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•contniiis tlie b.ill, and its tknlky ^ill be vfiy liulc 

 altered by expanding l;hroiigli that narrow tube ; eonfc- 

 quently tlie ball will be urged by nearly the fainc uniforin 

 force with that of the lirll inllant : whereas the clallic fluid 

 of inflamed gun -powder, bears a fmall proportion to the 

 barrel of the gun, and by dilating from the fmall portion of 

 it near the but-end into a comparatively large fjiaco, its 

 eladic force will be proportionally weakened, and itr. aClion 

 on the ball in the barrel will become gradually lefs and lefs. 

 Hence it appears, that the air-gun will projcft its ball with 

 a much greater proportional degree of vefocitv than that 

 vhich is above llated ; infomuch that air condenfed ten 

 times will produce a velocity not much inferior t'> that 

 arifuig from the gun-powder. 



However, iii this kind of gun, and in all cafes which re- 

 quire a very confiderable condenlation of air, it will be re- 

 cjuiiite to have the fyrlnge of a fmall bore, I'iz. not exceed- 

 ing half an inch in diameter; becaufe the preQure agaiiifl 

 every fqiiare inch is about 15 pounds, and againil ever)- cir- 

 cular inch about 12 pounds. If, therefore, the fyringe be 

 one inch in diameter, when one atmofphere is injetted, there 

 will be a reiillance of !2 pounds againil the pillon ; when 

 two, of 24 pounds ; and when ten are injefted, tliere will 

 be a force of 120 pounds to overcome ; whereas ten atniof- 

 pheres aft againil the circidar half inch pillon, whofe area 

 is but one-fourth part fo big, with a force but one-fourth 

 as great, viz. 30 pounds ; or 40 atmoiphcres may be in- 

 jefted with fuch a fyringe as well as ten with the other. 

 Uefaguliers's Exp. Phil. vol. ii. p. 398, &c. Martin's Pliil. 

 Brit. vol. ii. p. 1H9, &c. Adams's Left, on Nat. and Exp. 

 Phil, by Jones, vol. i. p. 133. 



AiR-jaciet, a jacket of leather, furnillied with bags or 

 bladders of the fame material, inflated with air, and fer\ing 

 to buoy up the perfon who wears it, and to prevent his 

 finking in water, without any effort of fwimming. Thefe 

 bags communicate with each other, and are filled v\ith air 

 by means of a leathern pipe, having at the end of it a flop- 

 cock, accurately ground, fo as to adinit the injefted air, 

 and, when clofed, to prevent its efcape. The jacket muft 

 be well moiftened with water before the bags are filled : 

 otherwife the air will efcape through the pores of the lea- 

 ther. 



Ai?.-Iamf>, a pneumatic machine, formed by the com- 

 bination of inflammable air and eleftricity, which, by turn- 

 ing a ftop-cock, produces a flame that may be reitrained or 

 continued at pleafure. The contrivance of machines of this 

 fort was iuggefled by the experiments of Mr. Volta, Dr. 

 Ingenhouz, Sec. The air-lamp is now conftrufted in 

 the following manner. A, {P/a/em. Ptmtmatics, fg. 21.) is 

 a glafs jar for containing the inflammable air ; B, an open 

 glafs urn, that contains water, by the preflfure of which the 

 air is forced out of the jar A, through the brafs-pipe a ; 

 C, is the llop-cock, lo perforated, that the water may de- 

 fcend from B into A., and the air pals out through the 

 pipe a. By turning the bar of the llop-cock to an hori- 

 zontal pofition, the communication between the two velTels 

 is clofed, and the paffage of the air obflrufted ; and by 

 turning it into a vertical pofition, the communication is 

 opened. The lower jar, A, is fupplied with inflammable air 

 by means of the bladder, {^fg. 22) ; and two bladders of 

 this kind accompany each lamp. It is ufed in the follow- 

 ing manner : Take off the cover D, from the lamp, and 

 turn the ilop-coek upwards ; then pour as much clear wa- 

 ter into it as will till the vefleU A, up to the pipe a; un- 

 fcrew this pipe, and in its Head fcrew the fmall brafs piece 

 {^Jig- 23.) and to tliis fcrew one of the Hop-cocks and blad- 

 der, [Jig. t2.) ^^'ith the bli;ddcr under one arm, oiv.- hand 



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to th? cock at C, and the other to that of the bladder, 

