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barrel and riles to the top-plate, which is made air-light with 

 a collar of leather!, like Snn.taton's, it forces out all the air 

 above it ; and as the air cannot return into the band on 

 account. of the vhIvc in the top-plate, when the pitton 

 defccnds, there will be a vacuum between it and the plate ; 

 tvery thing being fuppofed pcrfedt. But in working the 

 pump, the pillon is not allowed to dcfccnd entirely into the 

 cilleni fo far as to leave the bottom of the banel open ; but 

 it defcends below a hole in the lidc of the banel near the 

 bultom, which opens a free communication between the 

 barrel, cilleni and receiver. Through this hole the air 

 rulhes from the cillern into the cxliaulled buntl, when the 

 pillon has dropped below it ; and by its next afcent this air 

 is forced out as the other was before. If the capacity of 

 the receiver, cidern, pipes, fee. belov.- the bottom of the 

 barrel, taken togcthe,-, be equal to the capacity of the 

 barrel, half the remaining air will be expelled by every 

 U:oke. Put as the working of this pump with a folid piftou 

 v.uuld be laborious, on account of tlie refillance it would meet 

 with iu its defcent from the air beneath, though it would 

 be lefTened by ever)' ftroke as the air became more rarefied, 

 Mr. Prince pierced three holes in the pillon at equal dillance., 

 from each other, and by a circular piece of bladder, tied 

 over the top of the piilon, formed a kind of valves over the 

 lioles, which opened with fufficient eafe to prevent any labour 

 in working the pump, by allowing the air to pafs through 

 the pillon fn its delcent. The efcape of the air does not, 

 however, depend upon a pafiage through the pillon into tlie 

 barrel ; for when the air, weakened by rarefaftion, cannot 

 open tliis valve, it. will ftill get into the barrel when the 

 Communication is opened by the hole at the bottom. This 

 pillon will therefore defcend as ealily as any other, nor will 

 the valves impede the rarefaftion. By this coiiiliuclion the 

 valves, made to open with inore cafe by Mr. Smeaton, are 

 rendered unneceflary for i-arefying the air ; and that at the 

 bottom of the barrel is entirely removed ; the valve on the 

 top-plate being the only one neccllai-)' in rarefying the air. 



Having fet afide the valves, which partly prevented the 

 air from entering the baiTel above the pillon, Mr. Prince's 

 next attempt was to expel the air more perfeftly out of the 

 barrel than Mr. Smeaton had done, by making a better 

 vacuum between the pillon and the top plate, lo that more 

 of the air might be allowed to expand itfclf into the barrel 

 from the receiver. Mr. Prince alfo contrived to conneft the 

 valves on the top plate with the receiver occafionally by 

 means of a pipe and cock, by the turning of which the 

 machine might be made to exhauft or condenfe at pleafure. 

 In order to remove the preffure of the stmoiphere from the 

 valve on tlie top plate, fo that this valve might open as 

 cafily as the pillon valve, he connefted with the dutl on the 

 bottom piece, which conveys the air from the valves to the 

 cock, a fmall pimip of the fame conilruftion as the large 

 one ; having the barrel opening into the ciftern, the piftou 

 rod, which is fohd, moving through a collar of leathers, 

 and a valve near the top, through which the air is forced 

 into the atniofphere. This pump with one barrel is called 

 the valve-pump ; its chief nfe being to rarefy the air above 

 the valves, or to remove the weight of the atmofphere from 

 them.' When this valve pump is ufed, the pafiiige through 

 the cock is (liut up ; and, therefore, inilead of placing three 

 duels at equal dillances round ttie cock in the manner cf 

 Mr. Smeaton's, Mr. Prince divided the whole into five equal 

 parts, leaving the diftance of onc-tifth part between the 

 dntts leading from the cillern and tlie valves to the cock, 

 and two-fift!is between each of thefe and the one leading 

 from tlie coek to the receiver. By this adjullment, v.-hen 

 the communication is open betv/een the receiver uni the 



valves for condenfation, the other hole through the cock 

 opens the cillenis to the atmofphere ; but when the com- 

 niunication is made between the ciftems and the receiver fur 

 exhauilion, a folid part of the key comes againll the dud 

 leading to the valve, and (huts it \:p, and the air which is 

 forced out of the barrel pafles throuf^h the atmofphere into 

 the valve-pump ; for the valve of the fmall pump may be 

 kept open while the great one is v.orktd. 



