ON THE THEORY OF RAIN. 405 



descend and accumulate in the lower atmosphere, until at 

 a certain stage of the process clouds and rain were formed. 

 This view of the subject is supported by the well-known 

 factj that the rate of decrease of the temperature of the 

 atmosphere with the height, is greater in rainy than in 

 fine weather ; and it appears likely to lead to a satisfactory 

 explanation of many important atmospherical phenomena. 



With respect to the second question^ it is difficult to 

 offer any plausible conjecture. There can, however, be 

 little doubt that vapour deprived of its latent heat often 

 exists to a considerable extent in the atmosphere without 

 sensibly affecting its transparency ; and, indeed, it often 

 happens that the atmosphere is unusually transparent 

 immediately before, and even during showers of rain, and 

 when, therefore, it is strongly charged with vapour in this 

 peculiar state. 



Notwithstanding the cooling by radiation of the upper 

 portion of a warm, moist wind, it is very probable that at 

 a station on the surface of the earth, the temperature 

 would be found to go on slowly increasing, in conse- 

 quence of the continual arrival of fresh warm air, until 

 the moment when rain began to fall ; the rise would then 

 receive a check, and if the rain continued, a decided fall 

 of temperature would take place. If, therefore, we take a 

 day of rain, the day before and the day after, the differ- 

 ence of the mean temperatures of the day of rain and the 

 day before, ought to be less than that of the mean tem- 

 peratures of the day of rain and the day after. It will be 

 seen that this conclusion is borne out by the following 

 results of the Greenwich and Oxford observations : 





Mean Temp. 



of Day 

 before Rain 



Mean Temp, 

 of Day 

 of Rain 



Mean Temp. 



of Day 



after Rain 



Grreenwieli observations 

 Oxford 



49-25 

 49"50 



49-27 

 49*63 



48-98 

 49"44 



