The Co-Efficient of Humidity. 
145 
The value of m is nearly the same as that already established, 
but b instead of being nil has a small negative value. No physical 
meaning can be attached to the negative sign, and its appearance 
only illustrates the extreme delicacy of the method of least squares. 
The curve obtained by it is sensitive not only to deviations from the 
mean but to their positions in the set of observations. Here it is 
undoubtedly peat No. 145 i. that having a value below the average, 
depresses the curve slightly below the origin because of its position 
at the end of the list. For the purpose in view and with the data 
available in soil analyses the method of least squares is almost too 
refined an instrument. With only half-a-dozen analyses the 
numbers obtained by its use may be quite misleading. Though 
further investigation may lead to a revision of the values set out in 
this paper, the material employed seems sufficient to furnish a fairly 
close approximation to the true magnitude of the coefficient of soil- 
humidity in the case of the Heather Moor Association, and to 
establish the principles involved in its determination. 
VIII. —The Influence of Clay on the Soil Moisture. 
If the method already applied to non-humous soils to determine 
the water held by the soil apart from the humus is sound, it follows 
that if a soil contains clay this will partially control the water- 
content even when present in much smaller quantity. Though such 
an effect has been anticipated no data are yet available for deter¬ 
mining the coefficient of soil-humidity in clay soils. Nor is it clear 
. . . m • . ... . . • water-content 
whether the coefficient could be taken as the ratio ,--—— 
humus + clay 
as the joint effect of the two colloids may not be equal to the sum 
of their individual water-holding power. There is also the difficulty 
that in clay soils the loss on ignition includes more than the humus. 
But an important paper on “ The Moisture Equivalents of 
Soils ” 1 by L. J. Briggs and J. W. McLane, that has only come to the 
knowledge of the author after the completion of this paper throws 
considerable light on the subject. Briggs and McLane’s “moisture 
equivalent” is the percentage of moisture a soil can retain when 
subjected to a centrifugal force about 3,000 times the force of gravity. 
The authors show that the magnitude of this moisture equivalent for 
any soil depends almost wholly and equally upon the clay and the 
humus present. The silt exerts an appreciable though smaller effect, 
but all the coarser particles show practically no retentive action. 
1 U.S. Department of Agriculture, Bureau of Soils, Bulletin No. 45, 
Washington, 1907. 
