134 PRELIMINARY WORKS. 



aside a portion of the area for ditclies does not reduce the 

 returns. 



There may be three kinds of ditches : — 



(1.) The collecting ditches, or feeders ; 



(2.) The connecting ditches, or leaders ; 



(3.) The main drain. 



See Fig. IG. 



The feeders receive the water from the soil and conduct it 

 to the leaders, whence it is taken into the main drain. In 

 some cases the feeders fall directly into the main drain. 



The first step in proceeding to drain an area is to take 

 levels, so as to ascertain accurately the fall of the locality ; 

 if the area is of some extent, a map showing contour lines 

 should be prepared. Unless this is done, mistakes are 

 likely to occur in laying out the system of drains. 



The second step is to lay out the main drain, if possible 

 along the natural line of drainage ; in other words, along the 

 lowest part of the area. According to the natural fall of the 

 land, the gradient of the main drain may have to be in- 

 creased by cuttings, or reduced by giving it a winding course, 

 so as to secure a suitable fall. 



The third step is to lay out a system of feeders, more or 

 less parallel to each other, at a suitable angle with the 

 general slope of the country, and to connect them at an 

 acute angle with the main drain, either directly or through a 

 leader, which is similarly connected with the main drain. 



In laying out such a system of drains, the gradient, 

 depth, shape and distance from each other require careful 

 consideration. 



The gradient, or fall, should be such that the water is 

 carried awa}' with sufficient rapidity, without causing injury 

 to the base and sides of the drains by the scouring action of 

 the water. AVliere the natural gradient of the ground is in- 

 sufficient, the base of the drain must bo sunk until the 

 necessary fall has l)een obtained. A\ here it is too great, tlie 

 drains must pursue a winding or zig-zag course, so as to 



