, 10 Ci'ijlruclisn of an Air Pump with MtiitUic Valves, 



iis place, and falleiieJ by the fcrews which ha<l before confined that piece. Figure 8 is 

 drawn in the plate as it is made for a double-barrelled pump ; but for a fingle barrel only 

 one piece is ufcd, rcprefciited by baa, the double piece being cut off at the dotted line a a. 

 In this piece is a female fcrew, to receive t!ie end of a long brafs tube ; to whicli a bladder, 

 if fufiicient for the experiment, mull be tied ; or elfe a «lafd, properly confined for this pur- 

 pofc, mufl be fcrewcd to it. Then the air, which is exliaufled out of a receiver (landing on 

 tlie plaie, will be forced into the bladder, or glafs conncded with the brafs tube. But if the 

 pump be double-barrelled, the apparatus, as reprefented by Figure 8, mufl be ufcd, and the 

 long brafs tube fcrewed into the female fcrew at C. 



Figures 9 and 10 rcprcfent the two gauges ; namely, the fyphon gauge, and the baro- 

 meter or long gauge. When thefe arc ufed, Figure 9 mull be fcrewed into the female 

 fcrew c b, or into that at the other end c Figure 7, and Figure 10 into the female fcrew a b 

 Figure 7. 



If it be ufcd as a fingle air pump, either to exhauft or condenfe, the fcrew K, which fallens 

 the rack to the cylindrical Mre H, mull be taken out ; then turning the winch till this wire 

 is deprefled as low as pofTible, the machine w'lll be rendered fit to exhauft as a fingle air pump : 

 and if it be required to condenfe, the direiflions given in the lad paragraph but one, with re- 

 gard to the bent tube T, and Fig. 8, mufl be obferved. 



Mr. Cuthbertfon's treatife contains a relation of various experiments made with this air 

 pump, which (hew its great power of exhaufting, and are in other refpecls entitled to the 

 confideration of philofophers. With the double fyphon gauge, and alfo with the long gauge, 

 compared with an attached barometer, in which the mercury had been repeatedly boiled, the 

 difference between the heights of the mercurial columns proved to be no more than one 

 fortieth of an inch, the barometer then ftanding at 30 inches. This gives an exhauftion of 

 1200 times. On fome occafions, when the air was in a very dry (late, he obferved the dif- 

 ference to be as low as one hundredth of an inch, which indicates more than double the rare- 

 fa£lion. 



On a review of thefe improvements of the air pump, I perceive in each inftrument fo 

 happy a combination of philofophical acutenefs and mechanical flcill, that it is with a de- 

 gree of diffidence that I venture to fpeculate on their refpeclive merits and blemifbes. There 

 is no proviflon to open the upper fixed valve of Prince's greater barrel, except the difference 

 between the prcflures of the elaftic fluid on each fide of the ftrip of bladder ; and this may 

 reafonably be inferred to limit the power of his fmall pump. In Cuthbertfon's pump, the 

 fame valve is expofed to the aiSlion of the atmofphere, together svith that of a column of 

 oil in the oil veffel. The mifchief in either inllrument is probably trifling ; but in both 

 the valve might have been opened mechanically. If this were done, the fmall pump of 

 Prince might perhaps be unneceffary in moll ftates of the atmofphere. With regard to the 

 lower valves, Cuthbcrtfon, by an admirable difplay of talents as a workman, has iufured 

 their atlion. Prince, on the other hand, has, by the proccfs of reafoning, fo far improved 

 the inftrument that no valves are wanted. In this refpecl he has the advantage of fim- 

 plicity and cheapnefs, with equal effefl. The mechanical combination of Cuthbertfon's 

 pump reduces the operation to one fimple aft of the handle: but Prince's engine requires 

 fome manipulation with regard to the play of the fmall pump; though this might have been 

 remedied by a more ikilful difpoCtion of the firft mover. 



The 



