TRANSACTIONS OF THE SECTIONS. 411 



the hottest months and near the ground, as compared with 

 those in the winter and on mountains ; finally, it is almost an 

 inevitable consequence from what is known of the gradation of 

 temperature in the atmosphere, that some effect of this kind 

 must necessarily take place. The very common observation 

 of the cooling of the air at the instant of the fall of rain, the 

 fact of small hail or snow whitening the mountains, while the 

 very same precipitations fall as cold rain in the valleys where 

 the dew point may be many degrees above freezing, is enough 

 to prove this. A converse proof of the dependence of the 

 quantity of rain at different heights on the state of the air at 

 those heights, is found in the rarer occurrence of a shower 

 falling from a cloud, but dissolving into the air without reaching 

 the ground. Lastly, I cannot forbear remarking, that this hy- 

 pothesis of augmentation of size of the elementary drops 

 agrees with the result that the increase of quantity of rain for 

 equal lengths of descent is greatest near the ground ; for 

 whether the augmentation of each drop be in proportion to its 

 surface or its bulk, the consequence must be an increasing 

 rate of augmentation of its quantity as it approaches the 

 ground. 



The direct mathematical solution of this problem, now that 

 the laws of cooling and of the distribution of temperature have 

 undergone such repeated scrutiny, may perhaps be attempted 

 with success ; but for the purpose of eliminating the effects of 

 periodical or local modifying causes, it is desirable that obser- 

 vations on the same plan should be instituted at many and di- 

 stant places, — both along the coasts and in the interior, — in the 

 humid atmosphere of Cornwall and in the drier air of the mid- 

 land counties. Always, at least three stations should be chosen, 

 as open as possible, one of them very near to the ground : 

 their relative heights, the mean temperatures, the mean 

 ranges of temperature, and the mean dew point for each 

 month should be ascertained. It would be useful to measure 

 the size of the rain-drops, and, if possible, their own tempe- 

 rature. The height of clouds, according to the plan of Mr. 

 Dalton, in his Meteorological Essays, and the direction and 

 force of wind should be noted, and distinctions made between 

 snow, hail, and rain. Some of these data I have not yet found 

 the means of procuring, partly in consequence of the great 

 labour and time required, and partly from the difficulty of well 

 arranging the experiments themselves. But since it is now 

 ascertained that the general results follow some settled laws, 

 and that the effects may be very well appreciated at moderate 

 heights, I hope not only to procure these, but also several 



