185 



the diminution of temperature would render the progression of the 

 tensions according to the presumed law impossible. We may there- 

 fore conclude generally, that the known diminution of temperature 

 in the atmosphere is incompatible with the existence of so large a 

 quantity of vapour in the upper strata as the theory in question de- 

 mands ; and, consequently, that the tensions observed at the surface 

 are neither dependent on, nor balanced by, the pressure of the vapour 

 in the higher parts of the atmosphere (in the way in which the entire 

 barometric pressure depends on the weight of the whole superincum- 

 bent column of air), for this would be insufficient to produce them. 

 To render an independent vapour atmosphere possible would, indeed, 

 require a fall of temperature in the air of about 1 for 1500 feet, or 

 less than a quarter of that which really takes place. 



It will also follow that, as the tension of vapour at any point ex- 

 ceeds the sum of all the pressures of the vapour above it, it must in 

 part be due to the reaction of the air particles, which must therefore 

 press upon those of vapour, contrary to the supposition with which 

 we started. This is, in fact, equivalent to saying that the air offers 

 a resistance to the diffusion of vapour, instead of having no effect 

 whatever in obstructing it ; and thus from an erroneous assumption, 

 based upon experiments made on very small quantities of air in 

 confined vessels, arises the fallacy of the theory I have been con- 

 sidering. 



I am aware of no systematic observations relative to the actual 

 distribution of vapour in the atmosphere, excepting those made by 

 Dr. Joseph Hooker, and published in his Himalayan Journals*. He 

 found in his journeys in Sikim, which extended to heights of 18,000 

 feet and upwards, that the quantity of vapour was dependent rather 

 on the temperature of the air than on anything else, and that it was, 

 in fact, simply a certain proportion of the maximum quantity that 

 can exist in accordance with the conditions of temperature at any 

 altitude, the relative quantity being pretty nearly constant through- 

 out the whole column. These conclusions of Dr. Hooker are alto- 

 gether corroborated by my own observations. In the annexed Tables 

 I have further illustrated this. In Table II. I have shown for a 

 considerable range of temperature at the earth's surface, the propor- 

 tion of vapour that would be found at various heights in the atmo- 

 * Himalayan Journals, vol. ii. p. 422. 



