86 FORESTRY WOBK 



When the proper course of the main drain is found it 

 should be marked out and the figures respecting the depth 

 given to the man in charge. A good average depth for 

 the main drain is 2 feet 6 inches, with a top width of 

 2 feet 6 inches and a bottom width of 10 inches. The 

 average angle of a slope for the sides will be about 

 72 degrees, which is enough for stifl soils. In hghter soUs 

 the slope should be greater — viz., from 60 to 65. The 

 small drains or feeders should be 6 inches shallower, with 

 a top width of 2 feet in stiff soils to 2 feet 6 inches in sandy 

 soils. A bottom width of 9 inches allows sufficient room 

 for a spade or shovel to be passed along easily when 

 cleaning them. 



When the quantity of water is greater, it is better to 

 widen the main drain rather than deepen it ; for although 

 water will run better in a narrow drain, it is apt to cut 

 into the sides and break them down. The gradient should 

 not be more than 1 in 300, if possible to avoid it. 



A wide drain is more easily kept clean than a deep one, 

 and it is often a mistake to lower the water-level in the 

 soil too much. Subsidiary main or side drains must be 

 cut from the main to any other low-lying parts and the 

 feeders cut from these. 



On ground where the same slope is maintained these 

 may be made parallel, but on very uneven ground this 

 cannot be done if a low gradient is to be preserved. Too 

 high a gradient will cause the water to rush downhill, 

 wearing the sides of the drains away and causing the 

 banks to break down and block them. 



The distance between the feeders varies according to 



