A I R 



A 1 R 



Hence it follow,', that the quantity of air which was at fivd; 

 ill tlic receiver before you began to pump, is proportionable 

 to the whole ilandard altitude ; and confequcntly the dirt'er- 

 aice of this air, which was at lirll in the receiver, and tliat 

 which remains after any certain number of turns, that is, 

 the quantity of air exhaulled, is proportionable to the dif- 

 ference of the ilandard altitude and the before-mentioned 

 defect, that is, to the altitude of the qiiic'ililvcr in the gage 

 after that number of turns. Hence again it apjiears, that 

 the quantity of air exhauiled at ever)- turn of the pump is 

 proportionable to the alcent of t'.ie quickillver upon each 

 turn, tiee Cotcs's Hydrolt. and Pneum. Leftures, leC.t. 



13. See Gage. 



Tiiere are fevcral inconveniencies attending air-pumps of 

 the common lorm, though much improved from what they 

 uied to be formerly, and many attempts have been made to 

 remedy them. It is a well-known fad, that pumps merely 

 ferve to rarefy the air to a eoniiderable degree, and that 

 none of them can produce a complete exhauilion ; as the 

 mercury in the gage is not railed by any of them to the 

 height which it occupies in the Torricellian tube, when well 

 purged of air. Few pumps will bring it within ["jth of an 

 inch. Haukd^ee's, fitted up according to his own inllnic- 

 tions, will leldom bring it within J-th ; pumps with cocks 

 of 'the belt conftruftion, and in the mod favourable circuui- 

 ftances, will bring it witliin ^'oth ; but none with valves 

 fitted up with wet leather, or to any part of which water 

 or any volatile fluids have accefs, will bring it nearer than -Jth. 

 Before we proceed to give an abridged account of the im- 

 provements that have been made in air-pumps, we obferve, 

 that the air-pumps moll commonly ufed are made cither 

 with brafs flop-cocks, or with valves of oil-fl-:in or of lea- 

 ther, lor preventing the return of the air into the receiver, 

 out of which It had been exhauded. Pumps with flop- 

 cocks, when well made and newly put together, are gene- 

 rally found to rarefy the air to a greater degree than thnfe 

 which are made with valves ; but after having been ufed for 

 fume time, they become lefs accurate than thofe with valves. 

 But the valves are alfo imperfeft ; as the external air, pref- 

 fing upon that in the pillon, prevents its rlijng, when tiie 

 elallic force of the air in the receiver, under exliauftion, is 

 much dimininitd. Attempts have been made, particularly 

 by the abbe Nolkt and Mr. Gravefandc, to pcrfedl the con- 

 ilruifliou of cocks. In Gravefande's double-barrelled pump, 

 the cocks at the bottom of the piltoris are turned by an 

 apparatus that is moved by the handle of the pump : the 

 pilloa has no valve, and the rod is conncAcd with it by a 

 flirmp, as in a common pump. This rod has a cylindric 

 part, which paffes through the flirrup, and moves fliffly in 

 it through the fpace of about half an inch, between a 

 (houlder above and a nut below. The flirrup fupports a 

 round plate, which has a fhort fquare tube, tliat fits tight 

 into the hole of a piece of cork, and w hich has alio a fquare 

 fliank, that goes into the fquare tube. Between the plate 

 and the cork is put a piece of thin leather, foaked in oil, 

 and another is placed between the cork and the plate which 

 forms the fok of the flirrup. When the winch is turned to 

 raife the piilon from the bottom of the bairel, the fritlion 

 of the pillon againfl the barrel keeps it in its place, and the 

 rod is drawn up through the ftiiTup. The wheel has thus 

 liberty to turn about an inch ; and this is fufficient to turn 

 the cock, fo as to cut off the communication with the ex- 

 ternal air, and to open that with the receiver. When this 

 is done, the continued motion ferves to raifc up the piflon 

 to the top of the barrel. When the winch is turned in the 

 oppolite direftioD, the pifton remains fixed till the cock is 



turned, fo as to Hnit the communication with the receiver; 

 and open that with the external air. The cock lias one 

 perfcnation diametrically th.rougii it, and anotlier in a per- 

 pendicular direction to this ; and after reaching the centre, 

 it pafies along tlie axis of the cock, and C(nnm\micatcs with 

 the open air. By tiiis communication, wlien it is opened, 

 the air ruflies in, and balances the pretfure on the upper fide 

 of the pillon in this barrel, fo that the preluire on the other 

 mull be countciv.cled by the pcrfon who works the pump. 

