EXPERIMENTS WITH MIXED GRADES. 



181 



not reproduced, is equally irregular. Fine- 

 ness seems to control capacity when the finer 



Mixture 



1.8 



(CDEFG) /' Natural 



FIGURE M. Logarithmic plots of capacity in relation to linear fineness, 

 for related mixtures of deVis. 



component of the mixture is the more im- 

 portant, but other factors mask its influence 

 when the coarser component dominates. 



In preparing the mixture (C 45 D 35 E 12 F e G 2 ) the 

 endeavor was made so to apportion the com- 

 ponents as to approximate a natural grade. 

 To whatever extent that effort succeeded, the 

 data obtained with use of the mixture are com- 

 parable with those afterward obtained with a 

 natural grade. The capacities and finenesses 

 of the complex mixture and the natural grade 

 are represented by two plotted points near the 

 bottom of figure 64, and the line drawn 

 through them gives 0.57 as a value of the 

 index. Unfortunately the range in fineness 

 covered by the two is small from 492 to 603 

 so that their determination of 7 4 has little 

 weight; its close agreement with other values 

 may be largely accidental. 



In Table 45 mean values of 7 4 based on work 

 with the sieve-separated grades are so arranged 

 as to show their control by slope, discharge, and 

 trough width. On making the indicated allow- 

 ances for differences in condition, and compar- 

 ing those values with the values indicated by 

 figure 64, it appears that the latter are some- 

 what smaller but that the differences are not 

 great. The index values from data of the 

 experiments with mixtures range from 0.57 to 

 0.70. Coordinate values from the experiments 

 with single, sieve-separated grades would range 

 from 0.70 to 0.90. 



500 

 Linear fineness 



1,000 



FIGURE 65. Curve illustrating the range and distribution of finenesses in natural and artificial grades of debris. 



In considering the bearings of these esti- 

 mates there is advantage in giving graphic 

 expression to the conditions they severally 

 represent. When the results of mechanical 

 analyses of sands and similar materials are 

 presented graphically, the usual practice is to 

 plot integrated quantities, or proportions, of 

 ingredients on a scale of fineness or coarseness ; 

 but for the present purpose it is convenient to 

 use the quantities without integration. In 

 figure 65 the horizontal scale is that of linear 

 fineness, and the ordinates are relative quan- 



tities of components of different finenesses. 

 The curve ABC represents the composition of 

 the d4bris constituting the tractional load of a 

 river and is based on a sample of alluvium 

 from a river bed. The area ABCD, between 

 the curve and the axis of fineness, represents 

 the total load and corresponds to the 100 per 

 cent of integrative diagrams. The range in 

 fineness is limited at A by competent fineness 

 for traction, coarser material on the river bed 

 not being moved by the particular discharge 

 to which the diagram corresponds. It is 



