154? Royal Society : — 



Hence, up to about .3000 feet, the temperature of the air would 

 be found to be higher than the dew-point, and the supposed tensions 

 might of course exist. But the temperature of the air, it will be 

 seen, diminishes much more rapidly as we ascend than that of the 

 dew-point ; and the former will therefore soon fall below the latter. 

 Thus at -1000 feet, the air being at G8°, the theory demands vapour, 

 with a dew-point of 69°*9, which is impossible; for any vapour, in 

 excess of that corresponding to the air temperature 0*8°, would be 

 instantly precipitated. In like manner it might be shown that, under 

 all conceivable conditions of heat or cold, and of damp or dryness at 

 the surface of the earth, we could always ascend to a height where 

 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 con- 

 fined vessels, arises the fallacy of the theory I have been considering. 



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- 

 sphere, as compared to that at the surface (which is in each case as- 

 sumed to be represented by TO), supposing the air to be everywhere 

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



