68 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The conditions under which heat transfer takes place, and the rapidity 
of this transfer, in soil of varying degrees of wetness would seem to be as 
follows : — 
Table VIII. — Transfer of Heat in Light Loam Soil. 
Condition of Soil. 
I. Very dry 
II. Increasing wetness up 
to point when free 
percolation begins. 
III. Further addition of 
water, so that — 
(a) There is freeper- 
colation ; or 
( b ) There is water- 
logging. 
Observed 
Values of ; 
R* ; 
R 0 
Mode of Heat Transfer. 
Remarks. 
•19 
By conduction from particle to par- 
See Sec- 
ticle, with considerable discontinuity 
owing to small number of points of 
contact and many air gaps. 
tion II (c). 
•19 to -42 
By conduction from particle to particle, 
See Sec- 
with gradually lessening discon- 
tions 1 1 (a) 
tinuity owing to growing water films. 
During this stage, either before free 
percolation begins or after it has 
ceased, fresh percolation may be in- 
duced by a rise in temperature of 
the soil. 
and III (a). 
•42 to '85 
j (a) By conduction and percolation. 
See Sec- 
tionsIII(a), 
III (6), and 
IV (b). 
•42 to *30 
(6) By conduction only, gradually 
diminishing in value to conduc- 
tion of water. 
Section VI. — Wind and Relative Humidity. 
The surface of the soil is almost always damp, and on overcast days or 
in the shade the surface temperature follows the wet-bwlh temperature 
very closely, and during the prevalence of strong winds of low relative 
humidity is often considerably below the air temperature as given by the 
dry bulb. 
In spring, particularly when the mean relative humidity is low and 
strong winds are frequent, the surface and underground temperatures may 
be reduced by excessive evaporation below the normal even though the air 
temperature as shown by the dry bulb is equal to or above the normal. 
Thus, under exceptional circumstances, the rule that the relation of the 
mean air temperature to the normal is a good index of the relation of under- 
ground temperature to its normal breaks down ; this happened at Hodsock 
Priory in April 1915, when an unusually low relative humidity with an 
excess of N.E. winds of more than average velocity was recorded, and under- 
ground temperatures were low in spite of the air temperature being above 
