Trottt Breeding. 109 



We reproduce the diagrams and give a translation of 

 part of the text below : 



" 'The most important factor in determining the cur- 

 rent of a river is its speed, which increases with the fall 

 and the quantity of water and diminishes with increase 

 of the width of the channel. The speed varies also 

 with the interior friction and with the friction of the 

 water against the banks. . . . 



" 'The result of this is that not all parts of the cur- 

 rent along a cross-section are moving at equal speed. 

 The velocity increases in a vertical direction from the 

 ground toward the surface, but it is greatest not at the 

 surface but a little distance beneath it; likewise it in- 

 creases at the surface itself from the banks toward the 

 middle. The lines of equal velocity in a cross-section 

 ' take the form of half-elipses convex downward. It 

 must be remarked further that the surface of the water 

 is not horizontal, but sometimes convex and sometimes 

 concave. It is the first in case a considerable mass of 

 water with higher velocity (as at high water) moves 

 in mid-stream, so that the middle of the river conveys 

 more water than the sides. When the water is falling, 

 a greater amount of water is flowing away at the mid- 

 dle, and the surface becomes concave. In the 

 Mississippi these oscillations of level measure as much 

 as two metres (six feet). 



" 'Owing to the shape of the bed of the stream, espe- 

 cially at the bottom, the water is deviated from a 

 straight line, so that the line of greatest depth in the 

 stream is curved. . . . If we observe the move- 

 ment of the water from the banks to the middle of the 

 stream we find that the water in the middle moves 

 downward and then in a spiral path approaches first the 

 bottom and then the bank,' 



