258 



IRRIGATION. 



For instance, it is a well established fact that, while 

 coarse gravel resists a current of two miles per hour, fine 

 gravel is moved by a current of one mile per hour ; ordi 

 nary sandy soil by a current of half a mile per hour, and 

 fine mud is carried away by a current that is almost im 

 perceptible ; so that the abrading action of flowing water 

 depends upon both of these contingencies. When a 

 stream, flowing with sufficient velocity, meets with a soft 

 spot in the bank, it soon excavates a concave outline 

 forming a bend, around which the current sweeps and is 

 deflected with violence against the opposite bank. Cut 

 ting away then begins in a new place, and a second bend 

 is formed here. The effect is continued, the banks are 

 hollowed out in opposite directions, the river, deflected 

 from bank to bank enlarges the bends and lengthens its 

 course. But as the course is lengthened, the fall is re 

 duced, the velocity is decreased, and the destructive 



stream becomes a placid, 

 gentle, harmless current ; 

 incapable of inflicting fur 

 ther injury upon its banks. 

 Besides, in time of floods, 

 the broad stretches be 

 tween the bends are swept 

 over by the spreading 

 stream, and the wide course 

 permits the waters to es 

 cape with rapidity, and 

 without dangerous veloci- 

 Fig. 137. PLAN OF A CUT-OFF. ty. When, therefore, it is 

 determined to reclaim one of these broad stretches, over 

 which the water flows, it must be remembered that it is 

 an effort to return to the former conditions when the 

 river was an active and destructive agent. The work, 

 therefore, requires to be done with care, caution, and 

 skill ; lest a new course of destructive action be caused, 



