CATALOGUE. JIETEOKOLOGY, IiyCROMETERS. 



471 



These experiments agree well enough with the formula, 

 as far as their evidence goes, to make us adopt the ex- 

 pression 2 Z — llzzn, and I'^l^.^ (l — n.) Upon this ground 



the depression required for producing deposition may. be 

 calculated as in the table. 



Novf, since the mean height of Deluc's hygrometer is 

 tbove 70 in London, and thepoint of deposition cannot, in 

 general, be supposed to be less than 5 or 6 degrees below 

 the mean temperature, it appears that the formula for re- 

 ducing Deluc's to the natural scale requires sonie alteration; 



we may therefore make it 1.5/ — 5 U^ln, Sind Izzi.s 



•1/ (s.25 — n) whence we obtain the numbers in the last co- 

 lumn, where 70° corresponds to a depression of about 6", 

 required to procure a deposition. 



To raise the natural hygrometer 1°, or .01, the tempera- 

 ture must be depressed as many degrees as the quotient of 

 the temperature reckoned froni — 133° divided by700»: 

 thus, at 62°, if n= .50, i?J =: .56°, if n = .30, .28°, which 

 is nearly the difference of the depression corresponding to 

 the different degrees of the hygrometer. For Deluc's scale 

 the inequality is greater ; a depression of .44° being required 

 to raise the hygrometer from 50° to 51", and of only .1 4° to 

 laise it from 99° to 100". Saussure's experiments on this 

 subject, must, as DaUon observes, have been affected by 

 considerable inaccuracy. 



On the specific gravity of Air. 

 We may now attempt the solution of a problem, which is 

 of some practical importance; that is, to determine the spe- 

 cific gravity of common air for any given state of the baro- 

 meter, thermometer, and hygrometer. Let the height of 

 the barometer, the temperature of the mercury being re- 

 duced, if necessary, to 32°, be b, that of Fahrenheit's ther- 

 mometer, reckoned from 32°,/, the height of the point of 

 deposition of raoismre above 32° g, the specific gravity of 

 vapour being to that of air as 1 toti.and thespecific gravity of 

 dry air to that of water, at 32°, when the barometer stands 

 at 30, as 1 to a. Now the space occupied by the vapour 



"'11 1^' — > " being -is+.oo? f + .00010 B*> and the 



quantity of matter contained in it • , at 32" 



30av 



the re- 



maining space being 1 — will contain — '—( 1 -) 



30 30a ^ 30/ 



and the sum of these will be ~ — l\ ) ^ 



iOa \ 30 / 30 a j; 



which must be reduced in the ratio of 1 to 1 -f .0021 yi 



and will become ■ • ( .( 1 ) -4 |. 



1 + .0021/ \ 30 a V 30/ 3000/ 



And if we employ Deluc's hygrometer, on the most proba- 

 ble supposition of the nature of its scale, its height in de- 

 grees divided by 100 being /, we have e^ (r. 5/ — ill). 

 (.18 -t- .O07/-t--000l9/'). By comparing this formula with 

 Sir G.Shuckburgh's experiments, from which thespecific gra- 

 vity appears tohave been -|j when b was 30,/z:2 1 ,and I proba- 

 bly about 80°, taking 11— 1 .4 from Saussure's experiment, in 

 which 7. sE. grains occupied j'jofacubicfootjor perhaps alittle 

 more, at 66", we have azi770; and the formula becomes 



(i *- i '" +i ' ) = ^"' + '■""•^^ '""'="'' °f 



which, we may employ, in common cases, l:781-f-i.C4y 

 + .05 f -J- .0014 i'S which gives, in the circumstances of 



the experiment, where g is about is. 



816.8 



At 55°, with 



vapour deposited at 50", the specific gravity becomes -\^, 

 which may be assumed as the mean at the sea side for . 

 England and France : at 80°, with vapour deposited at 62°, 

 the barometer being at 28 ; ^ : at 2°, when the air is 

 dry, and the barometer at 31, ^, ; so that the greatest pos- 

 sible variation at anyone place, is nearly in the ratio of 3 to 4 : 

 and the height of the atmosphere, supposed homogeneous, is 

 820X2.5X13.6=27880 feet. The weight of a cubic inch 

 of air will be .308 grains. This estimation of the air's va- 

 rying density is applicable both to barometrical measure- 

 ments, and to atmospherical refraction. 

 3 



