260 THE MOISTURE OF THE AIR; ITS FALL. 



during the winter. In Brazil, however, the rainy 

 season sets in at the same time as the dry season 

 of the northern hemisphere ; and conversely. A 

 great part of this summer rain is driven by the 

 west wind of the upper regions into inland Africa. 

 But since the belt of the calms in the Atlantic 

 always remains on the north side of the equator, it 

 is chiefly in the parts of Africa lying in the north 

 tropic zone that it rains in summer. The south- 

 west of Africa, on the other hand, is remarkable 

 for its extreme dryness. Over the parched soil, 

 too, of Sahara, the atmosphere is seldom cooled 

 down below the dew-point ; there, then, it scarcely 

 ever rains. 



The lower trade, the dry wind, is met with, as 

 you know, in the summer, even to the north of 

 the tropic. In the region, then, of the tropics, it 

 is dry in summer ; but in autumn the upper trade, 

 the rain-wind, comes down gradually lower and 

 lower, and reaches the earth in winter in the lati- 

 tude of the Canary Isles. On the borders, then, 

 of the torrid zone the rainy season answers to the 

 lowest position of the sun. On the north coast, 

 too, of Africa, and in the south of Europe, we 

 find the dry alternating pretty regularly with the 

 wet season ; but the latter becomes shorter as 

 the latitude increases, — becomes curtailed into 

 only a part of the summer, — because the south- 

 west wind reaches the ground earlier the farther 



