ConJlruB'ion of an Ah- Pump without the Lower Valve. i j, 



machine maybe made to exhauft or condenfe at pleafure. This is done in the following 

 manner : There is a crofs piece laid over the valves extending from one barrel to the other, 

 which has a du£l through it connefted with a fmall pipe (landing between the barrels ; 

 through this pipe the air partes into a du£t at the bottom piece leading to the cock. In this 

 piece is likewife the dud leading from the ciftern to the cock; and with this cock alfo is 

 conneded the pipe leading to the receiver. The key is pierced with two holes, in fuch a 

 manner, that one of them will conned the pipe coming from the receiver with the dufl io 

 the bottom piece leading to the ciftern, or with the other leading to the valves, as may be 

 required for exhaufling or condenfing. The other hole through the key will open, occa- 

 fionally, to the atmofphere either of thefe dufts round the cock: fo that, having the direc- 

 tion of the air which pafles through the valves under the command of this cock, the pump may 

 exhauR or condenfe at pleafure ; for when the key conne£ts the pipe from the receiver and the 

 du£t leading to the cifterns together, the pump will exhauft ; and when it connects the 

 pipe •!»ith the duft leading to the valves, it will condenfe, as the other hole in the key at the 

 fame time opens to the atmofphere the du£l leading to the cifterns, by which paflage the 

 air enters the barrel from the atmofphere, is forced out at the valves, and through the pipe 

 and cock into the receiver. In this part of the machine which is contrived for condenfa- 

 tion, the inventor has, by an additional part, endeavoured to get the air more perfcdly out 

 of the barrel. 



We have feen, fays he, that Mr. Smeaton, by making the pifton of his pump fit more 

 exaflly to the bottom of the barrel, and by fhutting up the top to prevent the preflure of 

 the atmofphere on the piflon valve, was able to get more of the air above it than could be 

 effeded in the common pump: but ftill the difficulty, though fo far removed, remains i i 

 the top of the barrel ; for, as the pifton cannot be made to fit fo exaftly to the top plate, 

 but that there will be Ibme lodgement for air, it is impoftible to expel it entirely. More 

 perhaps might be expelled if the valve on the top could bje made to open more eafily, by 

 removing the weight of air from it ; for tlie atmofphere prefting on this valve will prevent 

 its opening freely, in the fame manner as, when preffing on the pifton valve, it obftrufls the 

 opening of that in the common pump. 



The difficulty which Mr. Smeaton removed from the pifton vnlves, Mr. Prince endeavour- 

 ed to remove from the valve on the top plate ; that this valve, having the preffiire of the atmo- 

 fphere taken off, might cpcn with the fame eafe as the pifton valve does in his pump. To 

 effeft this, there is conneded with the dud on the bottom piece which conveys the air from 

 the valves to tlie cock, a fmall pump of the fame conftrudion as the large one ; having the 

 barrel opening into a cillern, the pifton rod moving through a collar of leathers, and a viilve 

 near the top, through which the air is forced into the atmofphere. This pifton is folid : be- 

 caufe the diameter being only half an inch, does not make it work hard. Thi:; pump, v.'hich 

 is of one barrel only, he calls the valve pump ; its chief ufe being to rarefy the air above the 

 VaWcs, or remove the weight of tlie atmofphere from off them. To ufc this pump, it is 

 ncccffary the key of the cock Ihould be pierced differently from that of Mr. Smcaton's ; for, 

 as the pipes round his are placed at equal diftances, when the one from the bottom of the 

 barrel is conncdtcd with that from the receiver to exhauft it, the other from the valve on the 

 top plate is opened tp the atmofphere by the other pafTage th.rough the cock. But in order 

 to rarefy the air above the valve in nny pump, it is ncccffiiry this laft paftage fiiauld be fliut 



K^ 2 up 



