70 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The small changes in underground temperature during a period of frost 
are due to the fact that when once the soil is frozen a layer of invariable 
temperature — the bottom of the frozen soil, where the temperature is always 
0° C. — lies between the surface and the lower depths. During a spell of 
frost the surface may fall to several degrees below 0° C., but, owing to the 
latent heat liberated at the bottom of the frozen layer, the temperature 
gradient in the frozen soil is much greater than in the unfrozen soil beneath, 
and the temperature variations in the surface are not conducted uniformly 
downwards, the only result of low surface temperatures being to slowly 
increase the depth of the frozen soil. 
Thus, during the process of freezing and thawing until the whole frozen 
depth is thawed and the temperature gradient again becomes uniform, surface 
variations of temperature hardly affect the 4-inch thermometer at all. 
These contrasted fluctuations may be well seen in Table X, where the 
movements of underground temperature at the 4-inch depth before and 
during spells of frost are given. No snow was on the ground on the dates 
considered ; the effect of a covering of snow will be dealt with later. 
Table X. — Fluctuations in Temperature at 4-inch Depth just 
BEFORE AND AFTER SURFACE IS FROZEN. 
4-inch Depth. 
Date. 
Air over Soil. 
Surface. 
Before Surface 
After Surface 
froze. 
froze. 
Minima. 
Minima. 
i Fall and Rate. 
Fall and Rate. 
f - 2-0" C. ! 
o-o° co 
- 3'0 U C. 1 
o-o° C. 
1919. 
-5*5°C. 
-2-8° C. 
1-6° C. 
i-o° C: 
Mar. 20-26 
- 3-5° C. | 
-1-5° C. 
in 12 hours. 
in 120 hours. 
-5-0°0. 
-1-6° C. 
v — 5*0° C. 
- 2-2° C. J 
Apr. 19-20 
1-2° C. 
o-o° c. 
J 3-9“ C. 
f in 12 hours. 
1 
Oct. 12-13 
— 0*4° C. 
o-o° c. 
f 3-6° C. 
\\ in 18 hours. 
(-V 4°C. 
-ro~ c.a 
Nov. 7-12 
| o-o° c. 
\ -0-8° C. 
-1-2 C.| 
0-0° 0. 1 
2-2° C. 
in 18 hours. 
1-1° C. 
in 84 hours. 
1-2*4° C. 
-0-8° C.i 
(-0-2° C. 
-0'4° C. \ 
Dec. 6-9 
| -1T°C. 
-1-0° c.| 
1-1° C. 
0-7° C. 
| - 3-0° C. 
— 1*2° C. 
in 12 hours. 
in 36 hours. 
1 - 4-0° C. 
-1‘6° C.J 
„ 24-26 
f - 1 -0° C. 
o-o° c. 
1-4° C. 
0-5° C. 
\ - 5*3° C. 
-2-4° C. 
1 in 24 hours. 
in 24 hours. 
\ -4-1° C. 
-2-1° Co 
„ 31 -Jan. 
1 - 2-4° C. 
-2-6° C. [ 
1-7° C. 
0-4° C. 
4, 1920 
1 -i-o°c. 
-1-8° C. 1 
in 24 hours. 
in 72 hours. 
1-2-0° C. 
— 0"5° C. 1 
