FLUME TRACTION. 203 



TABLE 67. Observations on flume traction, showing the relation of load to slope and other conditions Continued, 

 b. Over a surface of wood, rough-sawn, unplaced, and unpatnted. 



c. Over a surface of rectangular wood blocks, with grain vertical. 



d. Over a pavement of sand grains, grade (G), set in cement, the debris being also of grade (G). 



e. Over a pavement of pebbles, grades (H) and (I), set in cement. 



ADJUSTMENT OF OBSERVATIONS. 



FORMULATION. 



In flume traction, as in stream traction, 

 there is a finite slope competent slope cor- 

 responding to the zero capacity. An inspec- 

 tion of the observational data by plotting 

 served to show that they could advantage- 

 ously be adjusted by means of the formula 

 based on the theory of competent slope: 



Forty-two observational series were found to 



give information as to the value of a. Of these, 

 26 indicated positive values, three negative, 

 and the remainder values so small as to be of 

 uncertain sign. The mean of the 42 values is 

 + 0.29 per cent of slope. Eleven could be 

 compared directly with values adopted in the 

 adjustments of stream traction data, the mean 

 of the eleven values being, for flume traction 

 on a smooth surface +0.14 per cent, and for 

 stream traction +0.28 per cent. This differ- 

 ence is consonant with observed differences in 

 competent slope for the two modes of traction. 

 With the aid of this information, and with use 

 of considerations connected with modifications 



