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.'

It is scarcely possible not to feel that the investigations here
briefly sketched, possess unusual interest. As Ehrenberg says, the