Cu. XV.] 



ACTION OF RAIN. 



3 2 



of a renovating- nature ; in the former are included the erosion 

 of rocks and the transportation of matter to lower levels ; in 

 the renovating- class, the formation of deltas by the influx 

 of sediment, and the shallowing of seas ; but these processes 

 are so intimately related to each other, that it will not always 

 be possible to consider them under their separate heads. 



\ 



* 



i 



1 



') 



' 



i 



1 



• 



■ 





ACTION OF RAIN. 



It is well known that the 



Variations in average rainf* 

 capacity of the atmosphere to absorb aqueous vapour, and 

 hold it in suspension, increases with every increment of 

 temperature. This capacity is also found to augment in a 

 hiffher ratio than the augmentation of the heat. He 



e, as 



was first suggested by the geologist, Dr. Hutton, when two 



v 



olumes of air, of different temperatures, both saturated with 

 moisture, mingle together, clouds and rain are produced, for 



a mean decree of heat having resulted from the union of the 



two moist airs, the excess of va/pour previously held in sus- 

 pension by the warmer of the two is given out, and if it be 



■m 



As the temperature of the atmosphere diminishes gradually 

 from the equator towards the pole, the evaporation of water 



Br 



and the quantity of rain diminish also. According to 

 boldt's computation, the average annual depth of rain at the 

 equator is 96 inches, while in lat. 45° it is only 29 inches, 



But t 



here 



are so 



and in lat. 60° not more than 17 inches, 

 many disturbing causes, that the actual discharge, in any 



this rule. In 



om 



England, for example, where the average fall at London is 

 24^ inches, as ascertained at the Greenwich Observatory, 

 there is such irregularity in some districts, that while at 

 Whitehaven, in Cumberland, there fell in 1849, 32 inches, 

 the quantity of rain in Borrowdale, near Keswick (only 15 

 miles to the westward), was no less than 142 inches ! * As 



a rule the amount of 



r 



mountainous 



t 



Britain is more than double that which falls in the less 



elevated 



regions. 



The mean yearly fall of rain at TJpsala, 



* Miller, Phil. Trans. 1851, p. 155. 



