A 1 R 



(ize and fhapr, one being in the centre, and the other fix 

 round it, fo that the valve is fupportcd at proper dillancfs 

 by a kind of grating, formed by the lolid parts between 

 thefe holes, andrcfembling a honeycomb; and that the points 

 of conlaift between the bladder and grating may be as few 

 as pofTiblc, the holes are hexagonal, and the partitions are 

 fjcd almoil to an edge. Tlic breadth of thefe hexagons is 

 T?jths of an inch, and confequently the fiirface nine times 

 larger than common ; and as the circumference is three 

 times greater than tliat of the common valve, and the cohefion 

 to be overcome is, in tiie firil moment of the air's exerting 

 its force, proportional to the circumference of the hole, the 

 valve over any of thefe holes will be raifed with three times 

 more eafe. llefidts, the niifing of the valve over the centre- 

 liole is aided on all fides by thofe that arc placed round it ; and 

 as tliey all contribute as much to raife the bladder over the 

 centre hole, as the air immediately afting under it, the 

 valve will be raifed with double the eafe already iuppoled, 

 or with a fixth part of the force commonly neceflaiy. After 

 the bladder begins to riie, it will expofe a greater furfacc to 

 the air underneath, which will caufe it to move more eafily. 



The other defect in the common conftruftion would ilill 

 hinder the rarefaclion from being carried on beyond a certain 

 degree. For as the piilon does not fit fo clofely to the 

 bottom of the barrel, as totally to exclude the air, this air, 

 as the piilon rifes, will expand itfelf ; and preffing upon the 

 valves in proportion to its denfity, hinder the air within the 

 receiver from coming out. Hence, if the vacancy were equal 

 to the 150th part of the capacity of the whole barrel, no 

 air could pafs out of the receiver, when expanded 150 times, 

 though the piftcn v.-ere conftantly drawn to the top; becaufe 

 the air in the receiver would be in equilibrio with that in the 

 barrel, when in its moft expanded {late. In order to obviate 

 this inconvenience, Mr. Smcaton fliut up the top of the 

 barrel with a plate, having in the middle a coUar of leathers, 

 through wliich the cylindrical rod works that carries the 

 piilon. The external air is thus prevented from prefling 

 upon the pifton ; but for the difcharge of the air that pafles 

 from belo«' through the valve of the piilon, there is a valve 

 applied to the plate at the top, which opens upwards. By 

 this conftruction, when the piilon is put down to the bottom 

 of the cyhnder, the air under it will evacuate itfelf fo much 

 the more, as the valve of the piilon opens more eafily, when 

 prefled by the rarefied air above it, than when prefTed by 

 the whole weight of the atmofphere ; and as the pifton may 

 be made to fit as nearly to the top of the cylinder as it can 

 to the bottom, the air may be rarefied as m.ich above the 

 pidon, as it could before have been in the receiver. Hence 

 it follows, that the air may now be rarefied in the receiver 

 in duplicate proportion of what it could be upon the com- 

 mon principle. By this conilruftion, the pump, confifting 

 of a fingle barrel, may be worked with more eafe than the 

 common pump v.'ith two barrels, becaufe the prelTure of the 

 outward air is taken off by the upper plate ; and when a 

 confidcrable degree of larefailion is defiredj it will produce 

 it more fpeedily. 



Mr. Smeaton has alfo contrived a new gage, which mea- 

 fures the expanfion with certainty, to much lefs than tlie 

 lOOOth part of the whole. It confifts of a bulb of glafs, 

 in ihape refembling a pear, and fufficient to hold about half 

 a pound of quickfilver. It is open at one end, and the 

 other is a tube hennetically fealed at top. A fcale divided 

 into parts of about -p'^th of an inch each, and anfwering to 

 a 1 000th part of the whole capacity, is annexed to it. This 

 gage, during the exhauftionof the receiver, is fufpended in 

 It by a flip-wire. When the pump is worked as much as is 

 thought neceflary, the gage is pullied down, till th« open 



A I R 



end is immerged in a cillem of quickfilver placed under- 

 neath. The air being then let in, the quickfilver will 

 be driven into the gage, till the air remaining in it be- 

 comes of the fame denfity with the external ; and as the air 

 always takes the highcil pl.ice, the tube being uppentioft, 

 the cxpanlion will be determined by the number of divifions 

 occupied by the air at the top. See Gage. 



