§ 325. RED FOGS AND SEA BREEZES. 141 



They tell of a croas- Africa, but South America, and in the southeast 

 belts. trade-wind region of South America. Professor 



Ehrenberg has examined specimens of sea-dust from the Cape de 

 Yerds and the regions thereabout — from Malta, Genoa, Lyons, and 

 the Tyrol — and he has found a similarity among them as striking 

 as it would have been had these specimens been all taken from 

 the same pile. South American forms he recognizes in all of 

 them ; indeed, they are the prevailing forms in every specimen 

 he has examined. It may, I think, be now regarded as an estab- 

 lished fact that there is a perpetual upper current of air from 

 South America to Korth Africa ; and that the volume of air 

 which flows to the northward in these upper currents is nearly 

 equal to the volume which flows to the southward with the north- 

 east trade- winds, there can be no doubt. 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 occurrence of the falls of this 

 dust, Ehrenberg thinks it " necessary to suppose a dust-cloud to he 

 constantly swimming in the atmosphere hy continuous currents of air ^ 

 and lying in the region of the trade-iuinds^ hut suffering partial and 

 periodical deviationsJ^ It has already been shown (§ 295) 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. 

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

 season; instead, therefore, of its being "held in clouds suffering 

 partial and periodical deviations," as Ehrenberg suggests, it more 

 probably comes from one place about the vernal, and from anoth- 

 er about the autumnal equinox ; for places which have their 

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

 At the time of the vernal equinox, the valley of the Lower Ori- 

 noco 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 



* Humboldt. 



