246 IRRIGATION. 



tained by adding the width of the surface to that of the 

 bottom, dividing by two, and multiplying the sum by 

 the depth. This is explained by fig. 129, in which it is 

 seen that the two triangular side sections of the area of 

 the stream are equal to half the central section. If the 

 bottom of the stream came to a point and had no width, 

 then the two halves would be equal to a square with a 

 diameter equal to half the width of the surface. The 

 Telocity is found by floating a cork or piece of light wood 

 upon the stream, and accurately measuring the distance 

 traveled in a minute. The usual estimate of the water 

 required in extensive, continuous irrigation, is one cubic 

 foot or 7 1 ], gallons per second for 100 acres. Other 

 estimates double the quantity of water required, but it is 

 found, as the soil becomes saturated, that much less 

 moisture is required to supply the crops. This smaller 

 estimate may thus safely be taken as the basis of cal- 

 culation for the size of the main canals. 



In calculating the capacity of the canal, or rather the 

 amount of water that will be carried through it, allow- 

 ance must be made for the loss by filtration, and also by 

 evaporation. The total loss from these sometimes 

 amounts to 50 per cent of the water entering the canal, 

 during a flow of a few miles. The Platte Water Canal 

 Company, of Denver, loses 700 inches of water out of a 

 total inflow at the head of their canal of 1,700 inches. 

 Both filtration and evaporation may be reduced to a 

 minimum by giving to the canal the form shown in fig. 

 121, by which the bed is narrowed, and the surface expos- 

 ed to the atmosphere is decreased. 



The measurement of the water supplied should be ac- 

 curate. Generally this is done by means of a gate of given 

 dimensions, fixed in a sluice-way accurately constructed, 

 which is graduated so that it may be raised to a mark 

 designating the quantity of water passing through the 

 opening. The quantity issuing is regulated by the head 



