PUMICE. A substance frequently ejected from volcanoes, of various colors, gray, white, reddish brown, or black ; hard, rough and porous ; spe cifically lighter than water, and resem bling the slag produced in an iron furnace. It consists of parallel fibres, and is sup posed to be asbestos decomposed by the action of fire. Pumice is of three kinds, glassy, common, and porphyritie. It is used for polishing ivory, wood, marble, metals, glass, &c. ; as also skins and parchment. It is useful in cleaning cloth, and affording surface for decomposition in retorts ; it consists chiefly of alumina. PUMP. A machine for raising water. Though the forms under which this use ful engine is constructed, and the mode in which the power is applied, may be modified in an infinite number of ways, there are only three which can be consi dered as differing from each other in principle. These are, the sucking pump, the forcing pump, and the lifting pump, so called from the manner in which they act.
The sucking pump, or common hold pump, is an apparatus of which the principle and construction will be evident from the annexed figure. A A is a pipe of any convenient length, the lower end of which reaches below the surface of the water in the well or reservoir ; B is a barrel, generally of er diameter than the pipe ; C a valve opening upwards ; D a piston moved by the rod E: in this piston there is also a valve opening. upwards. When the piston is raised, the air in the ibarrel between the valves is panded, and its tension, consequently, diminished ; the pressure of the air in the pipe, therefore, opens the valve C, and the whole air in the pipe and barrel becomes less dense. In this state the at mospheric pressure on the surface of the water causes it to rise in the pipe, until the tension of the confined air becomes equal to the pressure of the atmosphere. On again depressing the piston, the valve in it opens, and the air passes through it from the barrel as it descends ; but the valve C is closed by the downward pres sure, and the volume of water which has entered the pipe remains. On again rais ing the piston, the same effect is repeated, and an additional quantity of water en ters the pipe. Thus, by the alternating motion of the piston, a column of water is raised in the pipe until it reaches the pis ton when at the bottom of the barrel, and the whole of the air below it has been ex cluded. On raising the piston when the
water has reached it, the fluid will be compelled to follow by the pressure of the atmosphere on its surface in the well. When the piston is again depressed, the water flows through the valve in it, and ascends into the barrel, and by the suc ceeding strokes of the piston is lifted up until it reaches and flows out of the spout F.
Although in theory the limit of the height to which water may be raised by the sucking pump, from the surface of the fluid in the well to the highest posi tion of the movable piston, is about 34 feet, (the height of a column of water which balances the pressure of the at mosphere), it is not found practicable, with pumps of the ordinary construction, to raise it more than about 28 feet. The difference arises from the difficulty of making the apparatus absolutely air tight.
The chain pump used in ships of war consists of an endless chain moving over a wheel on the gun-deck, which is turned by winches, and over a roller in the pump well, having saucers or flat circular pistons at certain intervals. Near the pump-well, on the side on which the chain on turn ing the winches ascends. are a few feet of pipe ; through this the saucers raise the column of water, which, being lifted over the upper orifice of the pipe, falls into the cistern, and thence into the waste-pipe, called the pump•dale, which carries it overboard. The descending portion of chain falls through another case called the back case. Chain pumps, in large ships, throw out a ton a minute. The forcing pump is represented at fig. 2. The piston-rod E D is attached to a solid plunger D, adjusted to the cavity of the barrel. A pipe G. H, furnished with a valve F, opening outwards, communi cates with the barrel at G. On elevating the plunger D, the water will ascend through the valve C, in the same manner as in the sucking pump, till the barrel is filled to D. Now, when the plunger is depressed, the valve C will shut, and the water between D and C be forced through the valve F into the pipe G II. When the plunger is raised, the valve at F shuts, the pressure on its under side being re-, moved, so that the water which was forced into the pipe by the previous stroke cannot return into the barrel. At the next stroke of the piston, more water is again forced into the pipe, and so on till it is raised to the height re quired.