A I R 



will refl upon a a, fy.. do, which will then be fo far open as 

 to peimit the air to pals freely through it, while at the fame 

 time the end oi g q is forced agaiiill the top of the hole, 

 and clofes it in order to prevent any air from returning into 

 the receiver. Thus tlie pillon, while moved downwards, 

 fufFers the air to pafs out Ijetween the fides oi fig. do. and fy. 

 6i, and when it is at the bottom of the hanel, will have the 

 column of the air above it ; and, confequently, when drawn 

 upward, it will fluit and drive out this air, and by opening 

 tlie hole L, give a free palTage to more air from tlie re- 

 ceiver. This procefs being continued, the air will be ex- 

 haulled out of the receiver as far as its expanfive power 

 will permit : for in this machine there are no valves, as in 

 tlie common air-pumps, to be forced open by the air in the 

 receiver, which, when its elalHcity is diminilhed, it becomes 

 unable to alfeft j nor is there any thing to prevent tlie air 

 from exp;uuling to the greatell degree. 



In uflng this machine for exhaultion, no direftions are nc- 

 ceffary belides thofe which relate to common pumps, nor is 

 any peculiar care required to keep it in order, except that 

 t)ie oil-veffel, G, be always kept about half fidl vi' oil. 

 When it has ftood for a confiderable tiiiie without being 

 ufed, it will be proper to draw a table-fpoonful or two of 

 oil through it, oy pouring it into tlie hole in the middle of 

 the receiver plate, when the pillon is at the bottom of the 

 barrel ; then, by moving the winch backward and forward 

 to raife and deprefs the pillon, the oil will be drawn through 

 all the parts of the machine ; and the fiiperfluous part will 

 he forced out through the tube T, into the oil-ve(l'el G. 

 Near the top of the cylindrical wire H, is a fquare hole, 

 which is intended to let in fome of the oil from the vefTel G, 

 tliat the oiled leathers, through which the wire q q Aides, 

 may always be duly lupplied with it. 



When the pump is required to condenfe, either at the 

 tjme when it exhaulls, or feparatcly, the piece which con- 

 tains the bent tube T, mull be taken away, and_/?f. 64. put 

 into its place, and fallened by the fame fcrcws. In the 

 plate, jig. 64, is drawn as it is made for a double-barrelled 

 pump ; but for a Imgle barrel, one piece is ufed, reprefented 

 by baa, the double piece being cut off at the dotted line a a. 

 In this piece is a female fcrevv, for receiving the end of a 

 long brafs tube ; to which a bladder, if fufficient for the 

 experiment, mull be tied ; or elfe a glafs, properly confined 

 for this purpofc, mufl be fcrewed to it. Then the air, 

 which is exhaufted out of a receiver Handing on the plate, 

 will be forced into tlie bladder or glafs connefted with 

 the brafs tube. But if the pump be double-barrelled, 

 the apparatus, as reprefented by Jig. 64, mull be ufed, 

 and the long brafs tube fcrewed into the female fcrew 

 at C. 



The two gages are reprefented in Jig. 6^. and Jg. 66 ; 

 the one is the fyphon-ga;^e, and the other the barometer 

 or long gage. When thefe are ufed,^. 6^. mull be fcrewed 

 into the female fcrew, c h, or into that at the other end c. 

 Jig. 63. ; 7\\\AJig. 66 into tlie female fcrew n b, fig. 63. 



If it be ufed as a fingle air-pump, either to exhauft or con- 

 denfe, the fcrew K, which fallens the rack to the cylindri- 

 cal wire H, muil be taken out ; then turning the winch till 

 this wire is deprefled as low as pofiible, the machine will be 

 rendered tit to exiiaull as a fingle air-pump ; and if it be 

 required to condenfe, the direffions already given with re- 

 gard to the bent tube T, and_y^. 68, mufl be obferved. 



