DRAINAGE. 1G3 



cised by the surrounding soil upon the sides of the 

 ditches, danger exists of the embankments sliding down 

 and, ere long, to fill up the ditches. For this reason the 

 sides of the ditches have to slope at an angle of about 

 35° from the horizontal, whether the soil may consist 

 of sand, clay or loam. For, although the soil of the 

 last-named kind stands for a while firm, if the sides of 

 a ditch are all perpendicular, the drying effect of 

 the air upon the sides is such as to soon remove every 

 tenacity out of the soil, and make it crumble down into 

 the ditch. 



The slope of ditches in moors with tenacious soil 

 used to be from 15° to 30° from the perpendicular; with 

 loose soil 45° ; and if there is reason to assume that the 

 quantity of water carried through will be very large 

 and rapid, a slope of 60° to 70° is to be given. The 

 base of the ditches is commonly as vi^ide as the ditch is 

 deep, unless the locality, especially the elevation of the 

 soil, renders a change necessary. From the depth, the 

 width of the base and the pitch of the sides it is easy to 

 compute the amount of area occupied by the ditch. It 

 is of great importance to establish a uniform grade of 

 descent in the slope of the ditches. The larger the bulk 

 of the water is which has to pass off, the greater the 

 force of the water in the duct (or conduit) becomes, and 

 the less the gradual fall should be, especially in loose 

 soils ; otherwise the bottom and slopes of the ditches 

 will be exposed to the danger of washing out. In case 

 the natural descent should be too steep and, therefore, 

 encroachments upon the slopes are apprehended, it is 

 necessary to interrupt the descent from time to time, 

 and to establish little rajjids in the ditches at the foot 

 of which a new grade commences. But this operation 

 seldom occurs in the plains; it is aj)plied mostly in hilly 

 regions. 



