250 



THE ATMOSPHEEE AND METEOEOLOGT. 



the poles towards the burning latitudes of the equator. The zone 

 where this shock of the winds occurs is considered as the outer limit 



Fig. 100,— Variations in the Trade- Winds. 



of the trade- winds ; but it is incorrect to give it the name of the zone 

 of tropical calms, for if the complete equilibrium of the atmosphere 

 is more frequent there than in the regions bordering on the north 

 and south, yet, nevertheless, the calms hardly last more than a day in 

 the space of two or three weeks.* During the summer of the north- 

 ern hemisphere, when the sun is at the zenith of the tropic of Cancer, 

 the counter trade-winds may make themselves felt with tolerable 

 regularity, as far as the latitude of the north of Germany, or of St 

 Petersburg. In autumn and in winter the domain of these returning 

 currents is unceasingly restricted towards the north, and increases to 

 the south. Brest and then Lisbon are its extreme limits in the 

 northern hemisphere, till the sun resumes its march to the north. 



Why does the upper current thus descend from the heights of the 

 atmosphere during the greater part of the year ? Doubtless, because 

 it carries with it enormous quantities of watery vapour, which renders 

 it heavier than the cold dry air from the poles. Owing to its tem- 

 perature, it first rises higher than the Cordilleras, then, being 

 gradually cooled, it sinks under the weight of moisture that satu- 

 * Lartigue, Maury, Tilot Charts. 



