# Physical Geography Of The Sea 1855/4

CHAPTER IV. — RED FOGS AND SEA DUST.
Where found, § 157. — Tallies on the Wind, 158. — Where taken up, 160. — Humboldt’s Description, 163.—Information derived from Sea Dust, 165. — Its Bearings upon the Theory of Atmospherical Circulation, 167. — Suggests Magnetic Agency, 170.
157. SEAMEN tell us of “red fogs” which they sometimes encounter, especially in the vicinity of the Cape Verde Islands. In other parts of the sea also they meet showers of dust. What these showers precipitate in the Mediterranean is called “sirocco dust,” and in other parts “African dust,” because the winds which accompany them are supposed to come from the Sirocco desert, or some other parched land of the continent of Africa. It is of a brick-red or cinnamon color, and it sometimes comes down in such quantities as to cover the sails and rigging, though the vessel may be hundreds of miles from the land. Now the patient reader, who has had the heart to follow me in the preceding chapters around with “the wind in his circuits,” will perceive that proof is yet wanting to establish it as a fact that the northeast and southeast trades, after meeting and rising up in the equatorial calms, do cross over and take the tracks represented by C and G, PLATE I. Statements, and reasons, and arguments enough have already been made and adduced to make it highly probable, according to human reasoning, that such is the case; and though the theoretical deductions showing such to be the case be never so good, positive proof that they are true can not fail to be received with delight and satisfaction. Were it possible to take a portion of this air, as it travels down the southeast trades, representing the general course of atmospherical circulation, and to put a tally on it by which we could always recognize it again, 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.
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But the air is invisible; and it is not easily perceived how either marks or tallies may-be put upon it, that it may be traced in its paths through the clouds. The skeptic, therefore, who finds it hard to believe that the general circulation is such as Plate I. represents it to be, might consider himself safe in his unbelief were he to declare his willingness to give it up the moment any one should put tallies on the wings of the wind, which would enable him to recognize that air again, and those tallies, when found at other parts of the earth’s surface. As difficult as this seems to be, it has actually been done. Ehrenberg, with his microscope, has established, almost beyond a doubt, that the air which the southeast trade-winds bring to the equator does rise up there and pass over into the northern hemisphere.
158. The Sirocco, or African dust, which he has been observing so closely, has turned out to be tallies put upon the wind in the other hemisphere; and this beautiful instrument of his enables us to detect the marks on these little tallies as plainly as though those marks had been written upon labels of wood and tied to the wings of the wind. This dust, when subjected to microscopic examination, is found to consist of infusoria and organisms whose habitat is not Africa, but South America, and in the southeast trade-wind region of South America. Professor Ehrenberg has examined specimens of sea dust from the Cape Verde and the regions thereabout, from Malta, Genoa, Lyons, and the Tyrol; and he has found a similarity among them as striking as it would have been had these specimens been all taken from the same pile. South American forms he recognizes in all of them; indeed, they are the prevailing forms in every specimen he has examined. It may, I think, be now regarded as an established fact, that there is a perpetual upper current of air from South America to North Africa; and that the volume of air which flows to the northward in these upper currents is nearly equal to the volume which flows to the southward with the northeast trade-winds, there can be no doubt. The “rain dust” has been observed most frequently to fall in spring and autumn; that is, the fall has occurred after the equinoxes, but at intervals from them varying from thirty to sixty
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days, more or less. To account for this sort of periodical occurrence of the falls of this dust, Ehrenberg thinks it “necessary to suppose a dust-cloud to be held constantly swimming in the atmosphere by continuous currents of air, and lying in the region of the trade-winds, but suffering partial and periodical deviations.” It has already been shown (§ 128) that the rain or calm belt between the trades travels up and down the earth from north to south, making the rainy season wherever it goes. The reason of this will be explained in another place.
159. This dust is probably taken up in the dry, and not in the wet season; instead, therefore, of its being “held in clouds suffering partial and periodical deviations,” as Ehrenberg suggests, it more probably comes from one place about the vernal, and from another about the autumnal equinox; for places which have their rainy season at one equinox have their dry season at the other.
