66 IRRIGATION FARMING. 



cross-sedlional area than any other form, and also 

 keeps the depth of the water in the ditch nearly half 

 its width. The diameter of a pipe we will say is 4 

 feet. Its circumference would be, therefore, 3.1416 

 multiplied by 4, equal to 12.5664 feet; when it is 

 halved lengthwise half the circumference would equal 

 6.2832 feet. 



To get the greatest velocity and quantity of water 

 to flow in a redlangular canal it should be of such form 

 as to cause the water in it to flow exadlly one-half as 

 deep as wide, because the velocity of flow in such a 

 canal is proportional to the square root of the hydraulic 

 mean depth, and the hydraulic mean depth is at its 

 maximum when the breadth of the water is just twice 

 its depth. Fanning says that the variation of velocity, 

 with varying depth, is nearly as the variation of the 

 square root of the hydraulic mean depth. 



Grades and Slopes. — The grade is one of the 

 important things to be considered in canal construc5lion. 

 Ditches running from twenty to over one hundred miles 

 have widths from twenty to eighty feet, some being 

 built with and some without bermes — the grades rang- 

 ing from one foot to seven feet a mile. The steeper 

 grades are not common and are for short distances only. 

 The average grades for main ditches, carrying from two 

 to six feet of water, are from one and one-half to two 

 and three- fourths feet a mile. Such low grades will 

 answer only for the larger ditches carrying large 

 volumes of water, and where the ratio of volume to 

 resistance or fri(5lion on the sides is large. In smaller 

 distributing ditches, where the volume is smaller and 

 the resistance proportionately much greater, a steeper 



