AIR-PUMP. 



AIR-PUMP. 



m 



mid l.\ the time tin- piston has descended go much further that the 

 .idditional elastic force acquired from compression will suffice to lift 

 the valve A, the Utter will |nii. and the air will rush out. Thin con- 

 tinue* until the piston him quite returned to H. That in to nay, after 

 every stroke of tin- i.: -t..n. the nir in the vessel hi* only six-seventh- ! 

 the density which it had before the stroke, aince the air contained in 

 MX measure* is expanded into seven by the rim of the piston. There- 

 fore, at the end of the second stroke the density is f of f, or Jj, that is, 

 88 measures of common air would weigh a* much as 49 of the air we 

 have now got inside the vessel. At the end of the third stroke the 

 density is f of JJ, or UJ. Without going farther, suffice it to say, that 

 at the end of the twentieth stroke, the density of the rarefied air is 

 almt Jj; and t tin- nul of Ion strokes, it would take about five 

 million of measures of the rarefied air to weigh as much as one of 

 common air. But long before this time a limit would 1-c put t.- the 

 .exhaiwtion, in the present state of the apparatus. The air in the 

 veowl cannot escape into the tube unless it him force sufficient to lift 

 up the valve B ; which after a certain number of strokes will not be 

 the case, for the elastic force of the air diminishes in the same pro- 

 portion as its density, being at first fifteen pounds to the square inch ; 

 no that by the time the density is reduced to jjj, the valve, if it pre- 

 sent a surface of one square inch, will not rise, if it be so heavy as 

 half an ounce. Let us, then, suppose B to be fastened to the piston by 

 a loose string, so long that it becomes tightened just before the piston 

 reaches its greatest height. The string will then open ti 

 the rarefaction will take place as usual. 



The condensing instrument will now be easily understood. Let 

 the piston be rained, the valves will then be open; but the moment 

 tlie piston 1-egins to descend, the rush of air outward.* will shut c, 

 and the whole of the air in the tube will be forced into the vessel, 

 which admits it, since D opens inwards. If this be done quickly, HO 

 that hardly any air escapes, seven measures of air, after the stroke, will 

 occupy the space filled by six measures before it, so that the density of 

 the air in the vessel will be ] ; or six measures of condensed air will 

 weigh as much as seven of common air. When the piston returns, air 

 rushes in through c, and presses the valve D, which nevertheless, 

 unless made too heavy, does not open, because it is pressed with a 

 greater force from within. In every succeeding stroke an additional 

 measure of common air is added to the stock already contained in the 

 vessel. At the end of the second stroke the density is g, at the end of 

 the third \, and so mi. Kvery succeeding stroke will be more difficult, 



for tin- ait- (Ml liniil Uiwis-n r and P in the descent of tin- 

 will ti ' !>< M until it i i.. inpreseud tli.m ' 



nitliin the vessel. Also the condensation increases only in arith- 

 metical progrcss'-ii, while the corresponding rarefacti'-n in tlic 



exliausting syringe takes place in geometrical progression. It 

 take 30,000,000 of strokes, all but .-K. . t.. produce a t-.-n.li-: 

 the corresponding rarefaction to which is gained in a hnii.li"! It 

 is needless to say, that n-i m.-it. li.'l- il.l put t 



would bear such a pressure, and no force that v..- , .-M!.| c\.-rt would 

 create it. 



The exhausting syringe, as above described, is, in i-riiuip: 

 common air-pump. We shall now proceed to descriW iv.ihi.. 

 air-pump, containing the most recent material improvements. 



The circular plate or table at the top is of metal or of glass ground to a 

 perfect plane surface, on which is placed an inverted glass jai 

 which the air is to be exhausted, called the rrrtirer, the l-.tt.-ni ..f 

 which is also carefully ground : so that if the plate l.e >lii;htly smeared 

 with grease and the receiver placed ii]-n it. the junction .-f tin- twu i~ 

 air-tight. The hole in Uie middle i-f the plate is the end of a till*-. 

 which extends vertically downwards, until, curving at the l-.-tt.-m. it 

 passes through the front beam below the barrel*, wit h tin- interior "f 

 each of which it communicates. These barrels are exl 

 the construction of which will presently be more particularly tie- 

 scribed : they are worked by rack-work, communicating with a cog- 

 wheel and handles, s|>ace for the racks to play being cut in tin- upi-ci- 

 wood-work of the apinratus. On the left ore the gauges for ascer- 

 taining the degree of exhaustion obtained, and at A is a plan 



M'(,i:. See also SYPHON-OAUOK, as we shall here only describe 

 tin- most common, the Ixtrrnarlfr-yaitfff. The box attached to the 

 under beam on the 1. ft contains mercury, out of which rises a tube 

 and a graduated scale, as in the barometer. This tulw [ses tl 

 the higher wo.nl-w.irk, and also end" in the orifice which i* in tin- 

 middle of the plate, so that the communication In-ing free, the air in 

 the receiver, and that in the tul-e al-ove the mercury, are in the same 



sin.. II. -in.- M th. aii ii. i.ircli. .1. t)].- .'\ti-inal .iir will for.. 

 mercury up the tul-c, and the height to which it has risen will show 