U])on this coii{lruction, the pump with two bands n'ay 

 be m.ide like the common pump, which cannot be conve- 

 niently done where the lower valve is retained. In this pump 

 the pillons do not move the whole length of the barrels ; an 

 horizoi.tal fettion being made in them u little more than half 

 way ftom the bottom, where the top-plates are ii.ferttd. 

 Tiie pump is thus made more convenient and limple, as the 

 head of it is brought down upon the top of the barrels in 

 the fame manner as in the common air-pump. The bands 

 alfo Hand upon the fame plane with the receiver plate, and 

 this plane is raifod high enough to admit the common gage 

 of 32 or 33 inches to Hand under it without inconvenience 

 in working the pump : as the winch moves through a lefs 

 portion of an arch at each flroke than it would do if the 

 pillons moved through the whole length of the barrels. 



A gage for meafuring the degree of condenfation having 

 a free commimieation with the valves, cock, &c. is placed 

 between the barrels in this pump ; and the gage is fo con- 

 ftrufted that it will alfo fervc to meafure the rarefaction 

 above the valves when the air is worked off by the valve- 

 pump. It confills of a pedeftal, the die of which is made 

 of giafs, which forms a cillern for the mercury, a hollow 

 brals pillar, and glafs tube hermetically fcaled at one end, 

 which moves up and down in the pillar through a collar of 

 leathers. When the pump is ufed as a condenier, the degree 

 of condenfation is fhewn by a fcale marked on one edge of 

 the pillar ; when it is ufed as an exhauller, the degree of 

 rarefaction of the air above the valves is iliev.-n by a fcale on 

 the other edge of the pillar. This gage will alio (hew, when 

 the valves have done playing, either with the weight of the 

 atmofphere on them or taken off, in the manner which the 

 author ha? cefcribed. The degreeof condenfntion rriay be aL'b 

 meafured by the number of ilrckes of the winch. For the 

 piu"poftsof great condenfation, Mr. Prince has fitted acon- 

 denfer of a Imaller bore than the barrel of the great pump 

 to the ciAern of the valve-pump, to be fcrewed on occa- 

 fionally. Or, without this condenfer, the valve -pump may 

 be adapted to the purpofe by being made a little larger, and 

 by having a plate made to fcrew into the bottom of the 

 cylinder, v.-ith a valve on it opening into the cillera ; a hole 

 muft be made to be opened on the fame occafion near the . 

 top of the cylinder, to let air in below the pillon when this 

 is drawn up above it. 



The common gage, which is generally plf.ccd under the 

 receiver-plate, is placed in the front of this pump, that it may 

 be feen by the perfon who works it, and that the plate may 

 be left free for other ufes. The plate is fo fixed to the pipe 

 leading to the cock, that it may be taken off at pleafure,. 

 and ufed as a transferrer ; and it may alio ferve for other 

 purpofes. 



The head of this pump is made whole, except a fmall 

 piece on the back, where the wheel is let in ; and the v.heel 

 is freed from the pillon-rods by pufning it into the back part 

 of the head, and it is kept in- its place by a button fcrewed 

 into the focket of the axis behind. By this apparatus the 

 piflon-rods are diilodged from the wheel, and let down into 

 the cifterns, when the pump is not ufed ; and in thefe cillema 

 they may alfo have the advantage cf being covered with oil. 

 The principal joints of tLis pump arc fur.k iato fockets, that 



tlie 