'I'his ingenious artill has lucceeded io well in his con- 

 flru£lion of the air-pump, as to be able to rarefy air about 1000 

 times : whereas the bell of thecommon air-pumps, elleem.ed 

 good in their kind, and in complete oider, never rarefied it 

 above 140 times. Mr. Smeaton's air-pump acls alfo as a 

 condcnfing engine, by the very fimple apparatus of turning a 

 cock; fee CoNDENSKR. This air-pump is thus eafily made 

 an univcrfal engine, for fliewing any effect arifing from an 

 alteration in the denfity or fpring of the air ; and with a 

 little addition may be made to fliew the experiments of the 

 (W-FOUNTAIN, hiK-giin, &c. Phil. Traul. vol. xlvii. p. 



4«5— 42.1- 



A pcrlpeftive view of the principal parts of this pump 

 is exhibited in Plate vi. Piwutnalics, Jig. 45. A is the 

 barrel, B the ciilern, in which is included the cock, with 

 feveral joints, which are covered with water to keep them 

 air-tight ; and a little cock to let the water out of the ciftern 

 is marked 6. C cc 1% the triangular handle of the key of 

 the cock, which, by the marks on its anns, fiiews how it 

 muft be turned, that the pump may produce the effttl de- 

 fired. D H is the pipe of communication between the 

 cock aud the receiver. E is the pipe that communicates 

 between the cock and the valve, on the upper plate of the 

 barrel. F is the upper plate ot the pump which contain* 

 the collar of leathers d, and V is the valve, which is covered 

 by the piece y. G I is the fyphon-gage, which is fcrewed 

 on and off, and adapted to common puipofes. It confifts 

 of a glals-tube hermetically fealed at c, and furniflicd with 

 quickfilver in each leg, which, before the pump begins to 

 work, lies level in the line a b ; the fpace b c being filled 

 with air of the common denfity. When the pump exhaufts, 

 the air m b c expands, and the quickfilver in the oppofite 

 leg lifes, till it becomes a counterbalance to it. Its rife is 

 fliewn upon the fcale I f, by which the expanfion of the air 

 in the receiver may be nearly ellimatcd. When the pump 

 condcnfcs, the quickfilver rifes in the other leg, and the 

 degree may be nearly judged of by the contraftion of the 

 air in 3 c ; marks being placed at \ and j of the length of 

 b c from c, which Ihcvv v.-hen the receiver contains double 

 or treble its common quantity. K L is a Icrew-frame to 

 hold down the receiver in condenfing experiments, which 

 takes off at pleafure, and is fufficient to hold down a re- 

 ceiver, the diameter of whofe bafe is feven inches, when 

 charged with a treble atmofphere ; in which cafe it adls with 

 a force of about i 200 pounds againll the fcrew-frame. M 

 is a fcrew that fallens a bolt, which Aides up and down in 

 that leg, by means of which the machine is made to Hand 

 fall on uneven ground. The ftruClure, connection, and re- 

 lative ufes of the feveral parts of this pump will be further 

 perceived in the following account of Smeaton's air-pump, 

 conftrufted and improved by Mr. Nairne. 



A perfpedlive view of it appears in Plate vi. Pneu- 

 matics, Jig. 46. A, A, are the two barrels of a fimple 

 double-barrelled air-pump ; the tube q q conveys the air 

 from the receiver placed in the pump-plate T, and the cock 

 Q^ferves to cut off the communication between the receiver 

 and the barrels A, A, when the exhauftion is completed. 

 In the front of the pedeftal Z is a fcrew, fening to admit 

 air into the barrels, that the valves may not be preffed after 

 the cock Q^is turned ; the button i readmits air into the 



receiver ; 



