450 Irrigation and Drainage 



to crops, and for this reason much smaller tiles are 

 permissible than would otherwise be the case. It is 

 when the ground water in a cultivated field becomes 

 stagnant or stationary that poisonous principles are 

 developed and suffocation for lack of air occurs. 



The greater the gradient or fall of the line of 

 tiles, the greater will be its capacity and the smaller 

 it may be for a given area. The area of cross- 

 section of tiles increases in the ratio of the squares 

 of the diameters ; thus for diameters of tiles of 2, 

 3, 4, 5, 6, 7, 8 and 9 inches, the areas will be 4, 9, 

 16, 25, 36, 49, 64, and 81 square inches, and hence, 

 when running full with the same velocity, their 

 capacities would be in the relations of the second 

 series of numbers. The friction on the walls of the 

 tiles, and the eddies which the joints and other ine- 

 qualities tend to set up, reduce the velocity in the 

 small tiles more than they do in the large ones, 

 hence doubling the diameter of tiles considerably 

 more than makes its capacity four times as great. 



The longer the line of tiles the less it is able to 

 discharge when running full, but just how much the 

 capacity is decreased by the length cannot be simply 

 or accurately stated. 



In speaking of the proper size of mains, C. G. 

 Elliott* states : " For drains not more than 500 feet 

 long, a 2 -inch tile will drain two acres. Lines more 

 than 500 feet long should not be laid of 2 -inch 

 tiles. A 3 -inch tile will drain five acres, and should 

 not ba of greater length than 1,000 feet, A 4 -inch 



* Practical Farm Drainage, p. 57. 



