20 



AQUEOUS AGENCIES. 



\ 



C 



\ 



Fig. 12. 



seems so extraordinary a result that, before accepting it, we will try to 

 make it still clearer by an example. 



Let a (Fig. 12) represent a cubic inch of stone, which a current of 

 a certain velocity will just move. Now, the proposition is that, if the 



velocity of the current be doubled, it will 

 move the stone b, sixty-four times as large. 

 That it would do so is evident from the 

 fact that the opposing surface of b is sixteen 

 times as great as that of a, and the moving 

 force would be increased sixteen times from 

 this cause. But the velocity being double, 

 as we have already seen, the force against 

 every square inch of b will now be four times 

 that previously against a, and, therefore, the 

 whole force from these two causes would 

 be 16 X 4 = 64 times as great. But the 

 weight is also sixty-four times as great; 

 therefore, the current would be just able to move it. We may accept 

 it, therefore, as a law, that the transporting power varies as the sixth 

 power of the velocity. If the velocity, therefore, be increased ten 

 times, the transporting power is increased 1,000,000 times. 



We have seen that a current running three feet per second, or about 

 two miles per hour, will move fragments of stone of the size of a hen's 

 egg, or about three ounces' weight. It follows from the above law that 

 a current of ten miles an hour will carry fragments of one and a half 

 ton, and a torrent of twenty miles an hour will carry fragments of 100 

 tons' weight. We can thus easily understand the destructive effects of 

 mountain-torrents when swollen by floods. 



The transporting power of water must not be confounded with its 

 erosive power. The resistance to be overcome in the one case is weight, 

 in the other cohesion ; the latter varies as the square, the former as the 

 sixth power of the velocity. In many cases of removal of slightly coher- 

 ing material the resistance is a mixture of these two resistances, and the 

 power of removing material will vary at some rate between v* and v 6 . 

 There are certain corollaries which follow from the above law : 



A. If a current bearing sediment have its velocity checked by any 

 cause, even in a slight degree, a comparatively large portion of the sedi- 

 ment is immediately deposited. But if, on the other hand, the velocity 

 of a current be increased by any cause, in never so small a degree, it 

 will again take up and carry on materials which it had deposited ; in 

 other words, it will erode its bed and banks ; and these effects are sur- 

 prisingly large on account of the great change in erosive and transport- 

 ing power, with even slight changes of velocity. 



B. Water, whether still or running, has a wonderful power of sorting 



