258 Mr. Dallon on Meteorology . [April, 



Essays, a few years after this theory of rain had been made 

 known ; as far as I was then acquainted with it from one of the 

 Reviews, it appeared the most plausible of any I had seen ; but 

 on looking at my remarks, it is evident I had not been made 

 acquainted with its distinguishing feature, and that on which its 

 excellence depends ; namely, a higher solvent potver (if it may be 

 so called) in the air, than what is proportionate to the increase 

 of temperature ; and that the precipitation of vapour in the form 

 of clouds and rain is occasioned not by mere cold, but a mixture 

 of comparatively warm and cold air. 



At the time of my publication of the Essay on Rain, Sec. I had 

 a strong bias to the opinion, that the steam or vapour in the 

 atmosphere exists in a state of combination with heat, but with- 

 .'^ out any chemical union with the elements of the atmosphere ; 

 only it is subject to be wafted along mechanically by the great 

 body of the atmosphere in its ordinary currents. This opmion 

 was founded and supported on the authority of the late M. Saus- 

 sure in- part ; he having determined by direct experiment that a 

 •cubic foot of dry air of the temperature of 66° would imbibe 12 

 grains of water for its saturation. Now, from experiments on the 

 boiling of water in vacuo, I was persuaded that this quantity of 

 A'apour was nearly what would fill a cubic foot of empty space, in 

 the temperature of 66° ; and, by analogy, I concluded that the 

 quantity of steam necessary to saturate any given volume of air 

 at any temperature was the same that would be requisite to fill 

 ari equal void space at the same temperature. This reasoning 

 was of course hypothetical at that time, and unsupported by any 

 direct experiment. 



In 1801 a series of essays of mine were read before the 

 Society, and subseque^itly published in the fifth volume of 

 Memoirs ; one object of experimental inquiry Avas, whether 

 steam of any kind was the same in quantity in air and in a 

 vacuum, all other circumstances being the same. The result was 

 decidedly for the affirmative. 



Another object was to ascertain the true force of steam in all 

 atmospheric temperatures. This was clearly proved to be pro- 

 gressively increasing with the temperature, as Dr. Hutton had 

 .rightly conjectured. Indeed with a slight modification of the 

 thermometrical scale, the temperature is an (irit/imefica/ Tprogres- 

 sion, and the force of steam a geometrical one. Hence the curve 

 showing the force of steam is what mathematicians call the 

 logarit/niiic, one remarkably convex to its axis. 



The cause of rain, therefore, is now, I consider, no longer an 

 object of doubt. If two masses of air of unequal temperatures, 

 by the ordinary currents of the winds, are intermixed, when 

 saturated w itli vapour, a precipitation ensues. If the masses are 

 under saturation, then less precipitation takes place, or none at 

 all, according to the degree. Also the warmer the air, the 

 greater is the quantity of vapour precipitated in like circum- 



