:—The water is forced by the power of those who are pumping the engine into a vessel air-tight and full of air, having an opening which joins the “hose” at its lower part; the result is, that as the water is forced in faster than it can well escape, the air above it—becoming greatly compressed, and by its expansion between each stroke of the pistons—forces the water out, and so continues the stream or jet. [Fig. 3] shows this air-chamber; a joins to the hose c, and b is in union with the forcing-pumps of the engine. The air is represented as it would be compressed to about half its bulk, for it at first filled all the air-chamber down to the openings.