THE TRADE WINDS. 315 



est atmospheric movement is that of the Trade 

 Winds ; and we shall find in this phenomenon 

 another illustration on the large scale of the 

 fact of inequality of temperature producing 

 movements in large masses of air. There are 

 particular regions of the globe whose tempera- 

 ture is in the most violent contrast, as those of 

 the equator and the poles. Here, therefore, we 

 have all the conditions necessary for the pro- 

 duction of motion in the air. Under the beams 

 of a tropical sun, the equatorial regions become 

 heated to a high degree and over a vast area. 

 The result is, that an enormous body of air 

 rises from these regions, the place of which 

 must be supplied from cooler parts. Hence, if 

 the earth were not in revolution on its own 

 axis, a cold current would flow from both poles 

 directly to the equator, there rise with the 

 ascending heated current, and turning over, it 

 would proceed to the poles in the higher strata, 

 and would again descend to pursue the same 

 course. This is clearly indicated in the accom- 

 panying diagram. 



But the earth revolves on its axis. In so 

 doing it carries its atmosphere with it. The 

 equatorial regions being further from the cen- 

 tral axis of the earth, are in more rapid motion 

 than the polar, just as the rim of a wheel moves 

 faster in the same time than any part of the 



