TRANSPORTING POWER OF WATER. 233 



would gradually modify its bed till a permanent regimen 

 was established. But as the volume discharged is constant- 

 ly changing, and therefore the velocity, silt is deposited 

 when the velocity decreases, and scour goes on when the 

 velocity increases in the same place. 



It has been found by experiment * that a stream moving 

 with a velocity of 3 inches per second will carry along /me 

 clay and soft earth ; moving 6 inches per second, will carry 

 loam ; 1 foot per second, will carry sand; 2 feet per second, 

 gravel ; 3|- feet, pebbles an inch in diameter ; 4 feet, broken 

 stone, flint ; 5 feet, chalk, soft shale; 6 feet, rock in beds; 

 10 feet, hard rock. 



125. Transporting Power of Water. The specific 

 gravity of rocks varies from 2.25 to 2.64; when immersed 

 in water, therefore, they lose nearly half their weight. This 

 fact greatly increases the transporting power of water. The 

 pressure of a current of water against any surface varies as 

 the square of the velocity and as the area of the surface f 

 (Art. 97). But in similar figures, surfaces vary as the 

 squares of the diameters ; hence, the pressure of the current 

 varies as the square of the velocity and as the square of the 

 diameter, i. <?., the pressure of the current against a surface 

 varies as the square of its velocity multiplied by the square 

 of the diameter of the surface. Calling P the pressure 

 which the current exerts against a rock, v its velocity, and 

 d the diameter of the surface of the rock, we have 



P a & x d*. (1) 



Now the resistance to be overcome, or the weight of the 

 rock, varies as the cube of the diameter; i. e., calling W 

 the weight of the rock, we have 



W a d 3 . (2) 



* Experiments by Dubuat. Sco Kncy. Brit., Vol. XII., p. 503. 

 t Supposing that the area of the crosu-sectiou of the stream is at least large 

 enough to cover the surface. 



