A I R 



A I R 



c faHIng ilown, llic guinea and featlscr will hz obfci-vcd to 

 dffccnd with equal velocities, and by looking llcadily to the 

 bottom of the receiver, to fall to the pump plate at the fame 

 inftant. When air is in the ivceivcr, the guinea will fall in 

 an iiidant, and the feather will defcend gently and by an in- 

 direft motion. This apparatus is fonictimes fo conllruftcd 

 as to let three guineas with thr-r feathers fall A parately at 

 three different times, without taking it off or txhaulliug the 

 air afrefli. See Resistance of lie y/ii\ 

 IV. Mifcellaneous Experiments. 



1. Screw the fyringe H (Jig. 31.) to a piece of lead, 

 weighing at leall one pound ; pull up t'le pillon, which will 

 eaufe a vacuum in the lynnpe,aiid the air by its prcTure will 

 drive back the lead upon it ; raifing it and eouiu. rafting its 

 natural weight. But if the fyringe and annexed weight be 

 placed in an exhaufted receiver, they will fall upon the pifton 

 by their natural gravity, and upon readmitting the air, they 

 will be drove upward again, fo that the pilion will be at 

 the bottom of tlie lyringe. 



2. To a balance A li, Plate vii. Pin-timnl'ics, fg.$.:^- fufpend a 

 weight of lead, and let it be in equilibrio witli a piece of cork. 

 Place this apparatus under a receiver and exhauit the air, and 

 tlie cork will preponderate; but let the air be admitted, and 

 the equilibrium will he rellored. As the air is a fluid, all 

 bodies lofe as much of their weight in it as is equal to the 

 weight of an equal balk of the fluid ; and as the cork is 

 largert, it lofes more of its abfolute weight than the lead, 

 and of eourfe mull be heavier in order to compenfate tins 

 greater lofs ; but when the air is removed, all bodies gra- 

 vitate according to their quantities of matter, and therelore 

 the cork, which balanced the lead in air, will appear to be 

 heavier in vacuo. A more elegant apparatus for this expe- 

 riment, confilliag of a light glafs ball A, and a brafs weight 

 B, is exhibited u\Jig. 55. 



3. Set a clean receiver upon the plate of a pump, and 

 when you begin to exhaull it, hold a candle to the fide of 

 the receiver oppofite to your eye, and icveral colours, re- 

 fenibllng a halo, will appear about the candle, which are oc- 

 cafioned by the vapours that arife from the wet leathers and 

 their refradion of the light. 



4. Place a lighted candle under a tall receiver, and if it 

 holds about a gallon, the candle will continue to burn about 

 a minute ; and its light will gradually decay and at length 

 be extinguillied. The fnioke of the candle will afcend and 

 form a kind of cloud at the top of the receiver ; but upon 

 exhaulling it, the fmoke will fall down to the bottom ; thus 

 fliewing, that fmoke does not afcend becaufe it is pofitively 

 light, but becaufe it is lighter than air. 



5. Let the pipe reprefented in Plc!te\-m.Piteumatics,fg.6^, 

 be annexed to the top of an open receiver, and the air be ex- 

 haufted ; then place one end of the pipe in the middle of a 

 charcoal lire, and open the cock ; and the noxious air of the 

 charcoal will pafs tlirough the pipe into the receiver ; remove 

 the pipe from it, and let down a fniall lighted wax taper into 

 the receiver, and it will be immediately extinguidied. A moufe 

 or bird let down into the receive;' will be killed by the air 

 which It contains. If a candle be let down gently, it will 

 purify the air as it defcends. 



6. By connecting the wire that palTes through the collar 

 of leathers of a receiver with the trigger of a piilol lock, 

 placed under it, exhaulling the air, and then drawing the 

 trigger, the flint vyill ftrike the fteel and produce fparks of 

 fire, which will not be vifible as in the open air. Or, if 

 two iron bullets be made red-hot, and one of them be under 

 an exhaarted receiver, it will not appear luminous, like the 

 «tlier wliieh remains in tlie open air. 



