608 



PROTECTION AGAINST SWAMPS. 



When the gradient is slight and much water collects 

 (Fig. 277) in a swamp, besides the main drain A, a few con- 

 fluent drains should be cut, that lead into the main drain. 

 They should join the main drain at a very acute angle, so 

 that the passage of the water in it may not be interrupted, 

 nor its walls undermined by water from the lateral drains. 



ii. DIMENSIONS OF THE DRAINS. 



The breadth of the drains depends on the amount of water 

 in the soil,' the gradient of the ground, and the purpose of the 

 drains. The wetter the soil and the slighter the gradient, 

 the broader must be the ditches ; the leaders being broader 

 than the feeders, and the main drain than the leaders. From 

 1 to 2J feet is a sufficient width for the feeders, and 3 to 

 5 feet for the leaders and main drain. 



The depth of the drains depends on the depth to which the 

 land is to be drained, and on the physical nature of the soil 



Fig. 278. 



and subsoil. Clay requires a greater depth of drainage than 

 loam, and this again than sandy loam. In peaty soils, the 

 drains should go down to the mineral subsoil. In practice, 

 the depth of drains is usually half their width, and draining 

 to too great a depth is prejudicial, costing in excess of the 

 requirements of the case, and depriving the subsoil of reserve 

 water which will be required by the trees during dry weather. 

 The amount of water on the land during summer, not in early 

 spring, should be considered in fixing the depth of drains. 



The slope of the sides of the drains depends on the degree 

 of coherence of the soil, and on the gradient of the drain ; 

 the looser the soil and the steeper the gradient, the gentler 

 must this slope be. In Fig. 278, a is one meter, and the slope 



