fuhJtjTmg in the Atmufphire. 255 



heights, at which barometers would thuid fo low, that the 

 airTs of the fame temperature as the orif^inal experhnent is 

 made at, namely in tins cafe, as it is found at barometer 

 28-77 inches; but fince in reality air at great heights is ge- 

 nerally much colder llian below, to afcertain the real pro- 

 portion of vapour at thole heights it will be necefl'ary to find 

 the quantitv of vapour which a cubic foot of air is capable of 

 holding at that temperature barometer 2877, and the ratio 

 which the fpiantitv or weight of vapour actually found, bears 

 to the complement at that temperature. Then, ado. to find 

 tlie complement of a cubic foot of air at the temperature 

 which prevails at the given barometrical heiglit, and dimi- 

 nifli it in the fame ratio in which it was found dimini(hed 

 below ; and tinalty, diminidi it ftill further in the ratio which 

 that barometrical height demands. x\n example will fully 

 explain this rule. 



Thus Sauflure found, barometer 2S77 and thermometer 

 82^ of Fahr., a cubic foot of air contained about 10 grains 

 of moifture at (iencva. Now the complement of 82*^ is 

 nearlv 15 grains, and the ratio of 10 to 15 is X. Then at 

 Mount Blanc, on the lame hour, the barometer itood at 16" 

 and the thermometer at 26-8 : the complement of a cubic 

 foot of air at this temperature is 5*3 grains, which diminirtied 

 in the ratio of 2 to 3 becomes 3-5 ; and this, furtli-er dimi- 

 nifhed by the ratio which the barometrical height of, 16 

 inches demands, namely 78 = 3-5 X 78 = 27 grains, by 

 obfervalion it was found to be 17 : the difference is only one 

 grain. I'oy. aux Alpcs, § 2007. lk)w the temperature which 

 prevails at thofc great heights may be found, will be fiiowa 

 in the fequel. 



The celebrated Lambert of Berlin (Mem. Berlin 1772,) 

 has alio given an eliimatc of the jiroportion of vapour which 

 prevails in the atmofphere at different barometrical heights, 

 deduced from calculations founded on many fiAions — iuch 

 as that of a homogeneous atmoi'phere, of pure air diftin<St 

 from common air, and an erroneous fyftem of the afcent of 

 heat; yet, as it is much eafier in its application, and in the 

 inlhmc'e juft quoted approaches very near the truth, t have 

 calculated the rel'ults of his fydein, which is nothing more 

 than that the (piantity of vapour at ditferent barometrical 

 heitrhts above the earth is in tiie ratios of the fquares of ihofe 

 heights. By a homogeneous atmofjihere it is probable he 

 meant fuch a Hate of The atmoi'phere as prevails in icrcnc 

 unclouded weather; and it is certainly only in fuch an at- 

 mofphere that any caLalaliun can be inUilutud. 



Table 



