THE WORK OF RUXXIXG WATER. 117 



a small bit of mineral matter, the specific gravity of which is two or 

 three times that of water. Why does it not sink through the water 

 and come to rest at the bottom of the stream, or suffer transportation 

 as the gravel does? 



A particle of sediment in running water is obviously subject to 

 two forces, that of the current which tends to move it nearly hori- 

 zontally down-stream, and that of ._ . ^ 



gravity which tends to carry it to 

 the bed of the stream. In Fig. 

 100, the arrows ah and ac represent 

 respectively the relative force of 

 gravity and a current of 5 miles 

 per hour. As a result of these two ^ >|. :^ ^ 



forces the particle would tend to Fig. lOO.— Diagram to illustrate the relative 



descend in the 2:eneral direction of strength of the two forces acting on a 



ad, a line whidl represents the P^^^^^^^ ^ suspension. The arrows rep- 



resent the relative strength of the two 

 resultant of these forces, though forces when the stream's velocity is 5 miles 

 not the exact path which a particle per hour. No account is taken in the di- 



acted on bv them would take in ^f^'^' ^^ ^^^e viscosity of the water, or 

 " . .of the acceleration of velocitv of fall. 



water. It a river were the smiple 



straightforward current which it is popularly thought to be, a particle 

 in suspension would reach its bottom in the time it would take to sink 

 through an equal depth of still water, for the descent would be none 

 the less certain and none the less prompt because of the forward move- 

 ment of the water. The current would simply be a factor in deter- 

 mining the position of the particle when it reached the bottom, not 

 the time of reaching it. Very fine particles^ like those of clay, though 

 having the same specific gravity as grains of sand, would sink less 

 readily than coarser ones, because they expose larger surfaces, relative 

 to their mass, to the water through which they sink. But even such 

 particles, unless of extraordinary fineness, would presently reach the 

 bottom if acted on only by a horizontal current and gravity. Since 

 e^'en sediment which is not of exceeding fineness is kept in suspension 

 it is clear that some other factor is involved. This is found, in part 

 at least, in the subordinate upward currents in a stream. 



Where a bowlder occurs in the bed of a stream (Fig. 101) the water 

 which strikes it is in part forced up over it. If there be many bowlders 

 the process is frequently repeated, and the nmnber of upward currents 



