Some progress has already been made by a transatlantic investigator in the explanation so much desired by the distinguished naturalist.  Lieutenant Maury, of Washington—­an outline of whose views regarding the winds was given in No. 412 of this Journal—­finds 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 May—­and 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.’