122 Proceedings of the Royal Society of Edinburgh. [Sess, 
underground layers are still warm from the summer. This is clearly 
shown in fig. 1. 
Again, radiation is peculiarly hampered in its efforts to reduce the 
temperature of the soil ; not only has it to overcome the up-flowing heat 
conduction from the lower and warmer layers, hut in addition, by the very 
action of freezing the surface, it stores up against itself a reserve of latent 
$ = GROUND DRY AFTER 
DRY SPELL. 
O - GROUND WET AFTER 
RAIN. 
Fig. 1. — The above table is compiled from the observations at the Radcliffe 
Observatory, Oxford, during clear nights of April 1906-1915. 
heat which has to be counterbalanced before any further fall in tempera- 
ture of the soil can take place. The balance only of the radiation, after 
overcoming both these counter-influences, is available for lowering the 
temperature of the soil. 
Thus when the surface is dry and conductivity is reduced, or when the 
temperature of the underground layers is low, the radiation having less 
to counteract has a larger balance left over for cooling the surface. The 
temperature of the surface soil thus depends on : — 
