DRAINAGE. 603 



the present century to drain the peat-bogs in the Hannoverian 

 Harz districts and to plant them with spruce have proved 

 extremely costly, and given such poor results that they have 

 now been abandoned. 



On the above grounds the danger of drying up sources of 

 useful water-supply, and the fear that benefits resulting from 

 forest drainage works may not repay the outlay involved the 

 construction of such works on a large scale is to be depre- 

 cated, and the forester should not as a rule venture beyond 

 draining small local swamps, which may sometimes be ren- 

 dered innocuous or even useful when converted into fish ponds, 

 by excavating them or constructing a darn. 



From a general view, therefore, of the matter, it follows 





Fig. 269. 

 a. Vertical drain, b. Impermeable stratum, c. Permeable stratum. 



that the advantages of drainage are greatest and the dis- 

 advantages least for forests on fairly level ground, whilst the 

 reverse is true for mountain forests. The advantage of 

 draining swamps as regards sanitation, circulation of the 

 air and avoidance of malarial fever, need only be referred 

 to here. 



The following methods are employed in drainage : 



1. Vertical drainage. 



2. Surface-drainage : 



(a) By open ditches. 



(b) Kaiser's method of drainage. 



3. Underground drainage by covered drains : 



(a) By trenches. 



(b) By glazed pipes. 



(c) By ordinary draining tiles. 



