Aug. 28, 1916 Indirect Determination of Hygroscopic Coefficient 



843 



formula of Briggs and Shantz gives values too low for the coarsest 

 textured members of the series and too high for the finest textured. In 

 the modified formula the value assigned to the clay is lowered, that to 

 the "sands" much increased, and that, to the silt slightly raised. This 

 formula gives results in close concordance with the directly determined 

 values. The explanation of the need of altering the values is not far to 

 seek. As has already been pointed out in connection with the computa- 

 tion of the hygroscopic coefficients from the mechanical analyses of the 

 same samples (3, p. 406), the material coarser than silt is chiefly very 

 fine sand, consisting mainly of particles but little larger than the upper 

 limit for silt, while the so-called "clay" contains a very large proportion 

 of silt particles with a diameter not much less than 0.005 mm. 



Briggs and McLane (5, p. 21), in applying their generalized formula 

 based upon the analysis of 104 soils, found that for the Marshall series 

 it was necessary to give the clay a lower value and also to make allowance 

 for the content of organic matter. As has been mentioned above, the 

 samples in Table VII belong to the Marshall and Colby series. 



Thus, it appears that if the mechanical analyses are to be used for the 

 computation of moisture equivalents, it will be necessary, at least in the 

 case of some widely differing soil types, to employ several different formulas. 



Table VII. — Concordance of the values for the moisture equivalent obtained by com- 

 putation from the mechanical analysis with those directly determined 



WAUNETA 



I 



2 



3 



4 



5 



6 



Average . 



1 Moisture equrvBlent~"o.oa sands+0.22 silt+1.05 clay. 

 J Moisture equivalent=o.i4 sands+0.27 silH-0.53 clay. 



■°-3 

 - .6 



