THE IMPENETRABLE SEA 



be carried round by the earth's spinning motion as 

 though it were a soHd shell. But it is in a state of turbu- 

 lent motion everywhere as the result of the complicated 

 interplay of several forces. 



Most powerful of these is the sun's energy. Other forces 

 acting upon the atmosphere and supplementing the sun's 

 power are the gravitational 'Epulis" of the moon and the 

 sun (although the latter influence is slight) ; the centri- 

 fugal (or * 'throwing off") effect of the earth's rotation; 

 and the ascending and descending movements caused by 

 the natural law that heated air rises while chilled air falls 

 through the surrounding atmosphere. These up-and- 

 down movements contribute very largely to the hori- 

 zontal ones; for wherever the air ascends or descends 

 other currents rush in laterally to replace the rising or 

 falling columns. 



As the surfaces of the earth and sea are heated by the 

 sun's energy, air rises from them which carries water- 

 vapour from the moister areas of the land, or from the 

 seas, rivers and streams, and this water-vapour rises until 

 it reaches colder zones, where it is chilled and forms 

 clouds. On the other hand, wherever excess volumes of 

 air may have piled up, or where the air currents may 

 have cooled and therefore become heavier, masses of 

 air may descend towards the earth's surfaces, again 

 causing lateral winds. Such falling masses of air grow 

 warmer by compression, and as they absorb more water- 

 vapour the clouds tend to dissipate as they are affected 

 by them, usually causing clearer skies. Rising and con- 

 verging (low pressure) air currents are therefore accom- 

 panied by cloud-accumulation and rain, while descend- 

 ing and diverging (high pressure) currents are associated 

 with lack of rain and cloud-dissipation. 



The atmosphere is an invisible * 'ocean", and has its 

 waves and tides. The atmospheric tides are of two kinds. 

 One kind is due to the attraction of the sun and moon, 

 and is therefore similar to the oceanic tides. But this tidal 



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