The forcing pump is generally employed in mines or in situations where it is required to draw water from great depths. Pumps of this kind act by compression instead of exhaustion. Although atmospheric pressure is not necessary to the construction of forcing pumps, yet it is in most cases resorted to for raising the water, in the first instance, into the body of the pump where the forcing action commences and takes place; and when so constructed, such pumps are usually called lift and force pumps; and in all the machines of this description, the water may be raised to any required height, without any limit, consistent with the strength of the parts and the power at command. Forcing pumps do not differ materially in construction from the common pump already described; indeed, that pump, by a mere inversion of its parts, may be made into a forcing pump; that is to say, placing the piston below, and the stop-valve and delivering-pipe above, as shown in the subjoined figure, where h h shows the inverted working-barrel, and i the inverted piston and rod, with a valve opening upwards; k is the stop-valve placed at the top, instead of the bottom, and also opening upwards into the rising pipe l l, which may be continued to any required height; the lower end of the working-barrel is quite open, and must stand in, and be covered with the water it has to raise, so that no suction or feed-pipe is necessary to this pump; and the piston i may be worked by a frame o o, or in any other convenient manner.
After the description already given of the common lift pump it will be needless to say anything of the action of this machine, as it is presumed the figure will render it sufficiently obvious. While the lower end of the working barrel h h is immersed in water, and the piston i moves upwards and downwards, the barrel will be filled through the piston-valve at each down-stroke, and at each upstroke its contents will be expelled through the stop-valve k, into the ascending pipe 11; and whatever the diameter of this pipe may be, still its resistance will constantly be equal to the weight of a column of water of the size of the working-barrel, and of a height equal to the perpendicular altitude of the water in the ascending pipe; for this pipe may be placed horizontally or obliquely, so as materially to alter its length: but it is the perpendicular height between the 3urface of the water to be raised, and its point of discharge, which must alone be taken into account in estimating the load upon a pump; since increase of length without height in the pipe produces no other resistance than that of friction, which is easily overcome by increasing the capacity of the pipe.
It may appear that the preceding pump is applicable to every purpose and to every situation, such as raising water from mines and the deepest places; but this is not the case, owing to the almost imperceptibly small elasticity of water, and the effects of the vis inertia, which belongs to fluids in common with solid matter. In working the pump shown in the last figure, if we presume the pipe ll to be full of water, that water has not sufficient elasticity to permit the barrel h h to discharge its contents through the valve k, without putting all the water contained in ll into motion, while, when the piston descends, that motion will be at an end. The water in 11 will therefore be in an alternate state of rest and motion; and if the column is long, and its quantity great, the vis inertia will be very considerable; that is to say, it will require a considerable exertion of force to get it from a state of rest into motion; and when it has once begun to move, it will have no immediate tendency to return again to rest, but might be continued in its motion with less force than that which was originally employed to move it.
The descent of the piston, however, allows sufficient time for all the motion that was communicated to be completely lost; and hence, in working this pump, we not only have the weight of the column to overcome, but the natural inertia to combat with at every stroke. This may, in a great measure be removed, by keeping two, or what is still better, three pumps constantly at work by a triple or three-throw crank; and accordingly this expedient is generally resorted to in all small engines for throwing water to a great height, for by this means the water is never permitted to stand still in the pipes, but a constant flow or stream is maintained. No illustration is necessary to explain to the reader the combination of three pumps worked by a triple crank, each throw giving the alternating motion to one pump of the series, at equal distances of time and space throughout the revolution; but a mechanical arrangement, wherein a triple crank is employed to work one pump, containing three buckets alternating in the same working barrel, and producing the same effect as three pumps, seems to require the aid of graphic delineation; accordingly, we annex a cut, in which the process of raising water is thus conducted; it is the invention of Mr. Downton, of Blackwall, and was the subject of a patent granted to him in 1826. The figure in the margin may be called a front elevation, a portion of the working barrel or cylinder being broken away to show the buckets, etc. a is the uppermost bucket or piston, the rod of which b b is hollow, and being connected to a bent arm d, it is thereby attached to one of the limbs of a revolving three-throw crank e.
The middle bucket f has also a hollow rod g g, which, being of smaller dimensions than the former, slides freely through it, and is connected to the crank e by another bent arm h. The lowermost bucket i has a solid rod k k which passes entirely through the hollow rods of the other buckets, and is attached directly to the middle of the crank. Upon each of the limbs of the crank are placed anti-friction wheels, working in elliptical slots at the upper end of each rod, by which the attrition of the rubbing surfaces is considerably reduced.