 In order to obviate this inconvenience, Gravefandc put a 

 valve on the orifice of the cock, by tying over it a (lip of 

 wet bladder or oiled leather ; and by means of this the pif- 

 ton is preffed down, as long as the air in the barrel is rarer 

 llian the outward air, jull as if the valve was in the pillon 

 itfelf. Gravefandc, and alfo Mufchenbroek, extol the ope- 

 ration of this pump, as exceeding that of pumps with 

 valves. But it is evident tliat no precifc ellimate of its per- 

 formance can be obtained, whilll the pillons, valves, and 

 leathers of the pump are prepared by llceping them in oil, 

 and afterwards in a mixture of water and fpirits of wine. 

 With this preparation the gage could not be brought within 

 'th of an inch of the barometer. Bclldts, a eoniiderable 

 fpace is left between the pillon and cock, from which the 

 air is never expelled ; and if this be made very fmall, the 

 pump mull be worked very flowly ; othcrwife the air will 

 not have time to diiTnfe itfelf from the leceiver into the bar- 

 rel, efpecially when the expelling force or the clallicity of 

 the air, towards the clofe of the operation, is verj' fm;ill. 

 The rarefaftion vv'ill likewife be retarded by the valve, which' 

 will not open till the air below the piflon is confiderably 

 denfer than the external air. The cocks in pumps of this 

 kind are fubject to become loofe by ufc, and to admit air : 

 an inconvenience which might, indeed, be prevented by 

 placing the barrels in a difli filled with oil. For a figure and 

 defcription of Gravefande's pump, fee Gravefande's Mathem. 

 Elem.ofNatuia! Philofophy,byDefaguliers,vol. ii. p. 14. &c.- 

 Thefe pumps, if they were ever ufed in England, have been 

 long fupcrfeded by the cheaper and more fimple contrivance 

 of valves, formed by tying a llrip of bladder over a fma'l hole, 

 through which the air is allow'ed to pafs in one direction only. 

 Ill the year 1750, the ingenious Mr. Smeaton direft-' 

 ed his attention to the improvement of valve pumps.- 

 In confidering the flrufture of thefe pumps, he obfer- 

 ved, that the principal caufcs of their imperfeftion are, 

 partly, the difaculty of opening the valves at the bot- 

 tom of the barrels, and, partly, the pillon's not fitting ex- 

 actly, when put down to the bottom, which leaves a lodg- 

 ment of air that is of bad effcdl. The firfl of thefe imper- 

 feftions is owing to the fmallncfs of the common valves* 

 which are made of a piece of thin bladder flretched over a 

 hole generally much lefs than -rl,th of an inch in diameter, 

 and to the adhcfion of the bladder to the plate upon which 

 it is fpread, by reafon of the oil or water with which it is 

 moiflened : as the rarefaction of the air in the receiver is 

 continued by the operation of the pump, its fpring becomes 

 fo weak, that it is not able to overcome the cohefion of the 

 bladder to the plate, the weight of the bladder, and the re- 

 finance occafioned by its being flretched. The larger the 

 hole is, over which the bVjddcr is laid, a proportixiiiably 

 greater force is exerted upon it by the included air in order 

 to lift it up ; and yet the aperture of the hole cannot be 

 made veiy large, becaufe the prefTure of the incumbent air 

 would either burfl the valve, or fo far force it down into 

 the cavity as to prevent its lying flat and clofe upon the 

 plate. In order to avoid thefe inconveniences, inflcad of 

 one hole, Mr. Smeaton makes, ufe of fevcn, all of. equal 



fliC 



