1919-20.] Effect of Weather Changes on Soil Temperatures. 67 
soil, not only during or soon after actual rainfall, when water is percolating 
freely through the soil, but at other times when for any reason percolation 
starts afresh in soil that apparently had given up all its gravitational 
water.* 
If a sudden increase of temperature in the soil does bring about 
renewed percolation, it seemed reasonable to suppose that the effect of this 
percolation would be felt more fully and rapidly vertically underneath the 
source of the heat than at an equal lateral distance from it. To test this, 
I buried a hollow metal sphere in the soil and arranged thermometers 
3 inches vertically above and below it, and 3 inches laterally from it ; the 
sphere was then filled with boiling water and the behaviour of the ther- 
mometers noted. The thermometers below and to the side of the sphere 
were put in position without disturbing the soil ; the thermometer above 
the sphere was put in the replaced surface soil which had been carefully 
removed before digging the sphere in. The whole area was protected from 
atmospheric influence by a layer of sacks and earth ; the rise in temperature 
and lags of the various thermometers are given below : — 
Experiment I. 
3 inches below sphere . . Rise = 5'6° C. 
„ above sphere . . „ = 4 - 0° C. 
„ to side of sphere . „ = 4'3° C. 
Experiment II. 
3 inches below sphere . . Rise = 4*6° C. 
„ above sphere . „ — 3 2° C. 
„ to side of sphere . „ =3’3° C. 
Lag = 2’8 hrs 
= 35 
„ =3*2 
Lag = 2’5 hrs. 
„ =32 „ 
„ =3T „ 
Both experiments were made when the soil was in a drying condition 
after several dry days ; I thus hoped to catch the moment when there was 
a certain amount of “ transfer resistance” to heat due to discontinuities in the 
water films round the soil particles, though the soil was not yet too dry to 
prevent fresh percolation when these discontinuities disappeared in virtue 
of the decreased viscosity and surface tension of the soil water due to the 
applied heat. 
Whether I actually did catch the auspicious moment is difficult to tell, 
but the results show that in both experiments the heat from the sphere 
travelled more easily and rapidly vertically below the heat source than in 
any other direction, and I venture to think that this result must have been 
due to renewed percolation. 
* F. H. King, The Soil ; A. D. Hall, The Soil. 
