A direct pressure steam pump is one in which the liquid is pressed out by the action of steam upon its surface, without the intervention of a piston. A direct acting steam pump is an engine and pump combined.
A cylinder or reciprocating pump is one in which the piston or plunger, in one direction, causes a partial vacuum, to fill which the water rushes in pressed by the air on its head.
Note.—A suction valve prevents the return of this water on the return stroke of the piston, and a discharge valve permits the outward passage of the fluid from the pump but not its return thereto or to the reservoir through the suction pipe.
The force against which the pump works is gravity or the attraction of the earth which prevents the water from being lifted. This is shown by the fact that water can be led, or trailed, an immense distance, limited only by the friction, by a pump.
Note.—It may be noted that the difference between a fluid and liquid is shown in the fact that the latter can be poured from one vessel to another, thus: air and water are both fluids, but of the two water alone is liquid: air, ammonia, etc., are gases, while they are also fluids, i.e., they flow.
The idea entertained by many that water is raised by suction, is erroneous. Water or other liquids are raised through a tube or hose by the pressure of the atmosphere on their surface. When the atmosphere is removed from the tube there will be no resistance to prevent the water from rising, as the water outside the pipe, still having the pressure of the atmosphere upon its surface, forces water up into the pipe, supplying the place of the excluded air, while the water inside the pipe will rise above the level of that outside of it proportionally to the extent to which it is relieved of the pressure of the air.
If the first stroke of a pump reduces the pressure of the air in the pipe from 15 pounds on the square inch to 14 pounds, the water will be forced up the pipe to the distance of 21⁄4 feet, since a column of water an inch square and 21⁄4 feet high is equal in weight to about 1 pound. Now if the second stroke of the pump reduces the pressure of the atmosphere in the pipe to 13 pounds per inch, the water will rise another 21⁄4 feet; this rule is uniform, and shows that the rise of the column of water within the pipe is equal in weight to the pressure of the air upon the surface of the water without.
There are pumps (Centrifugal) especially designed for pumping water mingled with mud, sand, gravel, shells, stones, coal, etc., but with these the engineer has but little to do, as they are used mostly for wrecking and drainage.
The variety of pattern in which pumps are manufactured and the still greater variation in capacity forbids an attempt to fully illustrate and describe further than their general principles, and to name the following general