THE HEASOK WHY. 91 



' He bindeth up the waters in his thick clouds ; and the cloud is not rent under 

 them." JOB xxvi. 



They are probably kept near to the surface of the earth by a 

 superstratum of cold air. A cold air lying above, or a cold air 

 lying below, might equally contribute to keep a fog near the 

 surface of a particular part of the earth, until a flow of wind, or a 

 fall of rain, altered the atmospheric condition. 



There are many interesting facts connected with the history of dew. It has 

 attracted the attention of natural philosophers in all ages. But its true theory 

 was never understood until recently. The ancients imagined that dews were 

 shed from the stars; and the alchemists and physicians of the middle ages 

 believed that the dew distilled by night possessed penetrating and wonder- 

 working powers. The ladies of those times sought to preserve their beauty by 

 washing in dew, which they regarded as a " celestial wash." They collected it 

 by placing upon the grass heaps of wool, upon the threads of which the magio 

 drops clustered. 



CHAPTER XVIII. 



373. What are clouds ? 



Clouds are volumes of vapour, usually elevated to a considerable 

 height. 



Fig. 4. CIEBO-CUMULCS, OE SOXDEB CLOCD. 



374. Whence do clouds arise ? 



From the evaporation of water at the earth's surface. 



375. Wliy do we not see them ascend? 



We do, sometimes, in the form of what we call misti, but 

 generally the vapours that rise and contribute to the formation of 

 clouds are so thin that they are invisible. 



