Again, in March 1847, a coloured snow fell in the Tyrol, presenting a most singular appearance, and, when dried, leaving behind a brick-coloured dust. Most of the organised forms therein contained were European and American, with a few African; and again the microscope shewed it to be similar to the dust before examined, leaving no room to suppose it of local origin. 'The predominating forms, numerically, of one kind of dust, are also the predominating forms in all the rest,' as Ehrenberg observes; and says further: 'Impossible as it is to conceive of all the storms now compared from 1830 to 1847, as having a continuous genetic connection, it is equally impossible also to imagine the masses of dust transported by them, with such a degree of similarity, not to have a genetic connection .... The great geographic extent of the phenomenon of a reddish dust nearly filling the atmosphere, and itself filled with organisms so similar, many of which are characteristic of South America, not only admits of, but demands a more earnest attention to the probable cyclical relations in the upper and lower atmosphere, whereby very great masses of fixed terrestrial matter, earths and metals, and especially flint-earths, chalk, iron, and coal, apparently heterogeneous, and yet related by certain peculiarities, are held swimming in the atmosphere, now like clouds thinly spread by whirlwinds or electricity over a broad space, and now condensed, and, like the dust of the fir-blossoms, falling in showers in every direction.'
Ehrenberg, then, states his views as to the cause of the phenomenon. 'Although far from attaching undue weight to a hypothesis, I cannot but consider it a matter of duty to seek for a connection in the facts, and feel myself constrainedon account of the above-mentioned particulars, and in so far as they justify a conclusionto suppose an atmospheric current, connecting America and Africa with the region of the trade-winds, and sometimes, particularly about the 15th and 16th of May, turning towards Europe, and bringing with it this very peculiar, and apparently not African dust, in countless measure. If instead of attacking hypothesis by hypothesis, we strive with united effort to multiply scientific observations, we may then hope for a progressive explanation of these mysterious relations, so especially worthy of study.'
Some progress has already been made by a transatlantic investigator in the explanation so much desired by the distinguished naturalist. Lieutenant Maury, of Washingtonan outline of whose views regarding the winds was given in No. 412 of this Journalfinds in Ehrenberg's researches a beautiful and interesting confirmation of his own theory; namely, that the trade-winds of either hemisphere cross the belt of equatorial calms. Observations at the Peak of Teneriffe have proved that, while the trade-wind is sweeping along the surface of the ocean in one direction, a current in the higher regions of the atmosphere is blowing in the reverse direction. According to Lieutenant Maury, a perpetual upper current prevails from South America
to North Africa, the volume being equal to that which flows southward by the north-east trade-wind. This wind, it should be remembered, does not touch the African continent, but the limits of its northern border are variable; whence the fact, that the falls of dust vary between 17 and 25 degrees of north latitude, as before stated. As the belt of calms shifts its position, so will there be a variation in the locality of the descending atmospheric current.
The dust-showers take place most frequently in spring and autumn; that is, 'after the equinoxes, but at intervals varying from thirty to fifty days;' the cause being, that the equatorial calms, at the time of the vernal equinox, extend to four degrees on either side the equator; and as the rainy season then prevails between those limits, no dust can consequently be taken up in those latitudes. But the same period is the dry season in the valley of the lower Orinoco, and the surface of that extensive region is in a favourable condition to give off dust; and at the time of the autumnal equinox, another part of the great Amazonian basin is parched with drought, on which Lieutenant Maury observes: 'May not, therefore, the whirlwinds which accompany the vernal equinox sweep over the lifeless plains of the lower Orinoco, take up the "rain-dust," which descends in the northern hemisphere in April and Mayand may it not be the atmospherical disturbances which accompany the autumnal equinox, that take up the microscopic organisms from the upper Orinoco and the great Amazonian basin for the showers of October?' Humboldt gives a striking picture of the region in question, and, if the phrase may be permitted, of its dust-producing capabilities; so that the origin of this light powder, as regards one locality, may be said to be placed beyond a doubt.
As yet, the reason why the dust falls, as it were, concretely, and not generally diffused through the atmosphere, is not known; it is one of the obscure points waiting further investigation. Why it should travel so far to fall in a particular spot is, in the present state of our knowledge, not easy to explain. The coarsest dust is generally the first to fall; and it seems clear, that the descent occurs when and where the conditions are favourable. Lieutenant Maury considers, 'that certain electrical conditions are necessary to a shower of dust as well as to a thunder-storm;' and that, in the periodical intervals, we may get a clue to the rate of motion of the upper aerial currents, which appear to be 'remarkable for their general regularity, their general direction, and sharpness of limits.'