Rotary Pumps.

291

Chap. 5.]

drum or case, to which an ascending or forcing pipe is attached: thewater would rise through the pipe, provided the velocity of the arms wasincreased according to the elevation of its discharging orifice. In place oftubulär arms, two or more vanes radiating from a vertical axis and turnedrapidly in the case would produce the same efifect; the suction pipebeing connected to the bottom at the centre and the forcing pipe to therim or the top. Such pumps are in their construction simpler than otherrotary ones, besides which no particular accuracy is required in fittingtheir working parts; nevertheless, they are as liable to derangement asothers, for the velocity required to be given to the arms is so great, thatthe teeth of the wheels and pinions by which motion is transmitted to themare soon worn out.

Centrifugal pumps like those just described have been tried as Substi-tutes for paddle wheels of steam-vessels : i. e. the wheels were con-verted into such pumps by inclosing them in cases made air-tight, exceptat the bottom through which the ends of the paddles slightly projected;a large suction pipe proceeded from one side of each case (near its cen-tre) through the bows of the vessel and terminated below the water line:by the revolution of the wheels water was drawn through these tubes intothe cases and forcibly ejected below in the direction of the Stern, and bythe reaction moved the' vessel forward.

It must not be supposed that the preeeding observations include an ac-count of all rotary pumps. We have only particularized a few out of agreat multitude, such as may serve as types of the various classes to whichthey belong. Were a detailed descriptiön given of the numerous formsof these machines, modes of Operation, devices for opening and closing thevalves, moving the pistons, diminishing friction, compensating for the wearof certain parts, for packing the pistons,- &c. &c., those readers who arenot familiär with their history would be surprised at the ingenuity dis-played, and would be apt to conclude that all the sources of mechanicalcombinations had been exhausted on them. We would advise everymechanic who thinks he has discovered an improvement in rotary pumps,carefully to examine the Repertory of Arts, the Transactions of the So-ciety of Arts, the London Mechanies’ Magazine, and particularly theJournal of the Franklin Institute of Pennsylvania , before incurring theexpenses of a patent, or those incident to the making of models andexperiments.

Rotary pumps have never retained a permanent place amöng machinesfor raising water : they are, as yet, too complex and too easily derangedto be adapted for common use. Theoretically considered they are per-fect machines, but the practical difficulties attending their constructionhave hitherto rendered them (like rotary steam engines) inferior to others.To make them efficient, their working parts require to be adjüsted to eachother with unusual accuracy and care, and even when this is accomplished,their efficiency is, by the unavoidable wear of those parts, speedily dimi-nished or destroyed : their first cost is greater than that of coinmon pumps,and the expense of keeping them in order exceeds that of others; theycannot, moreover, be repaired by ordinary workmen, since peculiar toolsare required for the purpose : —a farmer might almost as well attempt torepair a watch as one of these machines. Hitherto, ä rotary pump hasbeen like the Psalmist’s emblem of human life :—-“ Its days are as grass,as a flower of the field it flourisheth, the wind [of experience] passethover it, and it is gone” Were we inclined to prophecy, we should pre-dict that in the next Century, as in the present one, the cylindrical pumpwill retain its preeminence over all others ; and that makers of the ordi-