PEEPABING THE PLANTING AREA 



85 



Therefore the height of the drain above the datum-line 

 at B 50 yards from A will be 10-00 X -60=10.60. The 

 level of the ground at B is 12-60. The difference between 

 12-60 and 10-60=2 feet=the depth of the drain. 



C is 50 yards from B, so -60 must be added to 10-60, 

 making 11-20. 



Ground level at C=13-70. Difference between 13-70 

 and 11-20=2-50=2 feet 6 inches=depth of drain at C. 



A section of the drain is made thus : 



The datum-line is drawn corresponding with the length 

 of the drain on a scale of, say, 1 inch per 50 yards. 



The distances of each point must be marked off on this 

 line and perpendiculars set up at each point (the scale of 

 depths may be 1 inch per 10 feet), the figures for each 

 point marked off on them, and the points of each line 

 joined. 



Level of the ground 

 above datum-line . . 



Level of the drain above 

 datum-line 



Depth of drain 



^ fOyds . Gradient 



Fio. 12. 



