72 Proceedings of the Boyal Society of Edinburgh. [Sess. 
several dates are compared ; as we have neglected the upward conduction 
from the 4-inch depth, we should expect the calculated values to be slightly 
too large, and this is found to be the case. 
Table XI. — Calculated and Observed Values of Depth Frozen. 
Date. 
(1) 
h. 
(2) 
*o. 
(3) 
x 2 = ‘99 0 h. 
(4) 
Calculated 
Value of 
Depth frozen 
in cms. 
(5) 
Observed 
Value of 
Depth frozen 
in cms. 
(6) 
Difference 
of Columns 
(5) and (6) 
in cms. 
(7) 
1919. 
Mar. 22/23 
10 
115 
10*35 
3-2 
3-0 
+ •2 
„ 23/24 
10 
•53 
4-7 
2*15 
1*8 
+ •35 
„ 24/25 
11 
•75 
7-4 
2-7 
2-6 
+ •1 
„ 25/26 
11 
•95 
9*4 
3*1 
3-0 
+ •1 
„ 28/29 
10 
1*23 
11*0 
33 
30 
+ •3 
Apr. 2/3 
10 
•20 
1*8 
1*35 
1*0 
+ •35 
» 3/4 
12 
•75 
8-1 
2-8 
2-5 
+ 3 
„ 4/5 
10 
•80 
7 '2 
2*7 
23 
+ '4 
Nov. 13/14 
10 
51 
459 
6-8 
6-4 
+ '4 
„ 14/15 
12 
2-5 
27*0 
5*2 
5-0 
+ •2 
„ 29/30 
12 
•8 j 
8-6 
2*9 
2*5 
+ *4 
Dec. 25/26 
19 
1*2 
20-5 
4*5 
4-2 
+ •3 
Dec. 31 /Jan. 1 
11 
1-0 
9*9 
3 2 
3-0 
+ •2 
1920. 
Jan. 1/2 
34 
17 1 
52-0 
7-2 
6-6 
+ •6 
1 
From the formula a good idea may be formed of the risk crops — such 
as potatoes — run from various degrees of frost at the surface. For frost to 
reach a depth of 4 inches in one night of 12 hours, an average surface 
temperature of — 9'2° C. is needed ; but in a continuous frost lasting over 
four days an average surface temperature of only — 11° C. is required to 
reach the same depth. 
During the continuous frost from December 31, 1919, to January 5, 1920, 
the frost reached the 4-inch depth after 100 hours, during which the 
average surface temperature was — T 24° C. 
Section VIII. — Snow. 
The effect of a covering of snow is to reduce the changes in underground 
temperature even more than frost does. When snow falls on unfrozen 
soil the surface now becomes the layer of invariable temperature ; and even 
when the snow falls on ground already frozen, the protecting influence of 
the snow causes the ground to thaw gradually, so that in either case the 
layer of invariable temperature tends to rise rather than to fall as in the 
case of frost. 
Snow is an extremely bad conductor, and as little as 4 inches of snow 
