HUMIDITY OF THE AIR. 157 



The annual fall of rain is not the same in amount on a plain 

 as on a hill; not the same on the hill-top as on the surface 

 sloping away from it : during any one shower it is not the same 

 on the windward side as on the leeward side of a mountain. Nor 

 is the same quantity of rain collected in a gauge placed at some 

 height above the ground, as in another placed level with the 

 surface of the ground. 



On the variation of the quantity of rain according to the height 

 of ground, the only information of value which we can produce 

 from researches in Yorkshire may be obtained by comparing the 

 depths of rain in the elevated and level districts, as given pp. 3-5. 

 From these data it clearly appears that more rain falls in hilly 

 districts, and we may add, in the immediate vicinity of hilly 

 districts, than in level tracts. Elevated ground, in fact, causes 

 deflection of the air-currents, and specially forces upward into 

 cooler regions the air which near the surface of the earth is both 

 warmer and more highly charged with moisture.. Thus carried 

 up to a higher region in which it is expanded by the diminution 

 of gravity, this damp air is cooled, the moisture which ac- 

 companies it cannot all be retained in a vaporous state at the 

 diminished temperature, and some portion is separated and floats 

 about the summits in mist, or falls in rain according to mo- 

 mentary circumstances. 



According to Mr. Miller's important experiments in the moun- 

 tain region which surrounds Sea Fell, it is neither on the summit 

 nor yet at the base of the mountains that the maximum of rain 

 is collected. The greatest quantity seems to be at a height some- 

 what below 2000 feet. As far as can be at present determined, 

 it is on the leeward side, with reference to the west and south- 

 west winds, that the maximum of rain is to be looked for. In 

 Borrowdale, for example, more rain falls than in Wastdale. 



Some experiments, commencing with 1839, at four points in 

 the great Penine ridge, between Glossop and Hathersage, the 

 two extremes being on low ground, and the two middle stations 



