On the Distribution of Aqueous Vapour in the Atmosphere. 157 



done in the case of the entire atmosphere, by the height of a column 

 of the density observed at the surface. The height of a homogeneous 

 atmosphere of vapour, equivalent to an independent vapour atmo- 

 sphere, on Dal ton's hypothesis would obviously be f- of the height 

 of the homogeneous air atmosphere, that is •§- of 26,250 feet, or about 

 42,000 feet. 



But the vapour actually existing is much less than this. Taking 

 the results of Dr. Hooker's observations, and considering the den- 

 sity'at the surface to be unity, the mean density of the whole vapour 

 below 20,000 feet will readily be calculated to be about "47 ; so that 

 the whole of the vapour up to this height would be equivalent to a 

 homogeneous column of 9460 feet of density TO. Now it may be 

 assumed approximately that the quantity of vapour above 20,000 feet 

 will bear the same relation to the entire quantity, as holds good be- 

 tween the densities at that height and at the surface ; and as we see 

 from the Table that the density at 20,000 feet is -y 1 -^ of what it is at 

 the surface, we may infer that this is the proportion of the vapour 

 above that altitude, the remainder, or ^\, being below it. Conse- 

 quently the whole quantity of vapour, according to Dr. Hooker's 

 observations, would be equivalent to a homogeneous column of ±££- 

 X9460, or 11,260 feet. Using the balloon observations, the height 

 would be rather less than this, viz. 10,050 feet, so that we may infer 

 that the actual pressure of the vapour in the atmosphere is to that 

 represented by the tension at the surface of the earth, as 10,500 to 

 42,000, or as about one to four ; and this ratio would also subsist 

 between the actual pressures and observed tensions at all eleva- 

 tions. 



The problem might otherwise be solved, by comparing the dimi- 

 nution of density as we ascend, according to Daiton's hypothesis, 

 and the observations, as shown by the series of figures in Table I. 

 This diminution, it will be seen, takes place in all the series, approxi- 

 mately in a geometrical ratio, so that the density is reduced nearly in 

 an equal proportion for each 2000 feet of ascent, namely, from TOO 

 to '96, that is by T f-g-, on Daiton's hypothesis ; from TOO to # 84, that 

 is by y 1 -^, according to Dr. Hooker ; and from TOO to '82, that is 

 by i^, according to Mr. Welsh. Now it follows, from an obvious 

 mathematical law, that the entire quantities of vapour in these dif- 

 ferent cases arc inversely proportional to the constant reduction of 

 density ; so that the quantity on Daiton's hypothesis, which is that 

 represented by the observed tension at the surface, is to the quantity 

 according to Dr. Hooker, as sixteen to four, and to the quantity ac- 

 cording to Mr. Welsh, as eighteen to four, a result nearly identical 

 with the former. The subtraction of the observed tension of vapour 

 from the total barometrical pressure, in the hope of obtaining the 

 simple gaseous pressure, must consequently be denounced as an ab- 

 surdity ; and the barometrical pressure thus corrected, as it is called, 

 has no true meaning whatever. 



In conclusion, I would remark that the consideration of the small 

 quantity of vapour that is disseminated in the upper parts of the 

 atmosphere, shows us that inequalities of level on the earth's surface, 

 which are insignificant when viewed in relation to the dimensions of 



