Page:Physical Geography of the Sea and its Meteorology.djvu/173

Rh America. Professor Ehrenberg has 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 similarity 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 prevailing 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 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 by continuous currents of air, and lying in the region of the trade-winds, but suffering partial and periodical deviations." 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 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 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