CH. V 



PRELIMINARY OPERATIONS 157 



holes, 1, 2, 3, 4, etc., are dug to the same depth as the 

 drain. The lower extremity of the drain is then 

 blocked, and water is allowed to collect within it. 

 When the drain is' nearly full, say up to the line cc, it 

 w T ill be found that the holes, 1, 2, 3, 4, etc., are full to 

 the same level. The drain is then opened and allowed 

 to empty itself, and after a short time the level of the 

 water in the holes will be examined. It will be 

 found that the nearer to the drain the less water will 

 remain in the holes. One of these will be empty down 

 to the level bb. This hole (No. 3 in the figure) will 

 determine the distance between the drains, for it will 

 be half-way between the drain made at a and the next. 

 The trace for the drains should be laid out with the 

 spirit-level, unless the area is very small, in which case 

 it can be laid out with the eye. 



The drains which are cut on hill-sides to prevent 

 erosion are laid out on a similar principle to those which 

 are made to drain a water-logged soil. The main and 

 receiving drains are cut along lines of steepest descent, 

 while the collecting drains are laid obliquely to the 

 slope. These are usually given a steeper gradient 

 than is given to those in swampy soil, especially in 

 places where there are torrential falls of rain, as it is 

 necessary to have the water led off quickly, otherwise 

 the drains will soon overflow. In Ceylon, where these 

 drains are much used on tea and other estates, they are 

 often given a fall of 1 in 15, their width being 18 

 inches (45 cm.) and their depth 12 inches (30 cm.). 

 In softer soil, however, their width at the top should 

 be greater. These drains can be laid out with a level 

 or with some kind of clinometer or " road tracer." 



As regards the distance between the drains, no hard- 

 and-fast rule can be made. It will depend on the 

 amount of clearing made, on the slope, on the com- 

 position of the soil, and on whether the rain comes in 

 moderate, evenly distributed showers, or in sudden, 

 violent downpours. 



The drains are cut during the dry weather, usually 



