PUMPS. Machines for raising water and other fluids; usually consisting of' a tube or tubes, in which valves and pistons, or buckets, are made to operate, to pro duce the effect. Engines differently constructed, and particularly those upon a larger scale than ordinary pumps, are generally termed HYDRAULIC MACHINE., which we have already treated of under that head. The ordinary definition of pump is, " a machine in which water is raised by the pressure of the atmo sphere,' which accords with the prevalent but erroneous notion, that the atmo sphere does of itself raise water to a height of thirty feet ; notwithstanding it is known to those who have considered, the subject, that it does not, in fact, contribute in the slightest degree to raise it at all; and that the same force is requisite to raise a pound of water a given height, as to raise a pound of lead, or of any other substance, through the same space. Of the evident truth of this fact, the reader, if a novice, will be satisfied upon reading our article on hydraulics or hydrodynamics, and by attending to the following description of a Common, or puer.—This pump consists of two hollow cylinders, a b and b d, placed one under the other, and communicating by a valve a, which opens upwards. The cylinder a b is Called the suction pipe, and has its lower end immersed in the well, or reservoir, ftom which the water is to be raised. In the barrel b d a bucket orpiston 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 b el, and in close contact with the valve a; upon elevating it, the piston-rod is kept closed by the atmospheric pressure, and if the valve a 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 lb lbs. multiplied by as many square inches as are in the section of the pis ton. But the moment the piston begins to ascend, the elasticity of the air in the suction-pipe beneath opens the valve a, 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 coun terpoise for this, the water is necessarily pressed up into that Ipe. The height to which the water will rise in the suc tion 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 toge ther 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 u. 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, Prom the commencement of the process, continually increases, until the level of the water rises to the discharging spout i, and thenceforward remains uniform.