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" Common Pumps. [Book II.

the depression of the handle or lever, the sucker is raised, this equallty isdestroyed, for the atmospheric column over the cylinder, and consequently

over the vaive O is lifted up, andsustained by the sucker alone; ittherefore no longer presses on theupper orifice, while its action on thelower one remains undiminished.Then as the external air cannot en-ter the pipe to restore the equili-brium except through its orifice im-mersed in the well; in its efforts todo so, (if the expression is allowa-ble) it necessarily drives the waterbefore it on every ascent of thesucker, until the air previously con-tained in the pipe is expelled, andboth pipe and cylinder become filledwith water.

The subsequent Operation is ob-vious. When the sucker descends,the clack on its upper surface is rai-sed by the resistance of the waterthrough which it passes ; and whenat the bottom of the cylinder, thisclack closes by its own weight: sothat when the sucker is again eleva-ted, besides overcoming the resis-tance of the atmosphere, it carriesup all the water above it, and which it discharges at the spout—at thesame time the atmosphere resting undisturbed on the water in the well,pushes üp a fresh portion into the vacuity formed in the cylinder, and thevaive O prevents its return.

Ttie horizontal distance between the cylinder or working part ofthe pump and the well is, in theory unlimited, but in practice it seldomexceeds one or two hundred feet. In all cases where long pipes areused, their höre should be enlarged in proportion to their length, or thevelocity with which the sucker is raised, should be dimimshed ; and forthis reason —time is required to overcome the inertia and friction of longcolumns of water in pipes; hence a sucker should never be raised fasterthan the pipe can furnish water to fill the vacuity formed by its ascent.In pumps whose pipes have too small a bore, it frequently happens thatthe sucker is forcibly driven back when quickly raised, because the waterhad not time to rush through the pipe and fill the vacuity in the cylinderas rapidly as it was formed. The bore of wooden pumps being equalthroughout, the water is not pinched or wire-drawn while passingthrough them, as in most of those of metal. This is one reason why theygenerally work easier than the latter. It is immaterial in what part ofthe pipe the vaive O is : it is usually placed at the upper end for the con-venience of getting to it when requiring repairs. When it fits close toits seat, the water always remains suspended in the pipe, (unless the lattershould be defective) as mercury is sustained in a barometer tube.

In cold climates it is a matter of some importance to prevent water inpumps from freezing. Metallic pumps are, from the Superior conductingproperty of their material, more subject to this evil than those of wood.Of various devices a few may be mentioned. The old mode of enclosing

No. 90. Common Metallic Pump.