 open the apertures and pnfs the bladder at the fame time ; 

 and thus the air will be forced upon the water in A, and 

 driven up the glafs pipe through the lube into B, with a 

 bubbling noife. When the veffel, A, is thus charged with 

 air, the llop-cocks are to be turned, fo as to cut off the 

 Communication with the external air. Care muft be taken 

 that the common atmofpheric air does not mix with tli'" 

 intiammable ; for if a mixture of thefe airs were fired, the 

 explollon would be iJr.'it and dangerous. 



Tiic apparatus for lighting this lamp is of the eleftrical 

 kind ; and it is as follows. The mahogany bafis, E E, if 

 a fort of box, about 1 2 inches fquare, and 5 inches deep ; 

 and in this is placed an eleftrophorus, confilling of a refi- 

 nous cake c, and metallic plate (/, wliich by a hinge at it» 

 back, admits of being pulled upwards and let down bv the 

 filkcn flring b, connefted both with it and with the ftop- 

 cock C. When this cake is once excited, its eleftrical 

 cflect upon the metal plate will be continued for a long 

 time. A metallic chain, G, communicates with a v.-ire 

 and ball r, jjafllng through a glafs tube below, in the boK 

 over the plate, and above with a fine wire palFing through a 

 glafs tube. This upper wire is bent lo about ^tii of an 

 inch diftance from the flame-pipe. It is evident tliat when 

 the eleftrophorus in the box is previoufly excited, and the 

 ilop-coek, C, turned, the lllken llring, h, will raife the 

 metallic plate ; and this will give an cletlric fpark to tlic 

 ball and wire above, which will convey it inftantly to the 

 flame-pipe, and inflame the air ilfuing out of the pipe, in 

 confcquence of the preflfure of the water in its defcent into 

 the vefi'el A. The cock, C, being turned back, the flame 

 ceafes ; and turned again, appears ; and will fcrvc to light 

 a candle, matcii, ixc. whenever it may be thought proper. 

 The number of times in which light may be produced will 

 be very great, and will depend on the quantity of the in- 

 flammable air in the veffel A. If tiie cock is not turned 

 back, the flame will continue till the whole of the inflam- 

 mable air is confumed. The light thus produced will be 

 fufficient for reading a large print in the night, or feeing the 

 hour by a watch. When the eleftrophorus is to be excited, 

 the filken firing, b, is unhooked from the plate, and the 

 apparatus taken out of the box ; and the metallic plate is 

 lifted up, whilil, with a filken or diy cat-fliin rubber, you 

 briflcly rub the furface of the rcfinous cake. Al>out 20 

 revolutions in rubbing will be fufficient, fo that the plate will 

 give a fpark to the knuckle about the diflance of an inch ; 

 and by the llrength of the fpark the degree of excitation is 

 to be eilimated. The filken ftring and fmall glafs tubes, 

 through which the wire, G, paflfes, fliould always be very- 

 dry, that the paflTage of the eleftrical fpark may be quite 

 perfeft. The whole length of this apparatus is about 22 

 inches ; but it may be made of any dimenfions. Dr. In- 

 genhouz ufed a fmall apparatus, conllrufted upon a fimilar 

 principle, in obtaining light for domeflic purpofcs, both 

 when at home and on his travels. Adams's LecT.urs by 

 Jones, vol. ii. p. 99, &c. 



AiR-pipes, a contrivance invented by Mi;. Sutton, a 

 brewer of London, for clearing the holds of fliips and 

 other dole places of their foul air. The principle upon 

 which this contrivance is founded is well known. It is 

 no other than the rarefying power of heat, which, by 

 caufing a diminution of the deufity of the air in one place, 

 allows that which is in contact, with it to rufh in, and to 

 be fiiccceded by a conllant fupply from remoter parts, 

 till the air becomes every wliere equally elailic. If a 

 tube, then, be laid in the well, hold, or any other part 

 (if a iliip, and the upper part of this tube be fufFicicntly 

 3 U 2 htate4 



