A I R 



A I R 



into the cavity of tlieir alKiomeii, fervcs to make them pre- 

 ponderate the water, and dtfcend ; wlieii they would aiceiid 

 again, a comprcfnon is inadc by the mufcltu of their abdo- 

 men, and the water forced out again, at leall io much as 

 Suffices to give the degree of levity wanted. Ray's Wifd. 

 of God, part ii. p. 346. 



A.i'Si-bljdders, or j\\K-ba^s, in Ornitholn^y, are cells or 

 receptacles of air in the bodies of birds, which communicate 

 with the lungs, and which are loJijcd both among the 

 flediy parts, and in the hollow bones of thefe animals. Mr. 

 John Hunter, F. R. S. (Philof. Tranf. vol. Ixiv. part i. 

 .p. 205, &c.) has publiflicd fome curious obfervations on this 

 fubj-.'ift:, leading to a more particular inquiry into the I'.nal 

 caufe of this peculiarity in the Rrufture of birds, whirh had 

 not been fnfilcien'ly noticed and examined by anatomills and 

 natural hillorians. He informs us, that the air-cells, which 

 are found in the foft parts of birds, have no communication 

 with the cavity of the common cellular membrane of the 

 body ; fome of them communicate immediately with one 

 another ; and all of them may be faid to have a communi- 

 cation together, by means oi the lungs as a common centre. 

 Some of them are placed in larger cavities, fuch as the ab- 

 domen ; othe/s are fo lodged in the interfticts of parts, that 

 they would, at firll, appear to be the common conneiSling 

 membrane, as about the bieaft, axilla, &c. The bones 

 which receive air are of two kinds ; fome, as the fternum, 

 ribs, and vertebras, have their intemal fubftance divided into 

 innumerable cells; whilft others, as the os humeri and the 

 OS femoris, are hollov.-ed out into one larj^e canal. Thefe 

 bones may be diftinguifhed from thofe tliat do not receive 

 air by their lefs fpecihc gravity ; by being Itfs vafculiir, and 

 confequently whiter, thaji others ; by containing little or no 

 oil ; by having no marrow nor bloody pulpv fubftance, 

 even in their cells ; by not being, in general, fo hard and 

 firm as other bones ; and by the eafe with wliich the paffage 

 that conveys the air into the bones may be perceived. 



The meehanifm by which the lungs are adapted for com- 

 municating air to the above-mentioned parts confifts prin- 

 cipally in the attachment of the lungs to ti;e diaphragm and 

 their connexion with the ribs and fides of the vertebrx. 

