1919-20.] The Cooling of the Soil at Night. 
15 
(c) Soil Dry. 
R 
To measure the values of ^ and the lag in completely dry soil, I 
K 0 
prepared a bed from soil which had been kept under cover for six months 
and installed thermometers at the surface and at a depth of 4 in. in this 
bed, which had a total depth of dry soil of 8 in. 
As capillary action is very slow in dry soil, I hoped to obtain good 
results in the first few days of the trial, so long as the weather remained 
fine and no rain fell. Every day but one of the trial was sunny and no 
rain fell ; the results are given in Table III. 
Table III. — Yalues of and Lag. 
Rq 
r 4 
Day. 
Lag. 
Remarks. 
1 
T9 
hrs. 
Sunny 
2 
•20 
H „ 
3 
•20 
Overcast, foggy 
4 
5 
6 
•25 
•28 
•28 
H » 
5 „ 
5 55 
1 
Sunny ^ jy[ 0 j sture r i s j n g by 
” ( capillary action 
55 J 
j> 
III. Values of the Lag corresponding to Various Values of 
R 0 
R 
On all days when the values of were found, the approximate value 
±V 0 
of the lag of the maximum and minimum at the 4-in depth was observed. 
R 
When had large values it was possible to read the lag to the nearest 
R 0 
R 
15 minutes, but on those days when ^ was small the temperature at the 
H 0 
4-in. depth arrived at and departed from its maximum and minimum values 
so slowly that the error in reading the lag may amount to 30 or 45 minutes. 
Table IV gives the average values of the lag corresponding to various 
R 
values of and shows the number of observations on which these average 
R(j 
values are based. 
