or some other parched land of the continent of
Africa. Professor Ehrenberg calls it "sea-dust."
It is of a brick-red or cinnamon colour; and it
sometimes comes down in such quantities as to
obscure the sun, darken the horizon, prevent a
ship at mid-day from being seen beyond a quarter
of a mile, and cover the sails and rigging with
a coating of dust, though the vessel may be
hundreds of miles from the land.

Now, were it possible to take a portion of air,
as it travels in the general course of
atmospherical circulation, and to put a tally on it, by
which we could follow it and always recognise
it, then we might hope actually to prove by
evidence the most positive the channels through
which the air of the trade winds, after ascending
at the equator, returns whence it came. But
the air is invisible; and it is not easily perceived
how marks may he put upon it, that it may be
traced in its path through the clouds. As
difficult as this seems to be, it has actually been
done. Ehrenberg's microscope has established
almost beyond a doubt, that the air, which the
south-east trade winds bring to the equator,
does rise up there and pass over into the northern
hemisphere. The Sirocco or African dust has
turned out to be tallies put upon the wind in
the other hemisphere, as plainly as though marks
had been written upon labels of wood and tied
to the wings of the wind.

This dust when examined with the microscope,
is found to consist of infusoria and
organisms whose habitat is not Africa, but South
America, and moreover in the south-east trade
wind region of South America. Specimens of
sea-dust from the Cape de Verd and the regions
thereabouts—from Malta, Genoa, Lyons, and
the Tyrol—are found to have a similarity as
striking as if all of them had been taken from
the very same spot. South American forms
prevail in every specimen examined. The dust is
probably taken up at two remarkable periods of
the year.  The vernal equinox is the dry season
of the valley of the Lower Orinoco. Everything is
parched up with drought; the pools are dry,
and the marshes and plains become arid wastes.
All vegetation has ceased; the great serpents
and reptiles have buried themselves for hibernation;
the hum of insect life is hushed, and the
stillness of death reigns through the valley.
The light breeze, raising dust from the bed of
dried-up lakes and lifting motes from the brown
savannahs, bears them away, like clouds, in
the air. The surface of the earth, strewed
with impalpable and feather-light remains of
animal and vegetable matter, is swept over
by terrific whirlwinds, gales, and tornadoes.
At the autumnal equinox, another portion of
the Amazonian basin is parched with drought,
and liable to winds that fill the air with dust,
consisting of the impalpable organisms which
each rainy season calls into being, to perish with
the succeeding drought. If, at such times, two
opposing currents of air, whose conflict produces
a rotatory motion, come in contact with the soil,
the plain assumes a strange and singular aspect.
The sand rises in inverted conical clouds whose
points touch the earth, through the rarefied air
of the whirling current, resembling waterspouts
at sea. The lowering sky sheds a straw-coloured
light on the desolate plain; the hot dusty
particles which fill the air increase its suffocating
heat; the horizon suddenly draws nearer, and
the steppe seems to contract, and with it the
heart of the wanderer.

We cannot pretend to prescribe the conditions
requisite for bringing the dust-cloud down
to the earth. The radiation of heat from smoke-
dust—as the visible particles of smoke may be
called—has the effect of loading each little atom
of smoke with dew, causing it to descend in the
black fogs of London. Any circumstances,
therefore, which may cause the dust that
ascends as a straw-coloured cloud from the
Orinoco to radiate its caloric and collect
moisture in the sky, may cause it to
descend as a red fog in the Atlantic or
Mediterranean.

As in the ocean, so in the air, there is a
regular system of circulation. "The wind goeth
towards the south, and turneth about unto the
north; it whirleth about continually, and the
wind returneth again according to his circuits."
We have, extending entirely round the earth.
two zones of perpetual winds, i.e. the zone of
north-east trades on this side, and of south-east
on that. With slight interruptions, these
winds blow perpetually, and are as steady and
as constant as the currents of the Mississipi
river, always moving in the same direction,
except when they are turned aside by a desert or a
rainy region here and there to blow as monsoons,
or as land and sea breezes. As these two main
currents of air are constantly flowing from the
poles towards the equator, we are safe in assuming
that the air which they keep in motion must
return by some channel to the place towards the
poles whence it came in order to supply the
trades. This return current must be in the
upper regions of the atmosphere, at least until
it passes over those parallels between which the
trade winds are usually blowing on the surface.
The agents concerned in producing the trade
winds are to be found in the unequal
distribution of land and sea, and rains, as between
the two hemispheres. They derive their power
from heat, it is true; but it is chiefly from the
latent heat of vapour which is set free during
the processes of precipitation. Halley's famous
theory of the trade winds, especially as regards
the cause of their easterly direction, is now
criticised by Captain Maury as not entirely
satisfactory.

Monsoons are, for the most part, trade winds
deflected. When, at stated seasons of the year,
a trade wind is turned out of its regular course,
as from one quadrant to another, it is regarded as
a monsoon; because it blows one half of the year
from one direction, and the other half from an
opposite, or nearly opposite, direction. The time
of the changing of these winds, and their boundaries
at the various seasons of the year, have
been discussed in such numbers and mapped
down in such characters that the navigator