Chap. 9.

Piston Pressure Engines.

353

an increase of friction in the passage of the piston. a And if another cylin-der extended from F to the bottom of the pit at O, and a solid piston fit-ted to it were used instead of the bücket A, with a contrivance at the bot-tom to allow the water to escape ; the apparatus would then be a pres-sure engine, although the principle of the motive part of it would not havebeen essentially cbanged. The cylinders in this example would performthe part of the buckets A B; they might be considered as permanent orfixed, and very long buckets with movable bottoms, i. e. the pistons, whichby ascending and descending in them received and discharged their Con-tents. And as with the buckets A and B, the quantity of water expendedfrom the motive or descending column would be proportionate to the di-mensions of the other and the elevation to which it was raised. Pressureengines may therefore be considered as a peculiar modification of thegaining and losing bücket machines, and as a combination of these withatmospheric and forcing pumps. They admit of various forms accordingto the location in which they are used and the objects to be accomplishedby them. As liquids press equally in all directions, the cylinders may beplaced in any position—horizontal, inclined, or vertical. Sometimes apressure engine consists of a single cylinder with its appropriate pipes andvalves like a double acting pump. The water to be raised enters at oneside of the piston, and the motive column at the other; but more com-monly a distinct cylinder and piston receives the impulse of the motivecolumn, and in Order to transmit it to the other, the two pistons are some-times connected to the same rod as in No. 161—at other times to oppo-site ends of a vibrating beam as in No. 162—so that while one cylinderand its apparatus act as a pump to raise water, the other is exclusivelyemployed to work it. In this respect pressure engines may be consideredrather as devices for communicating motion to machines proper for raisingwater, than as the latter, and they are sometimes used as propellers ofother machines, but in whatever light they may be viewed, they are toointeresting to be omitted.

There are two kinds of pressure engines, büt they differ from eachother only in that part which receives the impulse of the motive columnand tramsmits it to the other. In one, a solid body (a piston) is used forthat purpose—in the other, a volume of air; but while a slight Variation isthus caused in the two machines, the essential featüres, as well as themoving principle of both remains the same. Piston pressure engines aresaid to have been invented in the 18th Century by M. Hoell, a German en-gineer.» It is more probable that he improved them only, for they cer-tainly were known much earlier : still it may be that he was ignorant oftheir previous use, and was led to their re'invention by his efforts to raisewater from the Hungarian mines, in which he erected several pressuremachines on the principle of Heron’s fountain: the transition from theseto the other was easy and natural, and may have resulted from his endea-vours to avoid a defect to which the former are subject, viz : the absorp-tion of the air by the water. About A. D. 1739, an improved form ofpressure engines was devised, and introduced into some mines in France by Belidor , which he has described in his Architecture Hydraulique. Somewriters have considered him the author of piston pressure engines, but

a Pumps in certain locatiöns are sometimes worked in that rnanner : the pistons orrods being loaded with weights sufficient to depress them, are raised by a bücket ofwater suspended at the opposite end of the beam, which when it reaches the bottom itscontents are discharged, like the bücket A in No. 16, D in No. 17, or G in Nif. 162.

b “Machine ä colonne d’eau. German : Wasser Saulene. Machine ; inventee parM. Hoell, premier Machiniste d’L’Impferatrice.” Arts et Metieres. L’Art d’exploiterLes Mines. Folio ed., Paris 1779, page 1449. Quarto ed. tom. xviii, p 131.

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