Rotary Pumps.

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Chap. 5.

tlie pistons descend, the valve on the suction pipe closes, and die solidpiston drives the water in its cylinder through the hollow one in the other.so that whether rising or falling the liquid continues to flow. As bothcylinders are filled at the same time, the bore of the suction pipe shouldbe proportionably enlarged. The plate bolted over the opening at thelower part of one cylinder is to give access, in large pumps, to thelower valve.

In some pumps both a solid and a hollow piston are made to work inthe same cylinder. Such were those that constituted the “ single-cham-ber fire-engine” of Mr. Perkins. A plunger worked through a stuffingbox as in No. 123, and its capacity was about half that of the cylinder;consequently on descending it displaced only that proportion of the contentsof the latter. The apertures of discharge were at the upper part of thecylinder, and a single receiving one at the bottom. From the lower endof the plunger a short rod projected, to which a hollow piston or suckerwas attached, fitted to work close to the cylinder, so that when the plungerwas raised, this piston forced all the water above it through the discharg-ing apertures. To convert one of these pumps into a fire-engine, the cy-linder of the pump was surrounded by a shorter one of sheet copper, thelower end of which was left open, and its upper one secured air-tight tothe flanch of the pump ; the space left between the two forming a passagefor the water expelled out of the inner one. A larger and close cylinderencompassed the last, and the space between them was the air chamber, tothe lower part of which a hose pipe was attached by a coupling screwin the usual way.

Such pumps are more compact than those with two cylinders, but theyare more complex, less efficient, and more difficult to keep in Order and torepair. The friction of the plunger and sucker is much greater than thatof the piston of an ordinary double acting pump of the same dimensions;and the latter discharges double the quantity of water; for although doubleacting, the effecl of these pumps is only equal to single acting ones. Forthe above reasons they have, we believe, become obsolete or nearly so.

CHAPTERV.

Rotary or rotatory pumps: Uniformity in efforts made to improve machines—Prevailing custorn. toconvert rectilinear and reciprocating movements into circular ones—Epigram of Antipater —Ancientopinion respecting circular motions—Advantages of rotary motions exemplified in various machines—Operations of spinning andweaving; historical anecdotes respecting them—Rotary pump from Ser-viere—Interesting inventions of his-rrClassification of rotary pumps—Eve’s steam-engine and pump—Another dass of rotary pumps—Rotary pump of the 16th Century—Pump with sliding butment—Trot-ter’s engine and pump—French rotary pump—Bramah and Dickenson’s pump—Rotary pumps with pis-tons in the form of vanes—Centrifugal pump—Defects of rotary pumps—Reciprocating rotary pumps;A French one—An English one—Defects of these pumps.

No one can study tbe past and present history of numerous machinesand devices without perceiving a striking uniformity in the efforts madeto improve them in distant times and countries; the same general defectsand sources of defects seem always to have been detected, and similar me-thods hit upon to remedy or remove them: the same ideas, moreover, ledInventars to modify and apply machines to other purposes than those forwhich they were originally designed, and also to increase their effect bychanging the nature and direction of their motions. So uniform have been

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