206 



TRANSPORTATION OF DEBRIS BY SUNNING WATER. 



TABLE 68. Values of capacity for fume traction, adjusted in relation to slope of channel Continued, 

 e. Traction over pavement of pebbles, grades (H) and (I). 



PRECISION. 



Probable errors were computed from the 

 residuals of 25 series, the residuals being 

 measured on the plots as percentages. The 

 greatest probable error computed for a series 

 of adjusted values of capacity is 4.6 per cent, 

 and their average is 1.3 per cent. The aver- 

 age of the 25 determinations of the probable 

 error of an observation is 3.8 per cent. The 

 residuals number 139, and their average value, 

 which also is a measure of the precision of the 

 observations, is 4.3 per cent. 



A comparison of these measures with those 

 obtained from the data for stream traction 

 shows that the flume-traction data are de- 

 cidedly the more harmonious. The average 

 residual is more than twice as great for stream 

 traction as for flume. Part of this difference 

 may be due to the fact that the experiments 

 with flume traction came last and had the 

 benefit of previous experience, but it is to be 

 ascribed chiefly to the fact that in flume trac- 

 tion the slope is constant, while in stream 



traction it is subject to rhythmic fluctua- 

 tions. 



DISCUSSION. 



CAPACITY AND CHANNEL BED. 



Data illustrating the influence of the character 

 of the channel bed on the quantity of debris 

 which a stream can transport have been as- 

 sembled in Table 69. They are taken chiefly 

 from the preceding table, but a few items are 

 from Table 72 and Table 12. A single item, 

 marked as interpolated, is based on a combi- 

 nation of data from Tables 68 and 72. They 

 pertain to all the simple grades and mixed 

 grades with which experiments were made in 

 flume traction; and stream traction is repre- 

 sented, so far as possible, by coordinate data. 

 Comparisons are made for slopes of 2 and 3 per 

 cent and discharges of 0.363 and 0.734 ft. 3 /sec. 



The greatest capacity is in each case asso- 

 ciated with the smoothest of the tested channel 

 beds, the surface being that of a plank, planed 

 and painted, with the grain running parallel 

 to the current. 



TABLE 69. Comparison of capacities for flume traction associated with different characters of channel bed. 



[Width of trough, 1 foot.] 



