PUMP Fio. 1 Force Pump. pressure or by centrifugal force. It is usual to denominate them rotary force pumps and centrifugal pumps. The cylinder and pis- ton pump will be described first. The Force Pump. It is probable, as has been intimated, that the earliest valve pump was a force pump, and was similar in construc- tion and action to that shown in fig. 1 when the lower valve v is immersed in the reservoir, so that exhaustion, suc- tion, or atmospher- ic pressure has no essential connection with its working. When the piston P is raised, water will rush into the cham- ber through 0, and j when the piston is : depressed this valve will close, while the valve j w will be raised by the water, which is forced up into the pipe d. Upon raising the piston again, the pressure being removed from beneath the valve w, the weight of water above will cause it to close and thus prevent any return. But water from external pressure will again rush through the valve r, and the descending piston will again force it up through the valve w into the discharge pipe. The operation may be continued until there is enough water in the pipe d to exert a pressure per square inch equal to that exerted by the propelling power upon each square inch of the piston head. The Common Suction Pump. The functions of this pump depend upon the relative pressure of a column of water within the pipe and that of the atmospheric pressure upon the water outside of it. At the level of the sea the pressure of the atmosphere, when water boils at 212 F., is equal to sustaining a col- umn of mercury of 29-922 in. when at a temperature of 60. (See BOILING POINT.) The atmos- pheric pressure is therefore capable of sustaining, un- der the same condi- tions, a column of water 33'8 ft. high, or a little more than 13 times as high as the column of mercury, the specific gravity of the fluid metal being 13 -557 at 62-6 F. (See MER- CURY.) Consequently, if the lower end of a vertical tube of sufficient length is immersed FIG. 2. Common Suction Pump. in water and the tube completely exhausted of air, the water will rise to a height of 38'8 ft. above its level in the reservoir. The action of the common suction pump, fig. 2, will be easily understood from a consideration of this fact. The piston P, fitting the cylinder air-tight, on being raised will expand the air beneath it, and therefore diminish its pressure upon the water in the pipe beneath, according to the law of Boyle or Mariotte. (See PNEUMATICS.) "When the piston is depressed the lower valve will shut in consequence of the pressure being greater above than below, and the valve in the piston, opening upward, will open when the density of the air in the cylinder becomes greater than that of the external air, and its contents will thus be expelled. Succeeding motions of the piston will thus continue to ex- haust the air within the pipe until the pressure of the air on the water in the reservoir is suf- ficient to force the water in the pump up to the lower or suction valve. If the exhaustion is complete the water will rise to a height of 83-8 ft. This effect can be secured by filling the pump with water at the top before commencing. Now, as a column of water 83'8 ft. high ordinarily measures the extent of the pressure of the at- mosphere at the level of the sea, it follows that if the suction valve is placed at a greater distance above the water in the reser- voir the pump will not work. At an elevation, as upon the side or top of a moun- tain, the atmospheric pressure being less, the valve must be placed lower. At a height of 15,700 ft., where water boils at about 186 and the barometer stands at about IT'S inches, the lower valve requires to be within 19'7 ft. above the level of the water in the reservoir, this be- ing the height of a column of water which will balance the atmospheric column. The Lift Pump. By a slight change in the form of the suction pump, and the addition of a valve at z, fig. 3, the modern form of the lift pump is produced, and the water may be raised to a height corresponding to the amount of power applied. The form shown in this figure is that of a lift and suction pump combined. Remov- ing the lower valve e, and immersing the pump till the valve w in the piston is below the sur- face of the external water, the machine becomes simply a lift pump. The suction pump is also often called a lift pump. A form which is often figured in books employs an exterior frame supporting a piston rod which enters the pump at the lower end, pushing the piston Fio. 8. Lift Pump.
