127 
1918-19.] The Cooling of the Soil at Night. 
February 28, <p' m — <p m was 3° F. as against 2‘95° F. Formula (1) holds as 
long as the surface does not freeze; when the soil is frozen, the bottom of 
the frozen layer is always at 32° F. Conduction from the 4 in. depth is now 
1^(^ m — x £ x 00 x 60 x h calories 
10 -x m 9 
= calories . . . . . (2) 
10 — x m V 
(d) The Cooling of the Soil. 
Since the soil freezes at 32° F., so long as the surface does not fall to 
32° F. the temperature gradient is more or less uniform between the 4 in. 
Fig. 3. 
depth and the surface. This uniformity must obviously cease as soon as 
freezing sets in with its liberation of latent heat. 
Fig. 3 shows graphically the cooling of t h_e soil between the 4 in. 
depth and the surface : — 
(1) When the surface does not freeze. 
(2) When the surface freezes. 
When the surface does not freeze, the number of calories used up in 
cooling the soil is — 
'8 x 5 x area ABCD 
= •44 x™(0' + 0") 
z 
— •2'2[10(^' + 0")] 
(3) 