7. Set a bell upon acufliion under a receiver on the pump 

 plate ; and fliake the pump fo as to make the clapper 

 ilrike againfl the bell, and the found will be di(lin6\ly 

 heard ; but exhaull the receiver, and if the clapper be 

 made to ftrike with great foivc againll the bell, it will make 

 no audible found ; hence it is inferred, that air is neccflaiy 

 for the propagation of found. 



AiR-sHAFTs, among Miners, denote holes or (hafts let 

 down from the open air to meet the adits, and furnifli frefli 

 air. The damps, want, and impurity of air, which occu"-, 

 when adits are wrought 30 or 40 fathoms long, make it 

 neeeirai-y to let down alr-fliafts, in order to give the air 

 hberty to play through the whole work, and thus difchargc 

 bad vapours, and furmfli good air for refpiration : the ex- 

 pence of which (hafts, in regard of their vail depths, hard- 

 nefs of the rock, drawing of water, &c. fometimes equals, 

 nay exceeds, the ordinaiy charge of the whole adit. 



Sir Robert Murray defcribes a method, ufed in the coal- 

 mines at Liege, of working mines without air-fliafts. Phil. 

 Tranf. N"^ 5. 



When tlie miners at Mendip have funk a groove, they 

 will not be at the charge of an air-fliaft, till they come at 

 ore ; and for the fupply of air have boxes of elm exaftly 

 clofed, of about fix inches in the clear, by which they 

 carry it down about twenty fathoms. They cut a trench 

 at a little dillance from the top of the groove, covering it 

 with turf and rods difpofed to receive the pipe, which they 

 contrive to come in lide-ways to their groove, four feet from 

 the top ; which carries down the air to a great depth. 

 When tliey come at ore, and need an air-fhaft, they link it 

 four or five fathoms diftant, according to the convenience 

 of the breadth, and of the fame fafliion with the groove, 

 to draw as well ore as air. Phil. Tranf. N° yj. See 

 Mining. 



Air-threads of fpiJers. See Threads. 

 AiR-TRUN'K, a fiinple contrivance by Dr. Hales, for 

 preventing the ftagnation of putrid effluvia, and purifying 

 the air in jails and clofe rooms ; which eonfifts of a fquarc 

 trunk open at both ends, one of which is lixcd in the cieling 

 and the other is extended to a conliderable height above 

 tlie roof. The noxious effluvia, afcending to the top of the 

 room, efcape by this trunk. Some of thefe have been nine 

 and others fix inches in the clear ; but whatever be their 

 diameter, their length fliould be proportionable, in order to 

 promote the afcent of the vapour. As the prefTurc of 

 fluids, and confequcntly of the air, correfponds to their 

 perpendicular altitude, the longer thefe trunks are, fo much 

 the greater will be the difference between columns of air 

 prefting at the bottom and at the top ; and of eourfe fa 

 much the greater will be their eft'eft. See Ventilator. • 

 Air-vessel, \n HyJiaulics, is a name given to thofe me* 

 talline cylindei-s, which are placed between the two forcing- 

 pumps in the improved riRE-fw^/n.-j-. The water is injefted 

 by the aftionof th'^ piftons through two pipes, with valves, 

 into this vclfel ; the air prevloufly contained in it will be 

 compreflcd by the water, in proportion to the quantity aid- 

 mitted, and by its fpring force the water into a pipe, which 

 will difcharge a conllant and equal ftream ; whereas in the 

 common fquirting engine, the ftream is difctrntinued between 

 the feveral ftrokts. Other water-engines are furnifhed witU 

 veflels of this kind. 



AiR-vrffi-fs, in Botany, are certain canals, or dufts, where- 

 by a kind of abforption and refpiration is effefted in vege- 

 table bodies. 



Air-vefl'els havt been dl(lingui(licd from sA?-veff?ls ; the 

 fonnev being fuppoled to correfpond to the trachea, and 



3 Q_2 lungs 



