NOTE.—The action of a pump is as follows: The piston or plunger by moving to one end, or out of the pump cylinder, leaves the space it occupied, or passed through, to be filled by something. As there is little or no air therein a partial vacuum is formed unless the supply to the pump is of sufficient force to follow the piston or plunger of its own accord. If this is not the case, however, as it is when the water level from which the pump obtains its supply is below the pump itself, there being a partial vacuum produced, the atmospheric pressure forces the water into the space displaced by the plunger or piston, continuing its flow until the end of stroke is reached. The water then ceases to flow in, and the suction valve of the pump closes, forbidding the water flowing back the route it came. The piston or plunger then begins to return into the space it has just vacated, and which has become filled with water, and immediately meets with a resistance which would be insurmountable were the water not allowed to go somewhere. (See next page.) Its only egress is by raising the discharge valve by its own pressure, and passing out through it. This discharge valve is in a pipe leading to the boiler, and in going out of the cylinder by that route the water must overcome boiler pressure and its own friction along the passages. Water is inert and cannot act of itself; so it must derive this power to flow into the feed pipe and boiler from the steam acting upon the steam piston of the pump. The steam piston and pump piston are at the two ends of the same rod. Therefore the steam pressure exerted upon the steam piston will be exerted upon the pump piston direct. Between the upper and lower parts of the plunger a cup leather is introduced before these parts are screwed together. This cup leather, while it allows the plunger to move freely, also makes a water-tight joint. The lower valve consists of a piece of cast-iron flat on the bottom and circular in shape about three-eighths inch thick with a curved toe at one side. This iron disc is secured to the flat leather valve by a screw that passes through the valve and is threaded in the disc. [Illustration: FIG. 157.] The object of this toe upon the disc is to open the lower valve by means of raising the pump handle as far as it will go which lowers the plunger upon the toe and tips the lower valve upon its seat. This same operation also lifts the valve in the plunger off its seat, so that all the water in the barrel drains back into the well so the pump is kept from freezing up in winter. [Illustration: FIG. 158.] The leather which forms the lower valve is held in place by clamping the pump barrel upon it, so that it is held between the barrel and the base plate. Hand pumps are primarily divided into: 1, suction or lift pumps; 2, force pumps, and 3, suction and forcing pumps; 4, also pumps for exhausting air from vessels.