Recent scientific investigations in Europe and America have thrown
some interesting light on the nature of these very curious phenomena.
The results arrived at may be brought familiarly before our readers.
Mr Charles Darwin, in the narrative of his voyage in the _Beagle_,
states that while he was at St Jago, one of the Cape de Verd islands,
in January 1832: 'The atmosphere was generally very hazy; this appears
chiefly due to an impalpable dust, which is constantly falling, even
on vessels far out at sea. The dust,' he goes on to say, 'is of a
brown colour, and under the blow-pipe, easily fuses into a black
enamel. It is produced, as I believe, from the wear and tear of
volcanic rocks, and must come from the coast of Africa.' The same
opinion was held by scientific men generally, as well of the dust met
with in the North Atlantic, as of that which sometimes falls on the
islands and shores of the Mediterranean: Africa was supposed to be the
original source of the air-borne particles. Some of the dust, however,
having been sent to Ehrenberg of Berlin, that celebrated _savant_,
after a microscopical examination, laid an account of his inquiry
before the Akademie der Wissenschaften, in May 1844, in which he
shewed that the dust, so far from being inorganic, contained numerous
specimens of a species of flint-shelled animalcules, or infusoria,
known as polygastrica, and minute portions of terrestrial plants. The
investigation led him to certain conclusions: '1. That meteoric
dust-rain is of terrestrial origin. 2. That the same is not a rain of
volcanic ashes. 3. That it is necessarily a dust carried up to a great
height by a strong current of air or whirlwind from a dried-up
swamp-region. 4. That the dust neither demonstrably nor necessarily
comes from Africa, notwithstanding that the wind may blow from thence
as the nearest land when the dust falls, because there are in it no
forms whatsoever exclusively native to Africa.' These were remarkable
facts, but warranted by the evidence: one, if not more, of the
animalcules was proved to be peculiar to America, and that country was
naturally inferred to be the quarter from which they had been derived.
The inquiry once begun was followed up; other specimens of dust were
submitted to the same critical test, and found generally to contain a
much greater number and variety of infusoria than the first--mostly
fresh-water forms, but with a few of marine origin; whence the
conclusion, that they had been brought from a coast-region; and
especially remarkable was the fact, that among all the forms there was
not one peculiar to the African continent. One example was known to
belong to the Isle of France, the others were chiefly South American.
After an examination of six specimens, obtained at different
intervals, Ehrenberg discovered that they contained four organisms in
common. 'I now consider myself,' he observes, 'justified in the
conclusion, that all the Atlantic dust may come only from one and the
same source, notwithstanding its extent and annual amount. The
constant yellow and reddish colour of the dust, produced by
ferruginous matter, its falling with the trade-winds and not with the
harmattan, increase the interest of the phenomena.'
It had always been supposed, that the dust which traversed the
Mediterranean was borne from the Great Sahara; but in a quantity
collected on board the ship _Revenge_, at Malta, an infusoria peculiar
to Chili was met with, which, with other characteristics, proved the
dust to be the same as that observed on the Atlantic. Their colour,
too, was identical; while the Sahara is a 'dazzling white sand:' hence
the dust brought across the Mediterranean by the sirocco was not
peculiar to Africa. The conclusion here arrived at was still further
verified by another sirocco-storm in May 1846, which extended to
Genoa, and bore with it a dust that 'covered the roofs of the city in
great abundance.' This, as was clearly ascertained, contained
formations identical with those which had been collected off the Cape
de Verd; and it was shewn that the dust-showers of the Atlantic, and
those of Malta and Genoa, were 'always of a yellow ochre-like
colour--not gray, like those of the kamsin, in North Africa.' The
peculiar colour of the dust was found to be caused by iron-oxide; and
from one-sixth to one-third of the whole proved to consist 'of
determinable organic parts.' In the following year, 1847, Ehrenberg
had another opportunity of testing his conclusions, in specimens of
dust which had fallen in Italy and Sicily in 1802 and 1813; the same
result came out on examination; 'several species peculiar to South
America, and none peculiar to Africa.'
Thus, omitting the two last-mentioned instances, there had been five
marked falls of dust between 1830 and 1846; how many others passed
without notice, it would now be impossible to ascertain. The showers
sometimes occur at a distance of 800 miles from the coast of Africa,
and this region lies between the parallels of 17 and 25 degrees north
latitude, and whence, as we have seen, they extend to the northern
shores of the Mediterranean. In the dust collected from these various
falls, there have been found altogether nineteen species of infusoria;
of which eight were polythalamia, seven polygastrica, and two
phytolitharia, these chiefly constituting the flint-earth portion of
the dust. The iron was composed of the gaillonilla, and 'the carbonic
chalk earth corresponded tolerably well to the smaller number of
polythalamia.' The uniform character of the specimens obtained at
intervals over so long a course of years is especially remarkable.
