RECLAMATION OF THE SOIL. 137 



wards cleaning it. For the rest, the width of the drain 

 depends on the body of water which has to be carried away. 



The distance between two successive feeders depends on the 

 permeabihty of the soil, the depth of the ditches, the depth 

 to which the land is to be drained, and the general fall 

 of the locality ; it will accordingly range between 30 and 

 100 feet. 



c. Raising the Level of Fart of tlie Ground. 

 If it is undesirable or impracticable to drain a swampy 

 area, it may, if the water is of moderate depth, nevertheless 

 be rendered useful, by excavating part of it and using the 

 soil so obtained to raise the level of the rest to a sufficient 

 height so as to render the cultivation of trees practicable. 



a — a. Natural level of swampy groiind. 

 h, h, h. Ridges on which the trees are to be planted. 



In such cases the raised parts form generally parallel ridges, 

 as indicated in Fig. 19. 



Such works are expensive, and only species which stand 

 a good deal of moisture can be grown. 



3. Irrigation of Arid Land. 



In temperate Europe irrigation is only employed in 

 nurseries, but in more southern, tropical and arid countries 

 extensive areas are artificially irrigated for the growth of 

 forest trees. The details of irrigation must be studied from 

 a special work on the subject.* In a general way, irriga- 

 tion may be described as the reverse of draining; thus in 

 Fig. IG (page 135), d Avould represent the main supply 



* For instance. " The Roorkee Treatise on Civil Engineering in India." 

 Vol. ii., Section x. 



