20 THE FOUMS OF WATEK IN 



riseSj .like wood plunged into water, through the heaviei 

 air overhead. 



58. When the sunbeams fall upon the sea the water 

 is warmed, though not so much as the land. The 

 warmed water expands, becomes thereby lighter, and 

 therefore continues to float upon the top. This upper 

 layer of water warms to some extent the air in contact 

 with it, but it also sends up a quantity of aqueous 

 vapour, which being far lighter than air, helps the 

 latter to rise. Thus both from the land and from the 

 sea we have ascending currents established by the 

 action of the sun. 



59. When they reach a certain elevation in the 

 atmosphere, these currents divide and flow, part 

 towards the north and part towards the south ; while 

 from the north and the south a flow of heavier and 

 colder air sets in to supply the place of the ascending 

 warm air. 



60. Incessant circulation is thus established in the 

 atmosphere. The equatorial air and vapour flow above 

 towards the north and south poles, while the polar air 

 flows below towards the equator. The two currents of 

 air thus established are called the upper and the lower 

 trade winds. 



61. But before the air returns from the poles great 

 changes have occurred. For the air as it quitted the 

 equatorial regions was laden with aqueous vapour, 

 which could not subsist in the cold polar regions. It is 



