168 SEWER DESIGN 



rounding the flowing mass. According to Dubuat's experi- 

 ments the adhesive attraction of the walls seems to cease at 

 this layer, so that differences in the material of the walls pro- 

 duce no perceptible change in the resistance. That this reason- 

 ing is not good we now know; but since the early experiments 

 on the value of the coefficient were made under conditions in 

 which the wall-surfaces differed but little, and since no new 

 experiments were made until the middle of this century, engi- 

 neers, however much convinced of the unreliability of these 

 early formulae, were not in a position to construct a more 

 accurate one. It was left to M. Darcy, Inspecteur General 

 des Fonts et Chaussees, to whom the city of Dijon owes her 

 excellent water-supply, to open the way to a better under- 

 standing of this subject. In the Dijon water-pipes M. Darcy 

 noticed,* as had been observed by others, that those pipes 

 which presented the smoothest inner surface furnished the 

 greatest quantity of water in a given time, or, in other words, 

 that the greatest velocity was found in the smoothest pipes. 

 He argued that a similar phenomenon must take place in open 

 channels, and undertook to make a thorough and extensive 

 series of experiments upon this point. By special authority 

 of the government, he had constructed near Dijon, on the 

 Canal de Bourgogne, a special canal 6 feet wide, 3 feet deep, 

 and about 1850 feet long. It received its water from the 

 canal and discharged it into the River POuche. The water was 

 supplied by two reservoirs at a constant head, and the amounts 

 were measured by a series of carefully calibrated weirs. The 

 canal was furnished successively with different linings, viz., 

 neat cement, i : 3 mortar, boards, brick, fine and coarse peb- 

 bles, and laths, nailed transversely to the direction of the cur- 

 rent, .01 and .05 metre apart. The grades were varied from 

 .001 to .009 per unit of length. Besides these experiments, 

 all known data were collected and compared. Just as M. 

 Darcy had completed these arrangements, most of them pre- 



* Recherches expe*rimentales relatives au Movement de L'Eau dans les Tuyaux, 

 par Henry Darcy, 1857. 



