30 A MANUAL OF FORESTRY. 



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. In such cases the raised 

 parts form generally parallel ridges, as indicated in 

 Fig. 15. 



Fig. 15. 



a a. Natural level of swampy ground. 

 6, b, b. Ridges on which the trees are to be planted, 



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 (for which see page 100), 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, irrigation may be 

 described as the reverse of draining ; thus in Fig. 12 

 (page 27), d would represent the main supply channel, 

 situated in the highest part of the area, c, c, c the 

 leaders, and #, &, b the distributing trenches. The 

 watering of the land may be caused by overflow from 

 the trenches, or by percolation ; in the latter case the 

 trenches would only be just filled, and no more. 



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

 Vol. ii., Section x. 



