46 PHYSIOGRAPHY. [chap. 



In passing across England from east to west, it is found 

 that, as a rule, the rainfall increases. Thus, in the basin of 

 the Thames, the fall varies, from about 24 inches in the 

 eastern part, to between 30 and 40 inches among the western 

 hills, in which the river takes its rise. Looking at the entire 

 basin of the Thames, it may be said that the average rainfall 

 is about 26 inches. Now the area of the basin, as already 

 stated, comprises upwards of 6,000 square miles. Suppose 

 then that we measured out a square space a mile in length 

 on each side, and built upon this base a four-sided tower 

 two and a half miles in height, which we completely filled 

 with fresh-water ; this enormous column would represent the 

 quantity of water which falls upon the surface of the 

 Thames basin in the course of twelve months. And it 

 should be borne in mind that every drop of this water has, 

 at some time, existed in the atmosphere as invisible vapour. 

 In one sense, therefore, it may be truly said that the Thames 

 has its source in the air. 



Passing beyond the western limits of the Thames basin, 

 it is found that the rainfall becomes heavier, until, in the 

 western promontory of Cornwall, it exceeds 40 inches. The 

 maximum, however, is attained among the mountains of 

 Wales and Cumberland ; the wettest spot in England being 

 near Seathwaite in Cumberland, where the average rainfall 

 has been recorded as 165 inches. 



A general view of the rainfall throughout England and 

 Wales is presented by the accompanying rain-map" (Plate 

 IV.), which is reduced from the map published in the 

 sixth report of the Rivers Pollution Commission, for which 

 it was prepared by Mr. G. J. Symons. Such maps are 

 generally called Hyetographicar or Hyetological maps. In 

 the one given here, the differences of rainfall are indicated 

 "^ From ver&s, huetos, rain. 



