Chap. XIX.

ATMOSPHERE AS A MOVING TOWER.

1217

the weight of the atmosphere on every square inch of the earth's surface. The height ofthe atmosphere has been estimated at 328,021 inches, or 5-17 miles.

The elasticity of the air having been ascertained by the experiments of Boyle, it was notdifficult to construct the air-pump, though the one used by this celebrated philosopher wasinconvenient for many purposes, in consequence of the neces-sity of alternately opening and shutting the stop-cock andvalve, and the difficulty of making the piston descend whenthe air within the pump was greatly rarefied.

Hawksbee and Smeaton both turned their attention to itsimprovement: the latter found considerable difficulty inopening the valves at the l>ottoTn of the barrels, and fromthe pistons not fitting exactly when put close down to thebottom, leaving a lodgment of air that could not be pumpedout of the barrel. The first of these defects arose from thesmall orifice through which the air passed on raising thebladder, which opposed a considerable resistance, from the•necessity of keeping it moist with oil. To remedy this Mr.

Smeaton resolved to expose a greater surface to the actionof the air; with this view, instead of one hole he used seven,all of equal size and shape, one being in the centre, and theother six round it, so that the valve was supported at properdistances by* a kind of grating formed by the solid partsbetween these holes, which were hexagonal,and the partitions filed almost to an edge ;the metal was made a little protuberant,to prevent the piston from striking againstthe bladder. To prevent any lodgmentof air in the lower part of the barrel, heremoved the external pressure from thepiston-valve, by making the piston movethrough a collar of leather, and forced theair out by a valve applied to the plate atthe top of the barrel, which opened out-wards.

Cuthbertson, of Amsterdam , consider-ably improved Smeaton’s air-pump, byallowing the air to escape by opening apassage to it mechanically, without theassistance of its elastic force.

The construction of the air-pump is very simple,the essential part consisting in the exhausting syringe,a brass tube closed at one end, with the exceptionof a small orifice, to which a valve opening inwardsis attached; an air-tight piston is worked up anddown on the barrel by a rack and pinion turnedby a winch. Thepiston has also anorifice, with a valve,which opens upwards,or in the same di-rection as the valveof the tube. Thesyringe communicatesby means of a smallpipe fitted into theopening at its lowerextremity, with avessel called the re-ceiver, from which theair is to be pumpedout. The two ex-hausting barrels areworked by a rack andpinion, so that onepiston ascends whilethe other descends, and ...

_, j , Al _ lug. 1912. smeaton’s air-pomp.

attached to the pump

4 I

Fig. 1910.

Fig.1911.

HAWKSBEE S AIR-PUMP

Fig.1913.