The piston is the most important part of the machinery of a pump ; it is the part which requires the greatest skill in its manufacture, and upon whose correct action the success of the whole machine depends ; under these circumstances, as might have been expected, it has been modified in the greatest possible variety of manners. If the piston raise the water during its ascent, or its descent only, its action is said to be single ; if on the contrary, it should raise the water by both those motions, the action is said to be double. The ordinary suction pumps have wooden pistons, working hermetically against the sides of the cylinder, but bearing a clack in the centre, through which the water rises on the down stroke, ter be lifted on the up stroke to the point of discharge. A more complicated description of piston for the purpose of raising the water on the up stroke is used in the lifting pumps, and they have to support the weight of the column of water above them. In the ordinary forcing pump, however, the piston only forces the water forward on the down stroke; but in the double action forcing pumps, the water is raised by both the up and the down strokes, by a peculiar arrangement of the clacks and delivery pipes, to be noticed hereafter.
The pistons of common pumps are made ordinarily of a piece of hornbeam, to which the rod is attached by a kind of stirrup-handle, and which is pierced by a circular opening, a (fig. 1), whose diameter is equal to about half that of the piston. In the simpler forms of the suction and forcing pumps, the piston is made solid, and the water raised into the barrel or cylinder by the up stroke of the piston is forced on the down stroke to pass through a foot-valve placed upon the rising pipe fixed by the side of the cylinder, b (fig. 1); there is a valve also placed at the top of the arrival pipe, and both these valves are made so as only to open upwards. Under whatsoever circum stances the pistons may work, they are made to fit closely against the sides of the cylinder, by means of a species of packing, in order to cut off any connection between the air above and below them ; but of course, whilst thus excluding the passage of air, they must not give rise to such an amount of friction as to cause any serious waste of labour in pumping. The packing may consist either of leather, spun yarn, or of some anti-friction metal, acted upon by an internal spring.
The valves of pumps are known under the names of clack-valves, or of spindle-valves. The former are in fact flat plates of wood, metal, or leather, working on a hinge in only one direction, a and b (figs. 2 and 3), fitting closely upon the seating or face of the valve, and sufficiently stiff to resist deformation by the weight above them. The latter are either portions of cones, or small globes kept in their positions by means of spindles traversing their axes; upon the spindle a stop is placed to regulate the dimensions of the passage way. There are. however, some modifications of the spindle-valves, in which lateral guides are substituted for the central spindle, so that perhaps the term teat-valves would be more generally applicable to the class; and in the pumping-engines on the Cornish system a very complicated system of machinery is introduced, under the name of the equilibrium-valves, to regulate the admission of steam to the cylinders. The consideration
of these steam valves hardly comes under this portion of our subject, but it has been thought necessary to allude to them in passing, as being connected with pumping machinery. When very foul water has to be pumped, or when a very heavy column of water has to be raised, there are also introduced some special forms of valves, such as the ball-and-socket valves, &c., a (fig. 4), which it would be impossible • to describe in detail. It may suffice here simply to add that, inasmuch as the valves are precisely the portions of a pump which are the most likely to get out of repair, it is essential that means should be pro vided for obtaining easy access to them. This is usually effected by swelling out the pipes immediately above them, and by placing a moveable cover over the increased space thus enclosed.
The suction and rising mains do not require any special description ; for all that is required in the former, in addition to the ordinary con ditions of pipes, is, that they should have some provision made at their lower end for keeping out large extraneous substances, and that the valve-seats and chambers of the upper end should be provided for. In the rising mains, the junction pieces, the valve-seats, and the delivery cisterns, are the only portions which require special castings. The suction pipes can rarely be made of more than 26 feet in length, and their diameter should not exceed more than or 4 of that of the cylinder, without however falling below the latter dimension. The diameter of the ascending main should bear about the same propor tions; length need only he regulated. by the power able to be applid to it; in some mines, ascending pipes of 750 feet in length are used.
In D'Aubuisson's admirable treatise, ' De l'Hydraulique, the reader will find (at page 511, edition of Paris, 1840), the various conditions of the respective kinds of pumpiug machinery discussed in the most elaborate and philosophical manner. In this cursory notice it may suffice to state that D'Aubuisson shows that an ordinary suction pump cannot work efficiently at a greater distance, measuring from the underside of the piston to the surface of the water at the lower level, than 12 b, in which expression b will equal the height of the barometer at the place observed. This distance will vary between 30 feet and 26 feet 8 inches, according to the elevation ; but the effective height will be further himinished by the interval between the bottom of the stroke and the foot valve; the interval in question should be made as little as possible, and should not exceed one-tenth of the diameter of the cylinder.