34 PRACTICAL FORESTRY. 



should be in the line of lowest ascent from the outfall. 

 There being more water to discharge from the mains 

 than from the minor drains a less fall is required, as 

 the greater the quantity of water the more rapid will 

 be the flow. The main drain should never be against 

 a hill, as the flow would be too rapid, and therefore 

 inefficacious. 



The correct fall, too, will need the careful considera- 

 tion of the forester. Water will find its own level, 

 and, therefore, theoretically, the slightest fall will cause 

 a flow ; but something more than this will be required 

 to carry off with sufficient speed the discharged water, 

 and to overcome the capillary attraction which would 

 otherwise keep the soil, notwithstanding the drainage, 

 in a constantly wet state. Under no circumstance 

 should the fall be less than I in 400, and in clays 

 I in 300 will not be found too great. If there is 

 sufficient natural fall, the drains may be of uniform 

 depth; if not, a fall must be made when cutting the 

 drain. This can be done by increasing the depth at 

 the outlet, and decreasing it gradually as the work 

 proceeds. 



The length of drains will also engage attention. 

 On flat, or fairly level ground, the main drain may be 

 longer than under other circumstances, so that an 

 increased flow may arise from an increased bulk of 

 water. But main drains, as a rule, should be shorter 

 than the minor drains, and the latter should not 

 exceed 10 or 15 chains in length. 



The depth is a matter of great importance, but 

 must be left very much to the discretion of the 

 forester, and in accordance with the soil he is draining. 



