Page:Cyclopaedia, Chambers - Volume 1.djvu/99

 A I R

Some Authors take Airs, in a more extcnfive Senfe ; and divide 'em into loii), and high.

The Ifia Airs include the Natural Paces, as Trotting, Walking, Galloping, and Terra-a-Terra. See Pace, Trot, Gallop, tic. * . . r

The high, or m/jV ^7irs, are all fuch Motions as rile higher than the Terra-a-terra ; as the 'Demivolt, Curvet, Sec

AIRPvmp, a Machine, by means whereof the Air may be exhaufted out of proper Vcffcls. See Air.

The Ufe and 'Effea of the Air-Tump, is to make, what we popularly call, a Vacuum ; which, in reality, is only a Deorec of Rarefaaion fufficicnt to fufpend the ordinary Effeas of the Atmoiphere. See the Article Vacuum.

By this Machine, therefore, we learn, in fomc meaiure, what our Earth would be without an Atmofphere ; and how much all Vital, Generative, Nutritive, Alterative Power, depend thereon. See Atmosphere.


 * Principle on which it is built, is the EJafticity ot tl

(54) -«I R ,

fphere.a great part of which was to be removed at every Exac- tion, after a Vacuum was nearly arrived at. — But this Incon- venience has been fince removed by Mr. Haivksbce, who by adding a fecond Barrel and Pillon to the former, to rife as the other fell, and fall as it rofe 5 made the Preffure of the Atmofphere on the defcending one, of as much fervice as it was of differvice in the afcending one.

Some of the Germans have alfo brought the Air-Tump to do the oppofitc Office of a Cond.nfcr : But this is not to make the Inftrument fo much better, as more complex. See Condenser.

<rhe Structure of the Air Pump, as now made among us, with all its Advantages, is reprefented: in Tab. Tneuma- ticks, Fig. 16.

It confifts of two brazen Barrels or Cylinders, reprefented by aaaa ; which communicate with each other by a Canal paffing between them at dd ; and with the Receiver 0000, by means of the hollow Wire h h, one End whereof opens into the Canal of Communication, and the other into a like

Th lfr; P o n e :iirrco=rWyeSu m p^ fo W l C^aT^whi* penetrating the Plate it,

Air

ed, i's the Gravity of the fame Air. Sec Pump.

The StruBure of the Air-Tump is, in it felf, more fimple even than that of the Water-Pump.— The latter [uppofes two Principles Gravity and Elafticity likewiie : So that the Wa- ter-Pump mull firft be an Air-Tump, i. e. muft rarity the Air ere it raife the Water.—In effea, Water being a dor- mant unelaftick Fluid, needs fome external Agent to make it afcend ; whereas Air afcends in Virtue of its own elaftick Aaivity : its natural Tendency is, to feparate and leave a Vacuum ; and all that remains to Art, is to prevent the am- bient A r from fupplying the Place of what thus lponta- neoufly flies away.

To fay no more, to make Water afcend, the Force where- with it is prefs'd downwards, is cither to be diminifh'd or

increas'd in one Part, more than another ; like a Balance in /, catching by us Teeth into thejiac Equilibrio, one of whole Scales may be made to rife, either by dimini filing its own weight, or increafing that of the other : the Water, therefore, recedes from the common Cen- tre of Gravity, by the very Power wherewith it tends to- wards it, indireftly or fecondarily applied ; for that two fi- rnilar centripetal Forces being made to aa contrary to each other what the one overbalances the other, muft have the Eficci of a Centrifugal Force.— \Vhereas,thc Principle where- by Air ratifies or dimimfn.es, does nor refpea the Centre of the Earth, but the Centres of its own Particles ; being no other than a certain implanted Power, whereby they immediately tend to recede from each other. See Repel- ling Tower. . '

The Invention of this noble Inftrument, to which the prcfent Age is indebted for fo many fine Difcoveries ; is af- cribed to Otto de Guerick, the celebrated Conful of Magde- lourg ; who exhibited his firft publick Experiments there- with, bc'.bre the Emperor and the States of Germany, at the breaking. up of the Imperial Diet at Ratisbon, in the Tear ltJ 54. ,

Dr. Hook and M. in Hamel, indeed, afenbe the Invention to Mr. Boyle ; but that ingenious Author frankly confeffes de G"crick to have been beforehand with him. Some At- tempts, heaffures us, he had made upon the fame Founda- tion, bctore he knew any thing of what had been done abro.-d : but the Information he afterwards receiv'd from Scbottus's Mechamca Hydraulico-Tnevmatica, publilhed in 1S57, wherein was an Account of de Guerick' s Experi- ments, firft enabled him to bring his Defign to any thing of Maturity. — From hence, with the Affiltance of Dr. Hook, after two or three unfuccefsful Trials, arofe a new Air- Tump ; more eafy and manageable than the German one : and hence, or rather from the great Variety of Experiments that illuftrious Author applied it to, the Engine came to be denominated, the Macbiti'a Soyleana.

