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 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