1919-20.] Effect of Weather Changes on Soil Temperatures. 59 
depth and at the surface — is below the normal from minimum to maximum, 
and lies between the two values from maximum to minimum on either 
R 
side of it ; whilst on sunny days the corresponding value of — 4 is normal 
K 0 
and follows closely the two values from maximum to minimum on either 
side of it, only showing a very slight downward tendency owing to reduced 
conductivity from the drying of the soil. 
This is clearly shown in Table I. 
JL> 
Table I. — Relation of — 4 to the Normal — Sunny and Dull Days. 
R 0 
Sunny Day 
3. 
Dull Days. 
w 
W 
Date. 
Previous 
R 0 
Minimum 
Subsequent 
Previous 
R 0 
Subsequent 
r 4 
w 
w 
Minimum 
r 4 
R 0 - 
to 
R 0 
Rq 
to 
R 0 
! 
Maximum. 
Maximum. 
1919. 
April 19 
•42 
•40 
•44 
„ 20 
... 
•44 
•40 
•42 
„ ' 22 
•38 
•35 
•38 
May 2 
•41 
•36 
•39 
„ 3 
•39 
•38 
•39 
„ 16 i 
•31 
•31 
•31 
„ 19 i 
•40 
•39 
•38 
„ 21 
•35 
•33 
•35 
„ 24 
•30 
•30 
•30 
„ 26 
•30 
•31 
•30 
... 
„ 27 ' 
•30 
•30 
•30 
„ 30 
June 1 
•28 
! 
•28 
•28 
•32 
■29 
•30 
„ 8 
•42 
•39 
•42 
16 
*35 
•35 
•39 
„ 18 ! 
•32 
•32 
•32 
„ 27 
Oct. 9 | 
•36 
•37 
... 
•35 
•34 
•29 
•34 
„ 15 | 
... 
•37 
•30 
•36 
Averages 
•33 
•33 
•325 
•38 
345 
•38 
(b) Diffusion of Water Vapour. 
Dr John Aitken has clearly shown that diffusion of water vapour takes 
place in the soil, downwards by day when the surface is hot, and upwards 
at night when it is cold compared to the underground layers. This upward 
diffusion at night he shows to be one of the prime causes of the formation 
of dew.* 
* “On Dew,” by John Aitken, LL.D., F.R.S., Trans. Roy. Soc. Edin ., vol. xxxiii. 
