BAR 



BAR 



Hence wc hsve the mean expanlioii at loco parts of air 

 between 1 1" and 92° =: 2.29. 



The f.)I!owing table fhews the refnlt of general Roy's ex- 

 periments on airs mnch e.s.ceeding the common denfity. 

 The fir;l column contains the deiilities meafured by the 

 inches of mercury which they will fuppoit when they are of 

 the temperature 32'' ; tiie ftcond ci-lmnn iTievvs the cx- 

 panfion of ICOO parts of fuch air by being heated from o 

 to 212° ; and the third column is the mean e.xpanlion of l". 

 Table IX. 



Mean 



General Roy made many experiments on air much below 

 the common denfity, and he found, in f^entrr.l, that their ex- 

 parfibility by heat was analogous to that of air of ordinary 

 denfity, being greateft about the teir.peraturc of 60°. He 

 alfo found, that its expanfibility with heat decrcafed with 

 its denfity ; but he was not able to afcertain the law of 

 gradation. When reduced to al,out -Jth of the denfity of 

 common air, its expanfica was as iollows. 



Table X. 



From the experiments to which we have above referred 

 it appea—., that the expanfibi'ity of air is great ll when thtf 

 air is about it-s ordinary denfity, and that in fmall dcr»fiiie« it 

 is grcatU diir,iiiiO>ed. It appears, upoi. the whole, that 

 there is httle difference in tlie atlual expanfion or claltic force 

 of air, prclfcd with an atmofphere + or — one third part ; 

 yet, when it is rendered extremely rare, its elafticity is won- 

 derfully diminidied. It fhould feem, ind. ed, that the elaflic 

 force of common air is greater than when its denfitv is ccn- 

 fidcrably augmented or d;mini(lied by an addition to or fiiS- 

 traftion from the weight with which it is" loaded ; and lhi3 

 oblcrved difference contradifts the experience of Bovle, 

 Marriotte, &o. It alfo appears that the law of compitflion 

 is altered ; for in the preceding fpecimeii of the rare air half 

 ot the whole exp;infi<in happens about the temperature of 

 99', but in air of ordinary denfity at 105°. As this is the 

 cale, the experimerts of M. Amontons, in the Memoirs of 

 the Academy at Paris for 1702, &c. are not inconfillent 

 with thofe of general Roy. Ainontons found that what- 

 ever was the denfity of the air, at lead in cafes where it was 

 much denfcr than common air, the change of 180° of tem- 

 perature increalcd its clallicity in the fame proportion ; for 

 he found, that the column of mercury, which it fupported, 

 when of the temperature 50, was iucreafed { at the tem- 

 perature 212; and hence he haftily inferred, that its ex- 

 panfibility was inoreafed in the fame proportion : but this is 

 by no means the cafe, unlcfs we are certain that in every 

 temp-rature the elalHcity is proportional to the deuilty ; 

 which ftill remains to be decided. 



From another clafs of experiments made by general Roy, 

 we learn that the ela'lic foive of moift air is greatly fuperior 

 to that of dry air ; and that a very uniform increafino- pro- 

 greffion is perceived to take place from the 7.ero of Fahren- 

 heit, as far as 152° or 172'', and even to the boiling point. 

 From the mean rcfult of thtfe experiments, which are ar- 

 ranged in a table, it appears, tliat the cxpanfion of air, how- 

 ever moift, liaving that nioifture condenfed or feparated from 

 it by cold, differs not fenfibly from that of dry air. Thui 

 the rate for 32° below free-iing 2.22799 '^ nearly the fame 

 as in dry air ; but as foon as the moilhire begins to diffolv- 

 and mix with the air, by the addition of 20^ of heat, the 

 diflerence is perceptible ; for iufiead of 2.46675, the rats 

 for 20" above 32° in Avy air, we have 2.588 for that which 

 is moift. In the next ftep of 20°, the rate for dry air is 

 2.5S09; whereas that for moift is 2.97. In tins manner 

 the progreffion goes on continually iiicreafing, fo as to give 

 7.86854 tor the mean rate on each degree of the 212°, 



Mean cxpanfion 0.7R6 



which is near 3I times the cxpanfion of dry 



And, 



Vol. III. 



laftly, the rate for the 20' between 192° and 212" is twice 

 and one-half the mean rate, and about nine times that which 

 correlponds to the zero of ihe fcale , hut the comp,=.rifon 

 being drawn from the mean of fome particular experiments, 

 as being probably neareft the truth, the tctal cxpanfion of 

 moift will be more than four times that of dry air ; and the 

 rate for the temperature at boiling will be nearly 15 times 

 that which correfponds to the zero of Fahrenhe't. This 

 cireuniftance will probably account for the deviations from 

 the rules eftablifhed for determining heights by the baro- 

 meter, which take place in the province of Qii-to in Peru, 

 aTid at .Spitzbergen, within 10 degrees of the pole. In the 

 former fiti:atioii, which is at a great elevation above the level 

 of the ocean, the heights obtained by thrfe rules fall con- 

 fiderably ftiort of the real hcight.s ; and at the latter place 

 they coufiderably exceed them. Near the furface of the 

 earth there is a greater degree of hu.niidiry and heat in the 

 air than there is in the higher regions of the atmofphere ; 

 and the elafticity or expanfion of the lowermolt fedtirjo of 

 4 T every 



