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IV. — Rainfall, Water Supply, and Storage. By Prof. Ansted. 



Part II.* 



11. — Natural Surface-Drainage. 



Of the water falling on the earth as rain part will before long 

 be re-evaporated into the atmosphere, while part will run off in 

 streams, and so ultimately enter the earth, and there be lost sight 

 of, at least for a time. Since the history of the portion evaporated 

 belongs rather to meteorology than to our present inquiry, it is 

 enough to remark here that the proportion evaporated, though 

 differing in different places, is generally at least 14 inches. It 

 depends partly on the form of the ground, partly on the absorbent 

 or non-absorbent character of the rock, and partly on the general 

 climate of the country, but chiefly on the actual state of the 

 atmosphere. With these observations, we may now endeavour 

 to trace nature's provision for the circulation of water on the 

 earth's surface. 



Almost all definite natural divisions of the land consist of a 

 surface, partly mountainous or hilly, partly of plains lying between 

 these elevated tracts, at some height above the sea, and partly 

 of low flats near the sea-level. Each of these affects the natural 

 drainage, and beyond a doubt, the principal details, if not the 

 grand features of the land, are due to the action of water that 

 has fallen on the surface as rain. 



In any country on the higher ground, and among the moun- 

 tains if they exist, the rainfall will be heaviest, and the course 

 of the rain down the steep slopes will be most rapid. The 

 collected waters will form the head-waters of the principal 

 streams. Thus, in England, the sources of some of the largest 

 and most rapid rivers are to be found in the mountains of Wales. 

 But in all countries it is chiefly to the hills and lower undula- 

 tions that we must look for the minute history of the surface- 

 drainage, and the position of the hills governs in effect the drainage 

 of the country. In our own country the higher hills and mountains 

 are on the western side of the island, and we have already seen 

 that the rainfall is heaviest there. It is also an important fact 

 that in England the gentle slopes of the hills, and the longer dis- 

 tances from the hill tops to the sea, are everywhere rather to the 

 east and south than to the west and north. This is favourable to 

 the production of streams large in proportion to the area of the 

 land. 



Thus the natural drainage is clearly defined. The water 

 leaps down the mountain sides, and it runs briskly and rapidly 



* Continued from vol. ii., p. 79. ^ 

 VOL. III. — S. S. F 



