22 Proceedings of the Royal Society of Edinburgh. [Sess. 
being clear only the majority of the night, or, if totally clear, a wind 
blowing. The average error in the forecast under these conditions was 
(a) *30° C., (b) 1*0° C. 
We thus see that the forecast equation does give us the ensuing 
minimum temperature of the surface of the soil to a very close degree of 
approximation when ideal conditions exist ; and, as we should expect, that 
the forecasted minimum is low compared to the observed minimum when 
conditions are not ideal. 
From Table VI we see that the air temperature over open cultivated 
soil follows the surface-soil temperature very closely so long as the soil 
does not freeze ; it appears, therefore, from the above results, that we can 
forecast the minimum temperature over open soil on calm clear nights , 
so far as to say whether there will be a frost or not , with a remarkable 
degree of exactitude. 
(. Issued separately February 3 , 1920 .) 