 Thefe adhefions are peculiar to this tribe of animals. For 

 the communication of the air from the lungs to the other 

 parts the diaphragm is perforated with large holes, which 

 open a free paflage between the cells of the lungs and the 

 abdomen : and to each of thele perforations is annexed a 

 diftinft membranous bag, which is veiy thin and tranfparent, 

 ;ind thefe bags, which receive the air, are extended over the 

 whole abdomen. The lungs open at their anterior part, or 

 towards the llernum, into membranous cells, which lie upon 

 the fides of the pericardium, and communicate with the 

 cells of the fternum. The fuperior part of the lungs opens 

 into large cells of a loofe net-work, through which the 

 trachea, oefophagus, and large vefTcls, in their way from and 

 to the heart pafs. When thefe cells are dillcnded with air, 

 the fize of the part where they lie is enlarged, and this dif- 

 tenfion indicates paflion, as appears in the turkey-cock, 

 ■pouting-pigeon, &c. and in the breaft of a goofe, when it 

 cackles. Thefe cells communicate with others in the axilla, 

 imder the large peftoral miifcle, &c. ; and thofe again with 

 the cavity of the os humeri by fmall openings in the hollow 

 furface, near the head of that bone. The pofterior edges 

 of the lungs, which lie on the fides of the fpine, and projeft 

 backwards between the ribs, open into the cells of the 

 bodies of the vertebrx, ribs, canal of the medulla fpinalis, 

 facrum, and other bones of the pelvis ; and thus the air finds 

 .a palfage to the cavity of the thigh-bone. This fupply of 

 the bones with air is not wholly by means of the lungs ; for 



Vol.. L 



the cells of the bones of the head, in fome birds, arc filled 

 with it, as in the eafe of the owl, which has the diploe 

 between the two plates of the fcull cellular, and capable ot 

 admitting a confiderable quantity of air from the Eullachiati 

 tube. Mr. Hunter, in his conjeftures concerning the ufe 

 of thefe cavities, concluded at lalt that they were to be 

 confidered as appendages to the lungs ; and that they anfwer 

 the fame puipoi'e with the two bags that form the lungs of 

 amphibious animals which are continued down througli the 

 belly, the upper part of which performs the oflice of ref- 

 pirafion, and the remainder of which is a refervoir of air. In 

 confequcnce of this conformation thefe animals can breatlic 

 lefs frequently than others ; and birds are thus aided in their 

 flight, which muft render frequency of refpiration diiucuk, 

 and a refervoir of air Angularly ufeful. He farther fuggelis 

 that this conilruftion of the rcfpiratory organs may alJill. 

 birds in finging ; and that the long continuance of the fonjj 

 of a canai-y-bird between it;; breathings may be owing to 

 tliis caufe. Dr. Latham (Linnxan Tranfaftions, vol. iv. p. 

 94.) queries, whether this conilruftion may not enable fome 

 birds to dive and flay for a confiderable tiir.e under water. 

 Thefe air veficles, according to Dr. Monro, are of con- 

 fiderable ufe in two relptfts. They fen-e, by diilending the 

 huigs with air, to render the bodies of birds fpecifically light, 

 and alfo to Ib-aiten the trachea arteria, and thus return the 

 air ; and moreover to i'upply the place of a mufcular 

 diaphragm and fi;rong abdominal mulcles, and thus to aid 

 the exclufion of the eggs and fxccs. 



A-\v.-chimibcr, is a name given by Dr. Henfliaw to a room 

 contrived for obtaining the benefit of change of air, witiiout 

 going out of the houfe. According to his whimfical pro- 

 pofal, it is to be twelve fett fquare, and air-tight, furniflied 

 \vith a ver)- large pair of organ bellows, to or from which 

 air is to be conveyed through the wall by a copper pipe, 

 with valves for opening inwardly and outwardly as occafion 

 (hall require. With thefe bellows, the air in the room ii 

 eitlier to be condenfed and made heavier, by forcing air in, 

 or lighter and rarer by conveying air out of the room. See 

 his Aero-Chnlinos, or Phil. Tranf. N° 133. See Venti- 

 LATOR and Bio'zi'ing-W Ht.)iL. 



AiR-giw, or Il^ini/-i;iii!, a machine which fervcs to explode 

 bullets, and other fliot, with great violence, by the expan- 

 five force of the air. This fort of implement, charged with 

 air, has an effeft fcarcely inferior to that of a common fire- 

 arm charged with gun-powder ; but it difchargcs itfelf with 

 a mucii lefs report ; and it is this which probably gave oc- 

 cafion to the fable of white gun-powder. The firll account 

 of an air-gun, that has been noticed, is found in the FJemens 

 d'Artillerie of David Rivaut, who was preceptor to Louis 

 XHL of France. He afcribes the invention to one Marin, 

 a burgher of Lifieux, who prefented one to Henry IV. 



The common air-gim [Pncumr.t'ics, Plate iii. Jig. 14.) 

 is made of brafs, and has two barrels : the infide ban-el K 

 A of a fmall bore, from which the bullets are fliot, and a 

 larger barrel E C D R on the outfide of it. In the flock 

 of the gun there is a iyringe, S M N P, whofe rod M 

 draws out to take in air, and pifton SN drives the air before 

 it through the valve E P into the cavity between the two 

 ban-els. The ball K is put down into its place in the fmall 

 barrel with the rammer, as in another gun. There is another 

 valve at S L, which, being opened by the trigger O, per- 

 mits the air to come behind the bullet, fo as to drive it out 

 with great force. If this valve be opened and ftiut fuddenly, 

 one charge of condenfed air may make fevcral difcharges of 

 bullets ; becaufe only part of the injefted air will go out 

 at a time ; and a new bullet may be put into the place K ; 

 but if the wliole air be difcharged on a fingle bullet, the 



3O bai 



