the glaciers were much more perfect examples of streams 

 than they are at present, because during that period they 

 excavated channels adjusted to their strength and were 

 enabled to maintain their characteristics of flowage over 

 all interruptions of channel base made by themselves. 

 Similarly a river in drought is not a good example of a 

 stream, as there are too many points at which static con- 

 ditions practically obtain, whereas during the flood period 

 there is no opportunity for a static condition to occur at 

 any spot along the channel. 



The evident aim of a stream is to move vertically, that 

 is, in a direction perpendicular to the general plane of the 

 earth's surface. Nevertheless the resistance of the earth's 

 surface thus comes into play and by this means the stream 

 is deflected at some angle to the vertical and takes the 

 line of quickest descent. Thus the stream particles are 

 constantly under the influence of a dual action, one that of 

 the vertical force of gravity (practically constant at the 

 earth's surface) tending to produce a motion of 16 feet 

 during the first second <>f d. -scent, another that of the force 

 on the particles in the direction of motion at the point 

 considered at any time. The latter imparts a variable and 

 more or less horizontal velocity to the particles. To find 

 the resultant force and direction in which the stream 

 particles tend to move at this stage a parallelogram is 

 constructed with sides of such lengths and position as to 

 represent the amounts and directions of application of the 

 two forces under consideration. The diagonal O of the 

 parallelogram as shown in the occompanying diagram shows 

 the direction in which the stream tends to move. 



Tims the stream tends at any point to move in a parabolic 

 path. 1 In places where its movement is not cramped as 

 over a ledge, its path is actually parabolic. The knowledge 



1 Corrasion by Gravity Streams, pp. 216 and 323, 324. 



