IRRIGATION TOOLS AND DEVICES 



429 



would produce a velocity of 

 2 feet a second, while in a 

 ditch capable of carrying about 

 24 second-feet, the fall re- 

 quired to give the same veloc- 

 ity would be only M mc h to 

 the rod. The nature of the 

 soil determines chiefly the 

 grade that may be adopted 

 for farm ditches. In fine sand 

 or silt a mean velocity of 1 



FIG. 142. Dammer. 



FIQ. 143. Board 



da. in. 



