88 TERRESTRIAL ADAPTATIONS. 



nance of the vapoury state, some of the steam 

 will be condensed and will become water. It is 

 in this manner that the curl of steam from the 

 spout of a boiling tea-kettle becomes visible, 

 being cooled down as it rushes to the air. The 

 steam condenses into a fine watery powder, 

 which is carried about by the little aerial cur- 

 rents. Clouds are of the same nature with such 

 curls, the condensation being generally produced 

 when air, charged with aqueous vapour, is mixed 

 with a colder current, or has its temperature 

 diminished in any other manner. 



Clouds, while they retain that shape, are of 

 the most essential use to vegetable and animal 

 life. They moderate the fervour of the sun, in 

 a manner agreeable, to a greater or less degree, 

 in all climates, and grateful no less to vegetables 

 than to animals. Duhamel says that plants 

 grow more during a week of cloudy weather 

 than a month of dry and hot. It has been ob- 

 served that vegetables are far more refreshed by 

 being watered in cloudy than in clear weather. 

 In the latter case, probably the supply of fluid 

 is too rapidly carried off by evaporation. Clouds 

 also moderate the alternations of temperature, 

 by checking the radiation from the earth. The 

 coldest nights are those which occur under a 

 cloudless winter sky. 



The uses of clouds, therefore, in this stage of 

 their history, are by no means inconsiderable, 



