118 THE PHYSICAL GEOGRAPHY OF THE SEA. 



which flows to the southward with the northeast trade-winds, 

 there can be no doubt. 



275. The "rain dust" has been observed most frequently to 

 fall in spring and autumn ; that is, the fall has occurred after the 

 equinoxes, but at intervals from them varying from thirty to sixty 

 days, more or less. To account for this sort of periodical occur- 

 rence of the falls of this dust, Ehrenberg thinks it " necessary to 

 suppose a dust-cloud to he held constantly swimming in the at- 

 mosphere hy continuous currents of ah% and lying in the region 

 of the trade-winds^ hut suffering partial and jperiodical devia- 

 tions.^'' 



276. It has already been shown (§ 188) that the rain or calm 

 belt between the trades travels up and down the earth from north 

 to south, making the rainy season wherever it goes. The reason 

 of this will be explained in another place. 



277. This dust is probably taken up in the dry, and not in the 

 wet season; instead, therefore, of its being "held in clouds suf- 

 fering partial and periodical deviations," as Ehrenberg suggests, 

 it more probably comes from one place about the vernal, and from 

 another about the autumnal equinox ; for places which have their 

 rainy season at one equinox have their dry season at the other. 



278. At the time of the vernal equinox, the valley of the Lower 

 Oronoco is then in its dry season — every thing is parched up with 

 the drought ; the pools are dry, and the marshes and plains be- 

 come arid wastes. All vegetation has ceased ; the great serpents 

 and reptiles have buried themselves for hibernation ;* the hum of 

 insect life is hushed, and the stillness of death reigns through the 

 valley. 



Under these circumstances, the light breeze, raising dust from 

 lakes that are dried up, and lifting motes from the brown savan- 

 nas, will bear them away like clouds in the air. 



279. This is the period of the year when the surface of the earth 

 in this region, strewed with impalpable and feather-light remains 

 of animal and vegetable organisms, is swept over by whirlwinds, 

 gales, and tornadoes of terrific force ; this is the period for the 

 general atmospheric disturbances which have made characteristic 



* Humboldt. 