Mr. Cuthbertfon has, by a variety of experiments with 

 this air-pump, fliewn its great powers of exhauflion. With 

 the double fyphon gage, and alfo with the long gage, com- 

 pared with an attached barometer, in which the mercury 

 bad been repcaudly boiled,thc diflcrencc between the heights 



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of the merourial column proved to he no more than xsth of 

 an inch, the barometer Handing at 30 inches, which gives 

 an exhauflion of i 200 times. On fome occaiions, wlien the 

 air was in a ven,' dry Hate, he obferved the differenie to be 

 as low as -rV^t'^ "f •"> inch, which indicates more than dou- 

 ble the rarefadioii. See Delcription of an improved Air- 

 pump, by John Cuthbertfon, 8vo. London: for an ubIlraCt, 

 Nicholfon's Journal, vol. i. p. 128 — 130. 



^\'e Ihall clofc our account of the two pumps of Prince 

 and Cuthbertfon with the following judicious remarks of 

 Mr. Nicholfon (in his Journal, vol. i. p. 131.) on tlitir re- 

 fpcftive merits and imperfections. " There is no provifion 

 to open the upper fixed valve of Prince's greater barrel, ex- 

 cept the difference between the preffurcs of the elalllc fluid 

 on each fide of the itrip of bladder ; and this may rcafon- 

 ably be inferred to limit the power of his fmall pump. In 

 Cuthbertfon's pump, the fame valve is expofed tb the action 

 of the atmofphere, together with that of a column of oil in 

 the oll-velfel. The mifchief in either inllrument is probably 

 trifling ; but in both, the valve might have been opened me- 

 chanically. If this were done, the fmall pump of Prince 

 might perhaps be unnecedary in moll Hates of the atmof- 

 phere. With regard to the lower valves, Cuthbertfon, bv 

 an admirable dilplay of talents as a workman, has iiilurcd 

 their aftion. Prince, on the other hand, has, by the pro- 

 cefs of reafoning, fo far improved the inllrument, that no 

 valves are wanted. In this relpeit, he has the advantage of 

 fiinplicity and cheapnefs, with equal cffecl. The mecha- 

 nical combination of Cuthbertfon's pump reduces the ope- 

 ration to one fimple aft of tlie handle : but Prince's engine 

 requires fome manipidation with regard to the play of the 

 fmall pump ; though this might have been remedied by a 

 more Ikilful difpofition of the tirll mover." 



" The moll perfeft fcheme for an air-pump, taking 

 advantage of the labours of thefe judicious operators, feems 

 to be that in which two piilons of the conflrudion of Prince 

 (hould work in one barrel ; one pillon being fixed at the 

 lower end of the rod, and the other at the middle. The 

 lower pillon mull come clear out of the barrel when down, 

 and work air-tight through a diaphragm at an equal dillance 

 from the effective ends of the barrel. In the diaphragm 

 mufl be a metallic valve, of the form of Cuthbertfon's 

 lower-valve, but with a fhort tail beneath, that it may be 

 mechanically opened when the pillon comes up. Above the 

 diaphragm muil work the other piflon, fimilar to the firll ; 

 but as it cannot quit the barrel when down, a fmall portion 

 of the barrel mull be enlarged, jufl above the diaphracm, 

 fo that the leathers may be clear in that pofition. Laftly, 

 the top of the barrel mufl be clofed and fitted with a valve 

 and oil-veffel, according to the excellent contrivance of 

 Cuthbertfon." 



" If we fuppofe the workmanfhip of fuch a pump to 

 leave the fjiace between the diaphragm and lower pillon, 

 when up, equal to one-thoufandth part of the tpace pafied 

 througli by the ftroke of that pifton, the rarefaftion produced 

 by this part of the engine will in theory bear the fame pro- 

 portion to that of the external air ; and the lame fuppolition 

 applied to the upper pillon, would iucreafe the effedl one 

 tliouland times more : whence the rarefaclion would be 

 one million times. Plow far the pradical effefl might fall 

 fhort of this from the imperfeftions of worknianlhip, or the 

 nature of the air, which in high rarefaftions, may not dif- 

 fufc itfelf equally through the containing fpacts, or from 

 other yet unobferved circumllances, cannot be deduced from 

 mere reafoning without experiment.''' 



It Is obferved in the Encyclopa-dia Britannira, (vol. xv. 

 p. 107.) that a conftruftion of the air-pump, fimilar to that 



t>f 



