Matthew Fontaine Maury, Lieutenant, U.S.N.

“ Ehrenberg, with his microscope, has established, almost beyond a doubt, that the air which the southeast trade-winds bring to the equator does rise up there and pass over into the northern hemisphere.
158. The Sirocco, or African dust, which he has been observing so closely, has turned out to be tallies put upon the wind in the other hemisphere; and this beautiful instrument of his enables us to detect the marks on these little tallies as plainly as though those marks had been written upon labels of wood and tied to the wings of the wind. ”

“ This, therefore, is the zone in which the atmosphere that bears the “rain dust,” or "African sand," descends to the surface; and this, therefore, is the zone, it might be supposed, which would be the most liable to showers of this “dust.” This is the zone in which the Cape Verde Islands are situated; they are in the direction which theory gives to the upper current of air from the Oronoco and Amazon with its “rain dust,” and they are in the region of the most frequent showers of “rain dust,” all of which are in striking conformity with this theory as to the circulation of the atmosphere. ”

“ When, under the vertical rays of the never-clouded sun, the carbonized turfy covering falls into dust, the indurated soil cracks asunder as if from the shock of an earthquake. If at such times two opposing currents of air, whose conflict produces a rotary motion, come in contact with the soil, the plain assumes a strange and singular aspect. ”

“ The skeptic, therefore, who finds it hard to believe that the general circulation is such as Plate I. represents it to be, might consider himself safe in his unbelief were he to declare his willingness to give it up the moment any one should put tallies on the wings of the wind, which would enable him to recognize that air again, and those tallies, when found at other parts of the earth’s surface. ”

“ Professor Ehrenberg has examined specimens of sea dust from the Cape Verde 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 established fact, that there is a perpetual upper current of air from South America to North Africa ”

“ It is true that, in the present state of our information, we can not tell why this “rain dust” should not be gradually precipitated from this upper current, and descend into the stratum of trade winds, as it passes from the equator to higher northern latitudes; neither can we tell why the vapor which the same winds carry along should not, in like manner, be precipitated on the way ”

“ Hardly has the surface of the earth received the refreshing moisture, when the previously barren steppe begins to exhale sweet odors, and to clothe itself with killingias, the many panicles of the paspulum, and a variety of grasses. ”

“ 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.
160. At the time of the vernal equinox, the valley of the Lower Oronoco is then in its dry season — everything is parched up with the drought; the pools are dry, and the marshes and plains arid wastes. ”

“ What these showers precipitate in the Mediterranean is called “sirocco dust,” and in other parts “African dust,” because the winds which accompany them are supposed to come from the Sirocco desert, or some other parched land of the continent of Africa. It is of a brick-red or cinnamon color, and it sometimes comes down in such quantities as to cover the sails and rigging, though the vessel may be hundreds of miles from the land. ”