RECLAMATION OF THE SOIL. 



133 



and diverted by runniiif:; a ditcli of suitable dimensions 

 along the slope of the hill just above the swampy ground. 

 The ditch intercepts the water and leads it past the swamp. 

 In cases where the spring is situated in the swampy ground 

 itself, only draining can meet the evil. 



h. Drainage. 



A swamp may be drained : — 



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

 courses. This is practicable when the latter pursue a winding 

 course ; in such a case the course may be straightened, so as 



Fiff. 15. 



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 j&lter into the subsoil (Fig. 15). 

 Here a represents the water, h and d permeable 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 



