DRAINING. 227 



The collecting or primary drains are necessarily made at the 

 very seat of the swamp, and their number will depend on the area 

 to be drained and the distance apart they are made, if they wero 

 cut at right angles to the lines of steepest descent, i.e., along 

 horizontal contours, they would no doubt collect most water, but, 

 on the other hand, the outflow would be impeded ; while if they 

 were made to follow lines of steepest descent, they would collect 

 very little, as most of the water would drain away p irallel to them 

 through the soil, and flood-waters after heavy rain would rush 

 down and cause erosion. Hence the primary cuts should run 

 somewhat obliquely to horizontal contour lines, so as to collect as 

 much water as possible and yet allow it to run down freely without 

 a tendency to produce erosion. The other two classes of drains 

 should usually follow lines of steepest descent as nearly as practic- 

 able (Fig, 74) ; but when the ground is nearly a dead level, the 

 receiving drains also would best run obliquely to horizontal con- 

 tours, but in the opposite direction to the primary cuts (Fig. 75). 

 If the area to be drained is limited and the slope of the ground 

 pronounced, the net-work of drains may be laid out with the eye 

 alone ; but when the fall of the ground is not easily appreciable, 

 or the area is large and the surface configuration complicated, a 

 complete series of levels with an instrument should be taken and 

 an accurate map of the ground made showing contour lines at not 

 less than 10 feet vertical intervals. On such a map the system of 

 drains could then be easily laid out (Figs. 74 and 75). 



The cross-section of an open drain will depend on the nature of 

 the soil through which it is cut. The ratio of ab to be (Fig. 76), 

 which determines the cant of the sides, should never be less than ^ 

 and may go up as a high as 3. Heyer gives the following 

 figures ; 



For turf or peat ... ... ... ^ to ^ 



clay and stiff loams ... ... 1 



sandy loams ... ... ... 1 



,, sandy soils ... ... ... 2 to 3 



The fall to give to drains will also depend on the nature of the 

 soil, and is a direct function of the velocity of current permitted. 

 Heyer's figures for the maximum fall are 



In loose earth ... ... ... 76 per 1000 



stiff clay ... ... ... 152 



sandy soils ... ... ... 305 



,, gravelly soils ... ... ... 609 



shingly soils ... ... ... 914 



