THE CLOUDS 91 



we know what it is, we must be content to admire and 

 wonder at their marvellous beauty, assured that their 

 use is no less wonderful. One thing more must be 

 noticed before leaving this interesting series of cloud- 

 forms, and that is the important part played by the 

 low-lying clouds in preventing excessive radiation, 

 that is, in stopping the heat which has been absorbed 

 by the earth during the day from the sun's rays from 

 escaping too rapidly into the air. The process is 

 familiar to most of us. 3ow often do we say, when 

 the sky is very clear and the stars twinkle, not a cloud 

 being seen, " It will be frosty to-night." We realize 

 the effect while not thinking of the cause. The work 

 of the cloud is similar to that of the cosy on the tea- 

 pot (a most pernicious institution, by the way, and a 

 dire agent of indigestion), or the blanket on the bed 

 not to keep the cold out, according to the common 

 error, but to keep the heat in ; in scientific words, to 

 hinder radiation. Again, how frequently do we com- 

 plain, on a cloudy summer day, of the weather being 

 close or muggy, when we might (and do) wonder that, 

 the sun's rays being shut out from us, it is not cooler 

 than usual. Well, in the first place, the vaporous 

 cloud is co-operating with the moisture arising from 

 the earth to keep the air damp, while it prevents the 

 heat from escaping into the upper regions, and so we 

 are subjected to that most depressing of all physical 

 conditions, moist heat. 



These gentle, amiable forms of cloud are of the 

 summer, as the lightness of their appearance would 

 denote. They belong to bright and sunny conditions, 

 and as such are, however unconsciously, beloved by 

 us. But as the night is as beneficial in its way as 



