ON THE FORMATION OF CLOUDS. 3y5 



colder than over the intervening valley, where no 

 cloud was to be seen : and I cannot see any reason 

 to think, that the vapour either ascended or de- 

 cended during its transit over the valley. Descend 

 it could not, because its specific gravity, whilst 

 invisible, would not give it that tendency : and if it 

 ascended it would reach a colder region of air, and 

 still keep visible as cloud ; unless, indeed a column 

 of warm air over the valley, reached from the 

 ground to an altitude higher than the stratum of 

 air in which cloud appeared. In that case there 

 would not only be different degrees of temperature 

 in the same stratum, but vertical columns of air 

 of different temperatures also. Abiding by the 

 facts only, it is obvious that the coolest air was 

 over the highest land ; and consequently nearer 

 to that land than to the valley below. But the 

 general opinion amongst Meteorologists is, that 

 the cold air found on mountains, is not due to the 

 mountains themselves, but because their summits 

 are situated in high strata of air, which would be 

 cold whether the mountains were there or not. 

 Perhaps it might be said that the superior radiation 

 of heat from the valley, would warm the air to a 

 higher altitude than that over the hills. Such an 

 explanation would admit of a warm column between 

 two cold ones ; but it would not explain the reason 



