348 



TKANSPOKTING POWEK OF WATER. 



LCh. XV. 



composing the sides and bottom 



A velocity of 



three inches per second at the bottom is ascertained to be 

 sufficient to tear up fine clay, — six inches per second, fine 

 sand, — twelve inches per second fine gravel, — and three feet 

 per second, stones of the size of an egg.* 



It should be borne in mind that running water derives its 

 power of rounding off the angles of hard rocks and of under- 

 mining cliffs, by setting in motion much sand, fine and coarse, 

 and gravel, which it throws against every obstacle lying in its 

 way. The force thus acquired by torrents in mountainous 



i is more easil 

 How the more 



flowing on 



com 



level ground, can remove the 



prodigious burden which is discharged into them by their 

 numerous tributaries, and bv what means thev are enabled 



mass 



removing power 



to the sea ? If they had not this 

 their channels would be annuallv choked 

 up, and the valleys of the lower country, and plains at the 

 base of mountain-chains, would be continuallv strewed over 

 with fragments of rock and sterile sand, 

 prevented by a general law regulating the conduct of running 



But this evil is 



water, 



earns 



a bed of double surface. Nay 



of the principal 



river, after the junction of a tributary, sometimes remains 

 the same as before, or is even lessened. The cause of this 

 apparent paradox was long ago explained by the Italian 

 writers who had studied the confluence of the Po and its 

 feeders in the plains of Lombardy. 



The addition of a smaller river augments the velocity of 

 the main stream, often in the same proportion as it does the 

 quantity of water. The cause of the greater velocity is, first, 

 that after the union of two rivers the water, in place of the 

 friction of four shores, has only that of two to surmount ; 

 2dly, because the main body of the stream being farther 

 distant from the banks, flows on with less interruption ; and 

 lastly, because a greater quantity of water moving more 

 swiftly, digs deeper into the river's bed. By this beautiful 

 adjustment, the water which drains the interior country is 



* Encyc. Brit, art. Rivers, 





