86 PUMP by two galley slaves, who were relieved every hour. It is uncertain where the chain pump originated, but it was probably first used in China in the form of an inclined trough with drums at either end, giving motion to a chain or rope with scoops or blocks at- tached. The centrif- ugal pump is a ma- chine which acts upon an entirely different principle from that of any pump so far described. The force which elevates the water is the centrifu- gal force developed by the revolution of a fan Fio. 15. Massachusetts Pump. wheel. An early efficient form of centrifugal pumps was constructed in Massachusetts in 1818, and called the Massachusetts pump. It resembles an ordinary fan blower, as will be seen by the cut, fig. 15. It consists of a hori- zontal shaft to which are attached four eccen- tric blades, narrowed toward their extremities and located within a cylindrical-shaped box, from which a discharge pipe F passes upward. The water is received at the centre, around the shaft, which is so placed that the blades just graze the inner surface of the box at the junc- tion of the discharge pipe, into which the water is necessarily forced. The apparatus is placed below the level of the water, aa the vacuum power is small. A more recent form of centrifu- gal pump is Appold's, shown in figs. 16 and 17, which was first exhibited at the world's fair in London in 1851. The efficiency of a centrifu- gal pump depends upon the form of its blades, and Mr. Appold made a great improvement, nearly doubling the efficiency of the Massachu- setts pump, by giving them the form shown in section by the dotted lines in fig. 17. The re- volving fan wheel, shown in fig. 16 at c, is fixed FIG. 16. FIG. 17. Appold's Centrifugal Pump to the end of a shaft turned by the drum D. It plays between two circular checks, through the centre of both of which there is a circular opening to admit the water from the reser- voir, beneath the level of which the wheel is placed. The water enters at the central part of the fan, as shown in section in fig. 16 by the four curved arrows, two on either side, the whole being rotated in the contrary direc- tion. The lower part of the discharge pipe i enlarged into a drum somewhat similar to that of the Massachusetts and of the Gwynne pump, and the water issues from all parts of the pe- riphery of the fan wheel and is forced upward into the discharge pipe A. Calculations have been made as to the height to which water may be carried with one of these pumps, but they do not possess much practical value, as the power of each machine varies with its con- struction ; and 20 ft. is the practical limit, al- though by means of a very high velocity, not practicable for ordinary use, a height of 50 ft. has been reached. Gwynne and co.'s centrifu- gal pump is a modification of Appold's, and was shown at the same exhibition. A sec- tional view is given in fig. 18. Six equidistant arms, extending first in the direction of radii, but toward their outer ends curved and pointing backward as regards the direction of rotation, are fixed within a drum, which again moves, within an outer drum. The water enters at the centre, and taking the course of the ar- rows ascends the dis- charge pipe. Three of the arms commence at the axis, but the other three, alterna- ting, commence at the circle of admission. The two drums are only in contact at a small ring surround- ing the central open- ing. The arms dimin- ish in breadth toward their outer extremi- FIG. 16. tiwynne's Centrifu- gal Pump. ties to render the flow of water smooth, as the increase of centrifugal force at the pe- riphery causes an increase in the velocity of the water, and therefore it requires a less space through which to move. There are nu- merous practical points about the different kinds of pumps, to mention which would re- quire a too extended detail. It may be re- marked that a pump is one of the most diffi- cult machines to keep in order. It is exposed, if not constantly in use, to great changes of moisture and dryness, and its metallic parts, particularly if of iron, soon become rusty. It- is often convenient to have valves partly made of leather, but these cannot be expected to last long ; if constantly in use they soon wear out, and if they are allowed to become dry they shrink and cease to perform their office* well. A kind of steam pump without a pis- ton, called a " pulsometer," is the invention of Mr. C. H. Hall of New York. It consists of two long-necked chambers joined together at the top, where a ball valve by falling one way or the other opens one of the chambers to the admission of steam. The water is admitted at the bottom of the chambers, and passes into-
