188 



dency for the upper strata to contain a rather higher per-centage 

 with a high than with a low temperature ; a result likewise in ac- 

 cordance with fact, so far as we are able to judge from the compa- 

 rison of Dr. Hooker's Indian observations with those made in En- 

 gland by Mr. Welsh. 



The precise determination of the entire pressure of the vapour 

 thus shown by observation to be suspended in the atmosphere is a 

 matter of some difficulty ; but an approximation may be made to it 

 as follows : 



Let us suppose the weight of the vapour to be measured, as is often 

 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 Dalton'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 -^ of what it is at 

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

 above that altitude, the remainder, or T 8 ^, 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 elevations. 

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

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



