RED FOGS AND SEA BREEZES. 147 



America. Professor Ehrenberg Las examined specimens of sea- 

 dust from the Cape de Verds and the regions thereabout — from 

 Malta, Genoa, Lyons, and the Tyrol — and he has found a simi- 

 larity among them as striking as it would have been, had these 

 specimens been all taken from the same spot. South American 

 forms he recognizes in all of them ; indeed, they are the pre- 

 vailing forms in every specimen he has examined. It may, I 

 think, be now regarded as an established fact that there is a 

 perpetual upper current of air from South America to North 

 Africa; and that the volume of air which flows to the noithward 

 in these upper currents is nearly equal to the volume which 

 flows to the southward with the noii:h-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 thirt}^ 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 swim- 

 ming in the atmosphere by continuous currents of air, and lying in the 

 region of the trade-winds, hut suffering partial and periodical devior- 

 tions." 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 and back again, making the rainy season wher- 

 ever 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 

 sufiering partial and periodical deviations," as Ehrenberg sug- 

 gests, it more probably comes fi'om 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. At the time of the vernal equinox, the valley of the 

 Lower Orinoco is then in its dry season — everything is parched 

 up with the drought; the pools are dry, and the marshes and 

 plains become 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 the bed of lakes that are dried up, and lifting 

 motes from the brown savannas, will bear them away like clouds 

 in the air. This is the period of the year when the surface of 



* Humboldt. 



L 2 



