HECLAMATION OF THE SOIL. 



25 



b. Drainage. 



& swamp may be drained : 



(1.) By increasing the gradient (or width) of existing 

 watercourses. This is practicable when the latter pursue 

 a winding course ; in such a case the course may be 

 straightened, so as to increase the velocity of the current. 

 This method is only occasionally applicable. 



(2.) By breaking through an impermeable stratum in 

 the soil, so that the water can filter into the subsoil 



Fig. 11. 



(Fig. 11). Here a represents the water, b and d per- 

 meable strata, and c an impermeable stratum ; e, the 

 channel leading from the swamp to the lower permeable 

 stratum. 



(3.) By constructing a series of ditches, or laying 

 down a series of drainage pipes. The latter are rarely 

 used in forestry, because they are expensive and 

 liable to be choked by the roots of the trees. For 

 the same reasons covered ditches are only rarely 

 employed. The method usually followed consists in 

 the construction of a series of open ditches, because 

 they are comparatively cheap, whilst setting aside a 