SmiBure and Ufe of the AiR-Pump.

The Bafis or eflential Part in the Air-Tump, is a metal- line Tube, anfwering to the Barrel of a common Pump, or Syringe ; having a Valve at the bottom, opening upwards : and a moveable Pifton or Embolus, anfwering to the Sucker of a Pump, furr.ifii'd likewife with a Valve opening up- wards.— The whole, duely fitted to a Veffcl, as a Recipient. See Embolus, Valve, and Recipient.— See alfo SP- RINGE.

The reft, being only Circumftances chiefly refpeaing Con- veniency, has been diverfified and improved from time to time according to the feveral Views and Addrefs of the

Makers. That of Otto de Guerick being lefs artlefs, laboured

r feveral Defeas, in refpea of the Force neceffa

the Receiver.

Within the Cylinders arc two Emboli, or Suckers made of Brafs, and fitted with Cork and Leather to the Cavities of the Barrels, fo as exaaiy to fill the fame ; each being fur- nifh'd with its Valve, and terminating at top in a Racktpc, by which it is to be work'd.

At the bottom of either Barrel is another Valve; by which the Air may pafs out of the communicating Canal dd, and confequcntly out of the hollow Wire and the Re- ceiver it felf, into the Cylinder, below the Pifton ; from whence by the Valves of the Pifton it may proceed into the upper Space of the Cylinder, and thus into the open Air.

Fir the Application of this Mecbanifm. The Winch

bb being turn'd upward and downward ; its Spindle tchino by its Teeth into the Racks, will raife and de- p'refs the two Pi Lions, alternately.— Now, the Confequence of deprefltng a Pifton, is, that the Air before inclofed be- tween it and the bottom of the Cylinder, being thus crowd- ed into a lefs compafs, will, by its elaftick Force, which now exceeds the Preffure of the Atmofphere, pufh up the Valve of the Pifton, and thus efcape ; till what little re- mains be of the fame Denfity with the external Air incum- bent on the Valve.

This done, and the fame Pifton being again raifed in its turn, from the bottom of the Cylinder to the top ; the little Air before left, will of neceflity expand it felf, fo as to pof- feis the whole Space of the Cylinder thus deferted by the Pifton : Upon which, its Force or Preffure upon the Valve at the bottom of the Cylinder, being now inconfiderable ; the other, denfer Air of the Receiver, hollow Wire, and Canal of Conimnnication, by their fuperior elaftick Force, will lift up the Valve, and thus pafs into the Cylinder of rari- fied Air, till both be of the fame degree of Denfity.

And thus is the Air in the Receiver diminifh'd at each Elevation of the Pifton, by the Quantity of a Cylinder full; abating for what little remain'd between the deprefs'd Pifton, and the bottom : So that by thus repeating the O- peration again and again ; rhe Air in the Receiver is at length rarified to fuch degree, that its Denfity does not exceed the thin Air remaining in the Cylinder when the Pifton is raifed : which done, the Effea of the Air-Tump is at an end ; the Valve cannot now be open'd, or if it could, no Air would pafs it ; there being a juft Equilibrium between rhe Air on each fide.

To judge of the Degree of Exhauftion, there is added a Gage, //; confifting of a Tube, whofe upper Orifice communicates with the Receiver; the lower being im- mersed in a Bafon of Mercury, mm. — Hence, the Air in the Tube rarifying as fail as that in the Receiver ; in pro- portion as the Exhauftion advances, the Mercury will be -raifed by the Preffure of the Column of external Air, pre- vailing over that of the Column of Air included ; till the Column of Air, and Mercury together, become a Balance to that of the external Air. When the Mercury is thus rifen to the fame Height as it ftands in the Barometer, which is indicated by the Scale of Inches added to the Gage ; the Inftrument is a juft Torricellian Tube ; and the Vacuum may be concluded to be as perfeft as that in the upper End of the Barometer. See Barometer, and Torricellian.

To let Air again into the exhaufted Receiver, the Cock n is to be turn'd ; which makes a Communication with the external Air ; upon which the Air rufhing itnpetuoufly in, the Mercury in the Gage immediately fubfides into the Bafon.

To the Air-Tump belongs a large Apparatus of other Veffels, accommodated to the divers 'Kinds of Experiments.

under feveral Defeas, in reipett ot the i'orce neceuary

work it, which was very great ; and the Progrefs very How :

befide that it was to be kept under Water; and allow'd of See Apparatus,

no Change of Subjeas for Experiments £ s of Rarefaction in the Receiver of an Air-Pump.

Mr Soyle, by degrees, removed ieveral of thele lnconve- J.

niences ; and alleviated others : but ftill the Working of his 1°, For the Proportion of Air remaining at any time in

Pump w'as laborious, by reafon of the Preffure of the Atmo- the Receiver, we have the following general Theorem.--