86 ENGINEERING FOR LAND DRAINAGE. 



places limitation upon grades which cannot always be 

 changed by artificial work. While grades of 2, 3, or 4 

 inches per 100 feet may be desired as a minimum, they 

 cannot be obtained in very many tracts of land which 

 may be successfully drained. Grades as low as |- inch 

 per 100 feet are in successful operation, giving good 

 results on thousands of acres of land. Mains laid for a 

 distance on a level are sometimes used with success, 

 the flow through such mains depending upon the head 

 given by the free water in loose soils, and by lateral 

 drains having a grade greater than that of the main. 

 The lack of fall must be offset by increased size of 

 drains, and by the greatest degree of accuracy in their 

 construction. 



A uniform grade is the simplest. Having decided 

 upon the depth of drain at the outlet and also at the 

 upper end, as for instance 3 feet, subtract this from 

 the elevation of each of these points and obtain the 

 elevation of the grade line at the outlet and upper end 

 respectively. The difference of the elevation is the 

 available fall, which, divided by the number of stations, 

 gives the fall per station. 



Starting with the elevation of the grade line at the 

 outlet, add the grade per station to this elevation to 

 obtain the elevation of the grade line for the next suc- 

 ceeding one, and so continue to add the increments. 

 For intermediate stations use a proportional part of the 

 grade per station. 



The cut or depth of drain is found by taking the dif- 

 ference between grade and surface elevations. The 

 last column of figures under the head ** Cut " is the ob- 

 ject sought, and is that for which all other work so far 



