218 E. C. ANDKEWS. 



stream until the mass is surmounted. The downstream 

 slope offers a path for the surmounting stream. Gravity 

 asserts itself on the stream mass here partly as flow and 

 partly as a falling motion, and this new stream motion is 

 an expression of the steepness of the channel base traversed. 

 This change from inactivity to activity represents a net 

 gain to the power of the stream. 



Differential motion and shearing. — It is evident in 

 streams flowing under the action of gravity that every 

 obstacle in the channel causes differential motion to be set 

 up locally. The stream always follows the lines of the 

 least resistance as it must do from the nature of its origin. 

 Differential motion is thus set up. With increased mobility 

 comes increased ability to express eddyings. Under 

 sufficient pressure or gravitative stress the most refractory 

 stream may acquire wonderfully increased powers of 

 mobility with corresponding ability to emphasise differ- 

 ential motion. 



With increasing friction of the channel bottom and sides 

 comes rapid decrease of velocity among the basal portions. 

 The lower stream layers experience retardation while the 

 more superficial portions suffer much less from friction and 

 tend now by their momentum to move over the lower 

 layers. Strong reflections are caused from side and bottom 

 obstacles because of this nature of the stream to follow 

 the lines of least resistance. In many mobile streams 

 whole masses may thus be hurled bodily above the stream 

 surface. In the less mobile masses and those which are 

 apparently almost or quite solid under conditions of ordinary 

 atmospheric pressure, the differential motion often expresses 

 itself as shearing. The stronger the stream or the greater 

 the increase of its velocity, the more pronounced will be 

 these shearings and projections upwards and outwards 

 from the general surface. Nevertheless, as such shearing 



