very
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 constrained--on account of the above-mentioned
particulars, and in so far as they justify a conclusion--to 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 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 t