1878.] Uniform Regime in Rivers and other Open Channels. 117 



structed in wood, and were open above, and bad a flat bottom and 

 vertical sides, so that the current was rectangular in cross-section. 

 Channels of various other forms were also used, and the mode of flow 

 of the water in them was scrutinized. The results arrived at by these 

 experimenters tend very much towards establishing the supposition 

 which forms the subject of the present paper — the supposition namely 

 of the prevalence or frequent occurrence of a distribution of velocities 

 having the maximum velocity not at the surface but at some moderate 

 depth below. Boileau, by his experiments, was led to announce as 

 one of his conclusions (page 308), that in the medial longitudinal ver- 

 tical section of a rectangular canal with uniform regime, the maximum 

 of velocity is situated not at the surface, but at a depth which is a 

 fraction more or less considerable of the total depth of the current. 

 He also announced, as a conclusion, that the decrease of velocity, from 

 the place of maximum velocity up to the surface, must be attributed 

 to some new cause different from that which produces the diminution 

 of velocity from the place of maximum down to the bottom. This 

 new cause, he says, cannot be solely the resistance of the bed of air in 

 contact with the liquid surface acting like the face of a pipe or con- 

 duit ; and he assigns, in proof of this, the reason that the mobility of 

 this bed of air does not permit of our attributing to it a retarding in- 

 fluence so great as that which is implied in the rapid abatement of 

 velocities in approach towards the surface in the upper part of the 

 current. He recounts his own special experiments, made in 1845, on 

 the influence of wind on the velocities in currents, — a subject which 

 he says had up to that time been very little investigated by hydran- 

 licians. He deduces from his experiments conclusions (page 313) 

 to the effect that in spite of varied disturbances produced by wind 

 blowing over the water with varied intensity, yet there is manifested 

 a very sensible tendency to a decrease of velocities of the water for 

 approach towards the liquid surface ; and that the maximum velocity 

 is yet below the surface, even when the wind blows forward with the 

 current, and has a velocity greater than that of the current. Judging, 

 then, that resistance of the air cannot be the cause of the phenomenon, 

 he says that it is then principally in the mutual actions which bind 

 among one another the liquid particles, and in the oblique and rotatory 

 movements which result, under the influence of these forces, from the 

 difference of velocities of neighbouring particles, that it is necessary 

 to seek for the explanation of the phenomena of the decrease of velo- 

 cities in the approach towards the surface of currents. He goes on to 

 say that we have to conceive, in fine, that these oblique movements, 

 producing transverse living forces ("forces mves"), diminish according 

 to certain general laws, the living forces of forward motion which the 

 hydrometric instruments are adapted to indicate. 



I have cited this passage from Boileau very fully, because it seems 



VOL. XXVIII. k 



