28 A MANUAL OF FORESTRY. 



action of the water. Where the natural gradient of the 

 ground is insufficient, the base of the drain must be 

 sunk until the necessary fall has been obtained. Where 

 it is too great, the drains must pursue a winding or 

 zig-zag course, so as to reduce the fall ; or the base and 

 sides of the drains must be protected by a facing of 

 stone, or fascines ; in some cases the base may be 

 terraced. 



The maximum gradient which is admissible depends 

 on the nature of the soil. Where the latter is of 

 middling consistency an average fall of 1 per cent, would 

 probably be indicated ; on firm soil it may be greater, 

 on loose soil smaller. 



The depth of the feeders depends on the depth to 

 which it is intended to drain the land; the former 

 must be somewhat greater than the latter. The depth 

 to which the soil requires draining may vary from 18 

 inches to 3 feet, according to the species to be grown. 

 Ash, Hornbeam, and Elm can do with 18 inches, while 

 Scotch Pine, Beech and Silver Fir, prefer soil which is 

 drained to a depth of 3 feet. 



Figs. 13 and 14 illustrate the comparative depths to 

 which a locality is drained, and that of the feeders and 

 leaders. 



The shape of the drains depends on the fall and on 

 the nature of the soil ; the greater the cohesion of the 

 latter, the steeper may be the side slopes. In the 

 case of peat, the sides may be almost perpendicular, 

 in stiff loam they should form an angle of about 45 

 degrees, and they must become more and more slanting, 

 as the proportion of sand in the soil increases. The 

 base of the ditch should be at least as broad as the spade 



