This pump consists of two hollow cylinders, a b and b d, placed one under the other, and communicating by a valve u, which opens upwards. The cylinder ab is called the suction pipe, and has its lower end immersed in the well, or reservoir, from which the water is to be raised. In the barrel b d a bucket or piston p is moved, having a valve in it which opens upwards; this piston should move air-tight in the cylinder. At i is a spout for the discharge of the water. Supposing the bucket to be at the bottom of the cylinder bd, and in close contact with the valve u; upon elevating it, the piston-rod is kept closed by the atmospheric pressure, and if the valve u were not permitted to rise, a vacuum would be caused between it and the piston, the elevation of which would then require a force equal to about 15 lbs. multiplied by as many square inches as are in the section of the piston. But the moment the piston begins to ascend, the elasticity of the air in the suction-pipe beneath opens the valve u, and the air rushing through, it balances part of the pressure on the piston. Now, if the water at a were not permitted to rise, the air between the piston and the surface a would be rarefied by the ascent of the piston.
It would, therefore, press against the lower surface of the water with a force less than the atmosphere; but the entire force of the atmosphere presses on the surface of the water in the well; and the diminished elasticity of the air in the suction-pipe not being a counterpoise for this, the water is necessarily pressed up into that pipe. The height to which the water will rise in the suction pump will be proportioned to the length of the stroke of the piston p; but let us suppose it to have risen to the level of the dotted line e, there is then a compound column of air and water pressing on the level a; namely, the column of water a e and the elastic force of the air in e b. These two together balance the atmospheric pressure on the external surface of the water in the well. It consequently follows, that the air in b e must be rarefied, since its elasticity falls short of the atmospheric pressure by the pressure of the column of water a e. As a column of water about thirty-three feet in height balances the atmosphere, it follows that the elasticity of the air in b e is equal to the pressure of a column of water whose height is equal to the excess of thirty-three feet above b e.
At the next stroke of the piston, a further quantity of air is extracted, and the diminished elasticity under the piston causes the water to ascend to the level f, and the succeeding strokes raise it to the levels b and g. Hitherto, this machine has only operated as an air pump, but at the next descent of the piston, the water at g passes through the piston-valve, which closes and prevents its return; and upon the next ascent of the piston, the pressure of the atmosphere forces more water through the valve a. The succeeding descents and ascents are attended with like effects, until the water has reached to a level with the spout i, where it is discharged at every succeeding stroke afterwards. The force necessary to lift the piston is the weight of a column of water, whose height is that of the level of the water in the well, and whose base is equal to the section of the piston. This force, therefore, from the commencement of the process, continually increases, until the level of the water rises to the discharging spout i, and thenceforward remains uniform.
As the common, or sucking pump, operates by the production of a vacuum within the working-barrel, by which the external atmospheric pressure is called into action, and forces the water of the well up the suction-pipe, it follows, that the piston, at its greatest elevation, should never exceed the height of thirty-three feet from the surface of the water in the well.
Notwithstanding the common lifting pump, is incapable of raising water from more than thirty-three feet (in practice but thirty feet) below the place where it may be fixed, yet it may be made to deliver water at almost any required height above its piston, by the application of a continued straight pipe into the top of the working-barrel a b of the preceding figure. Thus, if we suppose twenty or thirty feet more of pipe to be so added to it, since the water once raised cannot pass downwards again through the piston valve, it must continue to rise with each stroke of the pump, until at length it will flow over the top of the pipe, or through a spout inserted in any part of its side. In this case atmospheric pressure has nothing to do with the elevation of the piston, consequently it may be carried to any height that the strength of the pump, or the force employed, is capable of; but the handle h, or any other contrivance by which the pump is-worked, must be fixed at the top of the additional pipe, and the piston-rod equally extended, in order that the working-barrel may be kept within the limits of atmospheric pressure, which makes a pump thus arranged inapplicable to very great depths, on account of the bending of the piston-rod. Where cast-iron pipes are used, this may in a great measure be prevented, by placing small pieces, with projecting arms of sufficient length to touch the inside of the pipe at each joint of the piston-rod, or about ten or twelve feet asunder, when this pump may be used for considerable depths with advantage.
In using pumps to draw muddy or sandy water, it is always advisable to set the bottom of the pump in a close wicker basket, or other strainer, because sand and small stones very soon destroy the leather and working parts of any pump; and when pumps are used for hot liquors, which is the case in many manufactories, thick hempen canvas must be substituted for leather, unless the valves and pistons are made entirely of metal, which is of course preferable.