Drainage by Gravitation. 25 



up the centre, with lateral drains on each side at intervals of 



every fifty chains ; ditches branching out of these lateral 



drains would be required at intervals of every ten chains, 



leaving the areas of the fields drained by them twenty-five 



acres each. Taking the main drain as having 26 feet bottom 



at its lower end, with side slopes of 2 to i, a mean depth in 



floods of 8 feetj and a rise in the bottom of 4 inches per mile, 



and 2 inches per mile on the surface, its discharging capacity 



would be as follows : — 



feet. 

 Bottom width , 26 



Width surface of water 58 



Mean width 42 



Mean depth 8 



Area 42-0 x 8'0 .. .. = 33^'0 h.m.d. 



-«* j±± — -. t ' 508 



Contour 17-6 + 17*6 + 26*0 =61 

 Fall per mile 0*166 X 2 = 0*332 



Vs *So8 X 0*332 = 1*352 vel. per second. 

 1*352 X 0*80 = I -0816 „ „ 



Area 336-0 x i*o8i6 = 363*42 cubic feet second. 



The dimensions in the drain at the middle would be 40 feet 

 6 inches top, 10 feet 6 inches bottom, 7 feet 6 inches depth, 

 and discharging capacity 182 cubic feet a second* The quan- 

 tity of drainage due to the rainfall on 10,000 acres, the quantity 

 discharging at this part of the drain would be 180*06 cubic 

 feet per second. At the top end the drain would diminish to 

 I -o bottom, 29*0 top, and 7 feet depth. The twenty latei'al 

 drains would each take the drainage of 1000 acres. The 

 mean dimensions of these drains in the middle of their length 

 would be 2 feet 6 inches bottom, 6 feet 8 inches top, slopes 

 I J to I, mean depth i foot 8 inches, fall in surface 6 inches 

 in mile, and in bottom i foot in mile, discharging capacity 

 S ' 19 cubic feet per second, the constant for friction, &c., 

 being taken at 070. The water due to the rainfall of 



