66 Rainfall, Natural Drainage, 



down the steep slopes of the valleys, but as It gets lower and 

 meets other stieams, and at length attains a more definite cha- 

 racter, it becomes a deeper and steadier body, and moves with a 

 more regulated pace until it reaches the lower plains. It then 

 advances more slowly, and is more easily turned aside by impe- 

 diments. It assumes a serpentine course, winding in and out, and 

 depositing from time to time part of the load of mud, sand, and 

 stone that has been brought into it by its numerous tributaries. 

 Throughout its course it appears to follow exactly the undula- 

 tions of the ground, but a careful search will show that this 

 course is really governed by the geological structure of the 

 country, and by the hardness or softness of rocks which are now 

 out of sight, and covered with a great thickness of transported 

 material. 



A knowledge of the laws that govern the natural surface- 

 drainage of a country is very essential to the agriculturist in 

 laying out a system of drainage that shall fall in with and not 

 oppose nature. In some parts of England, as in the upper part 

 of the valley of the Thames, the lower part of the valley of the 

 Severn, the middle and lower parts of the Trent valley, and 

 many of the streams in Lancashire and Yorkshire, the natural 

 surface drainage is especially instructive and exhibits clear proof 

 of the effect of weather, and the eroding action of running water. 

 Some of the streams that run into the sea on the east coast, between 

 the Norfolk coast and the Humber, exhibit remarkably well the 

 results produced when water runs over perfectly level tracts. 

 Many of our rivers have changed their channels considerably 

 within the historic period. 



That proportion of the whole rainfall of a district that is carried 

 by the streams to the sea must vary extremely. It varies, however, 

 according to certain natural conditions, and admits of modifica- 

 tion by the results of human labour and cultivation. It is calcu- 

 lable in a limited district, and various estimates concerning it, 

 more or less accurate, have been made in our own country and 

 elsewhere. Thus in the west of Lancashire, with a somewhat 

 heavy rainfall amounting to about 34^^ inches, and in a limited 

 area of hard non-absorbent sandstone rock, with very little vege- 

 tation, as much as 26 inches, or fully three-fourths of the total 

 rainfall, has been collected into a reservoir placed to intercept 

 and bring together all the natural channels of escape. This is 

 the result of observations ranging over several years, and taken 

 with great care, but it is no doubt an extreme example of maxi- 

 mum drainage. 



In the case of river systems, where the whole of the land 

 which would naturally fall towards the affluents of a river down 

 to a certain point, is assumed as the drainage area, the pro- 



