DISADVANTAGES FROM UNFAVOURABLE SOIL, ETC. 49 



at the mouth of the ditch, in order to hinder the ingress of 

 the water whilst it attains a higher level than that in the 

 ditch. 



Accurate levelling should take place before any extensive 

 drainage work is undertaken, and is also necessary even in smaller 

 cases when the difference in level is slight ; but otherwise, for 

 unimportant drainage works with a considerable difference 

 between the levels, the ditches may be marked off with the 

 naked eye, or with the use of the most simple instruments. 



The leading off of the water from a large area generally 

 takes place by means of small feeders, which conduct the water 

 that collects in them into side drains, and these again lead it into 

 the main drain. 



The main drain is best formed along the line connecting the 

 lowest points, and thence conducted at once in the direction of 

 the strongest gradient towards the pond or water-course into 

 which it is to debouch. If the difference in level is so great 

 that any scouring out of the ditch or damage to its walls may be 

 apprehended, owing to the velocity in the flow of the water being 

 drained off, the danger can be obviated either by a terrace-like 

 arrangement of the bottom of the ditch, in which case some sort of 

 paving is requisite, or, if possible, by prolonging the course of the 

 drain by sinuosities. Its width and depth depend on the quantity 

 of water to be drained off, the depth of ditches and drains being 

 reduced merely to what is absolutely necessary, not only to lessen 

 the expense, but also on account of the usual desirability of drain- 

 ing the upper layers of the soil only ; for anything like a thorough 

 drainage of the soil is very apt to exert an injurious influence on 

 the vegetation. The width of the ditches and drains depends on 

 their depth and their scarp, which can be steeper in proportion 

 to the tenacity of the soil ; they must be the more sloping, in pro- 

 portion as the soil is loose, and apt to get washed away or to 

 fall in. 



The side-ditches may debouch into the main drain either at right 

 angles or at an acute angle, the latter arrangement being prefer- 

 able when they have any considerable gradient, and are likely 

 to conduct large quantities of water into the main drain ; for in 

 that case, if debouching at right angles, the water would be apt to 

 scour out or undermine the opposite side of the main drain. 

 Width, depth, and distance between the side-ditches, as well as in 



D 



