RED FOGS AND SEA BREEZES. 141 



Soutli America to Nortli 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-^vinds, there can be no doubt. The "rain dust "has 

 been observed most fi'equently to fall in spring and autumn; 

 that is, the fall has occurred after the equinoxes, but at intervals 

 from them var3ring 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 sivimming in the atniosj^here hy continuous currents of 

 ah\ and lying in the region of the trade-u'inds, hut suffering j^cir- 

 tial and i^eriodical deviations''' It has already been shown (§ 295) 

 that the rain or calm belt between the trades travels up and do-\vn 

 the earth from north to south and back again, making the rainy 

 reason 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 23robably 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. 

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

 is then in its dry season — everythmg 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 smface 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 tor- 

 nadoes of terrific force : this is the period for the general atmo- 

 <spheric distm^bances which have made characteristic the equinoxes. 

 Do not these conditions appear sufficient to afford the "rain dust" 

 for the spring showers ? At the period of the autumnal equinox, 

 another portion of the Amazonian basin is parched with drought, 

 and liable to winds that fill the air with dust, and with the re- 

 mains of dead animal and vegetable matter: these impalpable 



* Humboldt. 