To turn, now, for a few moments to the second phenomenon indicated in
our title. In October 1846, a fearful and furious hurricane visited
Lyon and the district between that city and Grenoble, during which
occurred a fall of blood-rain. A number of drops were caught and
preserved, and when the moisture had evaporated, there was seen the
same kind of dust--of yellowish-brown or red colour--as that which had
fallen in a dry state on the occasions already referred to. The
strictest pains were taken to ascertain that it was not the common
dust swept from roads during a gale of wind; and when placed under the
microscope, it exhibited a greater proportion of fresh-water and
marine formations than the former instances. Phytolitharia were
numerous, as also 'neatly-lobed vegetable scales;' which, as Ehrenberg
observes, is sufficient to disprove the assertion, that the substance
is formed in the atmosphere itself, and is not of European origin. For
the first time, a living organism was met with--the '_Eunotia
amphyoxis_, with its ovaries green, and therefore capable of life.'
Here was a solution of the mystery: the dust, mingling with the drops
of water falling from the clouds, produced the red rain. Its
appearance is that of reddened water, and it cannot be called
blood-like without exaggeration.
Again, in March 1847, a coloured snow fell in the Tyrol, presenting a
most singular appearance, and, when dried, leaving behind a
brick-coloured dust. Most of the organised forms therein contained
were European and American, with a few African; and again the
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
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 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.'
It is scarcely possible not to feel that the investigations here
briefly sketched, possess unusual interest. As Ehrenberg says, the
subject is one 'of vast, manifold, and rapidly-increasing importance,
and is but the beginning of a future great department of knowledge.'
Now that it has been published in a connected form, and the attention
of scientific observers directed to it, we may hope soon to hear of
corroborative evidence from all parts of the world. We may mention, as
bearing on the question, that sand-showers are not unfrequent in
China. Dr M'Gowan of Ningpo, in a communication to the Asiatic Society
of Bengal, states, that at the beginning of 1851, three showers
occurred within five weeks; the last, which commenced on the 26th
March, and continued four days, being the heaviest. The wind during
the time varied from north-east to north-west, the breeze interrupted
by occasional calms. No rain had fallen for six weeks; and though, as
the doctor observes, 'neither cloud, fog, nor mist obscured the
heavens, yet the sun and moon were scarcely visible; the orb of day
appeared as if viewed through a smoked glass, the whole sky presenting
a uniform rusty hue. At times, this sameness was disturbed, exhibiting
between the spectator and the sun the appearance of a water-spout,
owing to the gyratory motions of the impalpable mineral. The sand
penetrated the most secluded apartments; furniture wiped in the
morning, would be so covered with it in the afternoon, that one could
write on it legibly. In the streets, it was annoying--entering the
eyes, nostrils, and mouth, and grating under the teeth. My ophthalmic
patients generally suffered a relapse, and an unusual number of new
cases soon after presented themselves. Were such heavy sand-storms of
frequent occurrence, diseases of the visual organs would prevail to a
destructive extent.'
These showers sometimes spread over several provinces at once, and far
out to sea. The Chinese call them yellow-sand. Their source is the
great desert of Gobi, or Sand-Ocean, more than 2000 miles long, and
from 300 to 400 broad, in the interior of Asia. Dr M'Gowan states,
that the fall amounted to ten grains per square foot, but without
specifying whether this quantity includes the whole duration of the
shower. During calms, it remains suspended. The dust thus raised from
the Mongolian steppes gives the peculiar tinge to the Yellow Sea.
Notwithstanding the annoyance of these dust-showers, they have a
valuable compensation. The Chinese, whose closeness of observation in
agricultural matters is well known, assert that they are always
followed by a fruitful season--not, it is true, as cause, but as
effect. The explanation is, that the soil of the provinces most
subject to the visitation, being of a compact character, is loosened
and lightened by the sand borne on the wind from the Tatarian plains,
and at the same time, the lighter fertilising matters carried away by
the great rivers are replaced; and thus, that which at first sight
appears an unmitigated evil, becomes the cause of good harvests, for
they invariably follow a fall of sand.