64 Proceedings of the Poyal Society of Edinburgh. [Sess. 
(1) The surface and 4-inch depth curves were both at about 14° C. for 
some hours during the rainfall. 
(2) The 4-inch depth temperature rose 3° C. between 1 and 2 p.m., the 
rise beginning only 50 minutes after the rise at the surface. 
Section IV. — The Relation of the Above. 
(a) Various Soils — Sand, Loam, and Clay. 
If we compare sand, loam, and clay in a similar wet condition after 
R 
rain, we find considerable difference in the values of ~ from one to 
R, 
R, 
another. Thus in sand seems to have a value of about *60, in loam of 
Jtln 
•42, and in clay of ’40. 
As the soil dries these values decrease — very rapidly in sand to *23, 
fairly rapidly in loam to *28, and slowly in clay to '37 after a considerable 
period of dry, sunny weather. 
At the beginning of the period the surface ranges should have been 
the greatest in sand and the least in clay, wdth an intermediate value for 
loam — due to the specific heats of equal volumes of wet sand, loam, and 
clay being ’34, '53, and *61 respectively. But when a dry mulch had 
formed the ranges should be in the following order — sand, clay, loam, 
since the specific heats dry are T2 for sand, T5 for clay, and T8 for loam. 
This ideal state of things is upset by two causes — the disinclination of 
clay to form a dry surface mulch, and the fact that both sand and clay, 
owing to their light colour, reflect back some of the radiant energy of the 
sun which is absorbed by the darker loam. 
In effect we find that the ranges at the surface are greatest in sand, 
though not so much greater as we might expect from its low specific 
heat, intermediate in loam, and least in clay, under practically all field 
conditions. 
In Table VI the surface and 4-inch depth ranges for these three soils are 
compared during a drought of 16 days in May and June 1919, and it will be 
seen that clay suffers in comparison to sand and loam by reason of its low 
R 
value of ^ when thoroughly wet — possibly due to the restricted move- 
Jtv. 
ments of soil air and water- 
R 
-and later on, when its value of ~ is greater 
rl 0 
than for either sand or loam, by its disinclination to form a dry surface 
and so attain a high maximum temperature in the surface. It does not 
compare equally with the other soils until after a period of about ten 
days’ dry weather — a state of affairs that is unlikely to occur frequently 
