FRESH-WATER STREAMS. 649 



or earth, uprooted trees, and the contents of the lake basin, far and 

 wide. 



Again, in some countries the rains are excessive. At Paramaribo, 

 in Dutch Guiana, the annual fall is 229 inches, or 19 feet; and south 

 of Bombay, in the Western Ghauts at Mahabaleshwar, at a height of 

 4,200 feet, the annual amount is 302 inches, equivalent to a layer 25 

 feet deep, which the rivers carry off and work with. 



3. Erosion processes ; the force concerned. — Erosion from the me- 

 chanical action of water is carried forward (1) through the impact or 

 blows of the water ; (2) through the abrasion of the bed and sides of 

 the channel by transported sand, gravel, etc. ; and (3) through the mu- 

 tual abrasion (or the corrasion) of transported particles or masses, by 

 which these are made more transportable and their removal facilitated. 



(1.) By impact. — The force of the impact of running water is ex- 

 pressed, in pounds, by the general equation P = 0*9702 n s v 2 , in 

 which v is the velocity in feet per second, s is the greatest transverse 

 section of the body in square feet, n a coefficient varying with the 

 form of the body ; and 0*9702 is the quotient from dividing the weight 

 of one cubic foot of water (6'2± pounds) by 2g (p. 639). Supposing 

 the greatest transverse area to be 1 foot : for a simple plate the valile of 

 n is 1*86 ; for a cube, 1*46 ; for a prism, placed end on, with square flat 

 base and length three times the breadth, 1*34; and the least amount, 

 for any prism with flat base, 1*25 ; for a sphere, 0*51 ; for some 

 rounded forms, only 0*25. If the cylinder has a hemispherical end to 

 face the current, the impact is half less than for one with a flat end ; 

 and if it ends in an equilateral cone, the impact is reduced to one 

 fourth of that where the end is flat. 



In accordance with the above, the force of impact against a flat 

 plate a foot square, in a current of five miles an hour (or 7-^- feet per 

 second), will be nearly 100 pounds ; in one of twenty miles an hour, 

 or four times five, 16 times that for five miles, and so on. 



On the other hand, if the surface struck be a hemispherical concav- 

 ity, the impact would be very much greater than for a flat surface, the 

 value of n being about two for a hollow hemisphere with the concav- 

 ity to the current. These results of experiment and mathematical 

 calculation show that it is not possible to measure the force exerted in 

 the movements of a river ; and also that the concavities and deep re- 

 cesses or channels among the rocks along the sides of a stream give the 

 blows great power. (See for the force of wave impact, p. 671.) More- 

 over, all rocks are more or less divided up by joints ; very many are 

 laminated in structure ; and most of them are in beds with some spaces 

 between the beds ; or they have interlaminations of weaker layers 

 (layers easily eroded or decomposed) whose removal makes the needed 



