444 - HENRY M. EAKIN 



reversed gradient gain headway. But as s,\\c^, lateral currents are 

 accelerated, friction increases. Friction in that part of the current 

 moving in a positive direction or with the lateral force consumes 

 part of the energy of the lateral impulse so that the actual gradient 

 is lowered. Friction in the part of the current moving in the negative 

 direction consumes part of the accelerative force of the actual gradient. 

 A condition of equilibrium is reached when the actual gradient 

 becomes such that it is just maintained by the excess of lateral impulse 

 over friction in the positive part of the current and whose accelerative 

 force is balanced by friction in the negative part of the current. This 

 adjustment is made by the shifting of the stratum of non-lateral 

 movement which alters the relative proportions of the positive and 

 negative parts of the current. The higher the actual gradient the 

 larger will be the negative part of the current. As acceleration' goes 

 on and the actual gradient becomes less, that part of the current 

 having a negative component of motion becomes more and more 

 closely confined to the bottom of the channel and its efficiency in 

 handling debris correspondingly increased. The condition of 

 equilibrium would represent the maximum of efficiency. That 

 such a condition is not represented in much of the course of a meander- 

 ing stream is sure. That it is represented in any part is not certain. 



Boring Currents in a Straight Stream 

 On straight reaches no reversal occurs. The condition of equilib- 

 rium between lateral forces and friction should be established. 

 Therefore F^, operating on straight reaches, should be immensely 

 more effective in directing lateral erosion than an equivalent amount 

 of lateral force operating in a meandering stream. 



As pointed out by Hagen,' Herschel,^ and others, the power of flow- 

 ing water to erode its channel depends very largely upon the swirls, 

 eddies, and such inner movements of particles among themselves. 

 Such minor movements might well be called the teeth of the current, 

 it being for the most part their action that wrests material from the 

 bank or the bed of the stream and places it in proper relation to the 

 current to be transported. These minor movements are due to 

 friction between parts of the current, differing in velocity. Their 



1 Hagen, Handbuch der Wasserhoukunst, Part II, Art. 21. 



2 Herschel, Jour. Franklin Inst., 3d ser., LXXV, 401. 



