Various
Chambers's Edinburgh Journal, No. 432
DUST-SHOWERS AND RED-RAIN
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.