274 
Proceedings of Royal Society of Edinburgh. [sess. 
almost identical with that found by Weilenmann from night 
observations. There is a comparatively very small, but distinct 
dependence on the cloudiness, which, by means of the preceding 
fundamental considerations, can easily be proved to be caused by 
differences in the specific heat of the soil in clear and cloudy 
weather. 
The quantity da must show a yearly range, having one maximum 
in midsummer and one minimum in winter ; twice near the 
equinoxes passing through zero. In every month there is a con- 
stant proportion between this quantity and the term for the mean 
solar radiation, a cos cf> cos S, so that we are able, by studying the 
curves at various stations, to give average factors expressing this 
quantity in units of solar radiation. The angle u , which expresses 
the epoch of maximum or minimum of moisture and cloudiness, 
must indicate in the months from April to September, a time very 
close to the culmination of the sun ; and from October to March, a 
time approximating to that of the maximum temperature, in 
accordance with the direct observations and the theory of the 
daily change of cloudiness and absolute humidity of air. 
From fifteen continental stations between 45° and 65° northern 
latitude, the daily curves of temperature of which were published 
by H. Wild in his standard work Die Temper aturverhaltnisse des 
russischen Reiches * I obtained the following values of da and the 
angle u for the months March to October, which show the perfect 
agreement between the theory of temperature and the directly 
observed daily changes of cloud and atmospheric moisture. 
Month. 
da. 
Time of Max. 
or Min. of 
March, . 
o-ooo 
a cos <b cos 8, 
Moisture. 
April, 
+ 0*060 
33 
12.2 p.m. \ 
May, 
+ 0-090 
33 
11.9 A.M. J 
June, 
+ 0*102 
3 ) 
11.9 „ ( 
July, 
+ 0-097 
33 
12.0 Noon/ 
August, . 
+ 0-069 
33 
12.1 P.M. 1 
September, 
+ 0*031 
33 
2 2 J 
October, 
-0*033 
33 
9 ^ 
-i.O ,, 
Maximum. 
Minimum. 
* Supplement!) and zum Repertorium fur Meteor ologie, St Petersburg, 1881. 
