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Chap. 7.] Limits of the perpendicular length of Suction Pipes.

the pump in a case containing tanners’ bark, charcoal, the düng of hor-ses, &c. is continued. Others are to prevent the valve O from sittingcloseto its seat, or to open it, by pressing the sucker upon a pin attached toit, so that the contents of the eylinder and pipe may descend into the well;hence every time the pump is used a fresh portion is required to ‘ primeit.’ A more common method is to connect the lower part of the eylinderwith the suction pipe by a stop cock and short tube, as at C. By openingthe cock the water in the pump deseends through it into the pipe. Butthe usual practice in this country, is to make the eylinder of such a lengththat two or three feet of it may be below the sui'face of the ground, andout of the reach of the frost; about a foot above the valve O or lowerbox, a plain cock is inserted : in winter this cock is left partially open,and the water above escapes slowly through it into the ground; whilethat below, into which the sucker is made to extend at its lowest position,serves instead of fresh ‘priming.’

A similar device is attached to the lateral pipes that convey the waterof the Schuylkill into the houses of Philadelphia .

Some persons can scarcely conceive how the atmosphere can have ac-cess to a well, while the latter is covered with slabs of stone or timber,and a thick bed of clay or mould over all. They forget that it is the ra-rity of air, the extreme minuteness of its particles, which enables it tocirculate through the finest soils, as freely as people pass through the va-rious chambers and passages of t.heir dwellings. Were the sides of awell coated, and its mouth covered with the best hydraulic cement—nosooner could the sucker or piston of a pump produce a partial vaeuumwithin it, than the air would stream through the cement as water througha colander or shower bath. And if the top and sides were rendered per-fectly air-tight, it would then enter the bottom and ascend through thewater without any perceptible obstruction. If it were possible to makea well impervious to air, no water could be raised from it by one of thesepumps: no movement of the sucker could then bring it up. We mightexamine the apparatus with solicitude—remove its defects with care—consult the learned with the Florentines, or get enraged like the Spanish pump maker of Seville ;—still, the water, like Glendower’s spirits of thedeep, would in spite of all our efforts refuse to rise.

When the atmospheric pump is required to raise water from a perpen-dicular depth, not exceeding 26 or 28 feet, (i. e. in those parts of the earthwhere the mercury in the barometer generally Stands at 30 inches) thelength of the eylinder need not exceed that which is required for thestroke of the sucker. In all cases, the perpendicular distance betweenthe sucker, when at the highest point of its stroke and the level of thewater, should never exceed the same number of feet as the tube of a ba-rometer, at the place where the pump is to be used, contains inches of mer-cury. But in the temperate zones where pumps are chiefly used, the pres-sure of the air varies sometimes to the extent of two inches of mercury, orbetween two and three feet of water; hence the distance should be some-thing less. And as the level of water in Wells is subject to changes, it is thelaudable practice of pump makers to construct the eylinder and rod of thesucker, of such a length, that the latter may always work within 26 or 28feet of the water.

By keeping the above rule in view, water may be raised by thesepumps from wells of all depths; for after it has once entered the cy-linder, it is raised thence by the sucker independently of the atmosphere,and to any height to which the eylinder is extended. This seems to havebeen well understood by old engineers. The remark of those who made