EXPERIMENTS WITH MIXED GRADES. 



179 



current. Every obstruction diversifies the 

 current. The deflection necessary to pass it 

 both constitutes and causes diversity of direc- 

 tion, and diversity of direction necessitates 

 diversity of velocity. If a pebble is placed on 

 the sandy bed of a small stream, the trans- 

 formation of the adjacent parts of the bed by 

 the diversified current is obvious. The build- 

 ing up of the bed in the lee of the pebble testi- 

 fies to lowered velocity, and the scouring at the 

 side to heightened velocity. In the same way 

 each coarser gram of a heterogeneous stream 

 load diversifies the current about it and gives 

 to such of the filaments as are accelerated 

 greater power for the traction of finer grains. 

 With reference to the transportation of the 

 finer debris, the coarser grains have the func- 

 tion of obstructions whether they are partly 

 embedded or lie on the surface or are rolled 

 along. 



If the factors concerned in the traction of the 

 finer components of mixtures have been cor- 

 rectly stated, it is not difficult to understand 

 that under most conditions the net result of 

 their influences will be a reduction of capacity, 

 and also that special conditions may deter- 

 mine an increase. 



VOIDS. 



The packing together of larger and smaller 

 grains which tends toward a smooth stream 

 bed tends also toward the reduction of inter- 

 stitial spaces within the bedded d6bris, thus 

 reducing the percentage of voids. It was sug- 

 gested in the laboratory that the percentage of 

 voids, used inversely, might serve as a sort 

 of index of mobility, and estimates of the voids 

 were accordingly made for most of the mate- 

 rials employed in the experiments with mixed 

 grades. Partly for this purpose, and partly to 

 obtain the factors needed to correct the weigh- 

 ings of load for interstitial water, a series of 

 special weighings were made. 



A vessel holding 535 cubic centimeters was 

 filled with saturated debris and weighed. Af- 

 terward the same debris was weighed in a dry 

 condition. The computation was made by the 



formula 



W W 

 Percentage of voids = -. 



in which W is the weight of saturated d6bris 

 in grams and W the weight of dry d6bris. 



Table 62 contains the estimated voids for the 

 binary mixtures. In each series the percent- 



ages are smaller for the mixtures than for the 

 component grades; and when the percentages 

 were plotted in relation to the proportions of 

 component grades (after the manner of the 

 capacities in figs. 55 and 56) each series was 

 found to indicate a minimum. The positions 

 of the minima correspond to mixtures with 30 

 to 40 per cent of the finer grade of debris. In 

 comparing voids with capacities, the minima 

 of the void curves are to be considered in rela- 

 tion to the maxima of the capacity curves. 



Capacity 



Percentage 

 of voids 



Linear 

 fineness 



Bulk 

 fineness 



FIGURE 63. Curves showing the relations of various quantities to the 

 proportions of fine and coarse components in a mixture of two grades 

 of ddbris, (C) and (G). The horizontal scale, when read from left to 

 right, shows the percentage of the coarser component in the mixture. 



Those maxima, however, are associated with 

 mixtures having 60 to 90 per cent of the finer 

 grade; and the attempt at correlation therefore 

 fails. A single void curve is reproduced in 

 figure 63, together with the corresponding 

 capacity curve. 



TABLE 62. Percentages of voids in certain mixed grades of 

 debris, compared with the percentages in the component 

 grades. 



