274 LIGHT SCIENCE FOR LEISURE HOURS. 



Of variable causes increasing temperature, the 

 principal are a serene sky in summer, and a cloudy 

 sky in winter. It may appear, at first sight, para- 

 doxical to assign opposite effects to a cloudy sky. It 

 must be remembered, however, that clouds considered 

 with reference to temperature, have two functions : 

 they partially prevent the access of heat to the earth, 

 and they partially prevent the escape of heat from the 

 earth. Now, in summer the first-named influence is 

 more important than the second: the days are longer 

 than the nights ; that is, the earth is receiving heat 

 during the greater part of the time in summer. A 

 cause, therefore, which affects the receipt of heat is 

 more important than a cause affecting the escape of 

 heat. On the other hand, in winter the nights are 

 shorter than the days, and the influence of clouds in 

 preventing the escape of heat becomes more important 

 than their effect on the receipt of heat. 1 In fact, we 

 may compare the influence of clouds to the effects of 

 certain kinds of clothing ; flannel, for instance, is as 

 suitable an article of dress for the cricketer as for the 

 skater. 



Now the climate of England is remarkably humid 

 both in winter and summer. And this humidity is 

 shown, not so much by the quantity of rain which falls, 



1 Gilbert White noticed long ago apparently without understanding 

 the influence of a clouded sky on the temperature. ' We have often 

 observed,' he says, ' that cold seems to descend from above ; for, when a 

 thermometer hangs abroad on a frosty night, the intervention of a cloud 

 shall immediately raise the mercury ten degrees ; and a clear sky shall 

 again compel it to descend to its former gauge.' 



