THE WORK OF RUNNING WATER. 115 



Transportation. 



The second element of erosion is transportation. The transporta- 

 tion of mechanical sediment is to be distinguished from the transpor- 

 tation of materials in solution. In so far as mineral matter is dissolved 

 it becomes, so far as flowage is concerned, a part of the stream. If 

 the quantity dissolved were large it might influence the mobility of 

 the water, but the amount is usually too slight to influence the flow 

 sensibly. 



The sediment transported by a stream is either rolled along its bottom 

 or carried in suspension at some higher level. The coarser materials 

 (gravel and sand) are carried chiefly in the former position, and the 

 finer (silt and mud) largely in the latter. 



Transporting power and velocity. — The transporting power of 

 running water depends on its velocity. The formula expressing the 

 relations between them is as follows: Transporting power, t, varies 

 as the sixth power of velocity, v, (tocv^); that is, doubling the velocity 

 of the stream increases its transporting power 64-fold. Strictly 

 speaking, this means that if a stream of given velocity is just able to 

 move a stone of a given size, a stream with double that velocity will 

 be just able to move a stone of the same shape 64 times as large as 

 the first. This may be graphically illustrated as follows : Let a current 

 be supposed just able to move the cube a (Fig. 97). If the current 

 be doubled, tmce as much Avater mil strike the same surface with 

 twice the force in the same time ; that is, the force exerted on the cube 

 a will be quadrupled. It will, therefore, be able not only to move 

 the one cube, but it will be able to move three other cubes {h, c, and d) 

 besides (Fig. 98). The same current against any other equal surface 

 would also be able to move four small cubes, and there are sixteen such 

 surfaces on the face of the large cube (Fig. 99). It follows that the 

 dimension of the cube which the stream with the doubled velocity can 

 move is four times as great as that of the cube which the original current 

 could move, and the cubical contents of such a cube is 64 times as great 

 as that of the first (64 = 2^) (Fig. 99). Swdft streams, therefore, have 

 enormously greater power of transportation than sluggish ones. It 

 does not necessarily follow that transportation keeps pace with trans- 

 porting power; that depends on the accessibility of materials suitable 

 for transportation. A stream of great transporting power, like the 

 Niagara at its rapids, may carry little sediment, because there is little 

 to be had. 



