DISTRIBUTION OF VELOCITY IN AN OPEN CHANNEL 335 



Since the inward surface drifts consist of water which has travelled up 

 the sides and has come from the region of minimum velocity, they will 

 evidently have the effect of reducing the surface velocity and of depressing 

 the filament of maximum velocity. 



The sketches in Fig. 149 a and ft show respectively the directions 



FIG. U9. 



of the transverse currents, and of the resultant motion of the stream, the 

 full lines a'a', aa, in Fig. 149 b representing the direction of the surface 

 currents, and the dotted lines b'b', bb, those of the bottom currents. The 

 presence of such currents in channels of various sizes has been 

 experimentally demonstrated by the author. 1 



In addition to this action of the sides, any retardation of the flow 



FIG. 160. 



being more marked over the central and swifter portions of the stream 

 tends to increase the super-elevation of the central surface and the 

 formation of these transverse currents, while an acceleration of the flow 



1 Proc. Hoy. Soc., A., Vol. LXXXIl., 1909, pp. H9 159, 



