92 THE PRINCIPLES OF 



lay before you the whole subject of drainage, as it would 

 itself take up the whole of the present volume. 



An important question with reference to drainage is 

 the descriptions of soils that require drainage. We shall 

 probably find that most of the land in this country is already 

 naturally drained. All chalk soils, all those light-topped soils 

 which I have described during the last chapters resting upon 

 fairly porous rocks, are naturally drained, but there are many 

 soils which the water cannot possibly percolate through. 

 They offer a resistance to the downward passage of water, 

 owing to their extremely fine state of division. The pores of 

 the soil, or the inner spaces between the particles of the soil, 

 are so minute that the friction or resistance to the downward 

 passage of water is excessive. That is the case in all the 

 great clay beds, which I need not again enumerate. Also in 

 many local beds of clay in which the particles are so fine, and 

 the inner spaces so minute, that there is a serious obstacle 

 placed in the way of the passage of water. The consequence 

 is that it is arrested at and about the surface, and becomes 

 an evil rather than a blessing. Such soils require artificial 

 drainage. 



A second class of soils is wet from position. They are 

 naturally capable of allowing the free percolation of water, 

 but at a greater or less depth beneath the surface there 

 is a more or less impervious substratum, and this impervi- 

 ous substratum causes an accumulation of water below the 

 surface soil which gradually forms what is called a "water 

 table," or, as it is sometimes called, a " reservoir." Water is 

 accumulated upon this tenacious and impervious substratum 

 until it reaches the surface and renders it wet. Notice that 

 there is a great difference between the two cases. One is 

 the case of stiff soil, which by its fine condition offers an 

 absolute resistance to the downward passage of water; the 

 other is that of light soils, which are wet simply because at a 