160. At the time of the vernal equinox, the valley of the Lower Oronoco is then in its dry season — everything is parched up with the drought; the pools are dry, and the marshes and plains arid wastes. All vegetation has ceased; the great serpents and reptiles have buried themselves for hibernation; [Humboldt ] the hum of insect life is hushed, and the stillness of death reigns through the valley. Under these circumstances, the light breeze, raising dust from lakes that are dried up, and lifting motes from the brown savannas, will bear them away like clouds in the air.
This is the period of the year when the surface of the earth in this region, strewed with impalpable and feather-light remains of animal and vegetable organisms, is swept over by whirlwinds, gales, and tornadoes of terrific force; this is the period for the general atmospheric disturbances which have made characteristic the equinoxes. Do not these conditions appear sufficient to afford the “rain dust” for the spring showers?
161. At the period of the autumnal equinox, another portion of the Amazonian basin is parched with drought, and liable to winds that fill the air with dust, and with the remains of dead animal and vegetable matter; these impalpable organisms, which each rainy season calls into being, to perish the succeeding season of drought, are perhaps distended and made even lighter by the gases of decomposition which has been going on in the period of drought.
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162. May not, therefore, the whirlwinds which accompany the vernal equinox, and sweep over the lifeless plains of the Lower Oronoco, 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 Oronoco and the great Amazonian basin for the showers of October?
163. The Baron von Humboldt, in his “Aspects of Nature”, thus contrasts the wet and the dry seasons there: “When, under the vertical rays of the never-clouded sun, the carbonized turfy covering falls into dust, the indurated soil cracks asunder as if from the shock of an earthquake. If at such times two opposing currents of air, whose conflict produces a rotary motion, come in contact with the soil, the plain assumes a strange and singular aspect. Like conical-shaped clouds, the points of which descend to the earth, the sand rises through the rarefied air on the electrically-charged centre of the whirling current, resembling the loud water-spout, dreaded by the experienced mariner.
The lowering sky sheds a dim, almost straw-colored light on the desolate plain. The horizon draws suddenly nearer, the steppe seems to contract, and with it the heart of the wanderer. The hot, dusty particles which fill the air increase its suffocating heat, and the east wind, blowing over the long-heated soil, brings with it no refreshment, but rather a still more burning glow. The pools which the yellow, fading branches of the fan-palm had protected from evaporation, now gradually disappear. As in the icy north the animals become torpid with cold, so here, under the influence of the parching drought, the crocodile and the boa become motionless and fall asleep, deeply buried in the dry mud...... “The distant palm-bush, apparently raised by the influence of the contact of unequally heated and therefore unequally dense strata of air, hovers above the ground, from which it is separated by a narrow intervening margin. Half concealed by the dense clouds of dust, restless with the pain of thirst and hunger, the horses and cattle roam around, the cattle lowing dismally, and the horses stretching out their long necks and snuffing the wind, if haply a moister current may betray the neighborhood of a not wholly dried-up pool...... “At length, after the long drought, the welcome season of the rain arrives; and then how suddenly is the scene changed!.....
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“Hardly has the surface of the earth received the refreshing moisture, when the previously barren steppe begins to exhale sweet odors, and to clothe itself with killingias, the many panicles of the paspulum, and a variety of grasses. The herbaceous mimosas, with renewed sensibility to the influence of light, unfold their drooping, slumbering leaves to greet the rising sun; and the early song of birds and the opening blossoms of the water plants join to salute the morning.”
164. The color of the “rain dust,” when collected in parcels and sent to Ehrenberg, is “brick-red,” or “yellow ochre;” when seen by Humboldt in the air, it was less deeply shaded, and is described by him as imparting a “straw color” to the atmosphere. In the search of spider lines for the diaphragm of my telescopes, I procured the finest and best threads from a cocoon of a mud-red color; but the threads of this cocoon, as seen singly in the diaphragm, were of a golden color; there would seem, therefore, no difficulty in reconciling the difference between the colors of the rain dust, when viewed in little piles by the microscopist, and when seen attenuated and floating in the wind by the great traveler. It appears, therefore, that we here have placed in our hands a clew, which, attenuated and gossamer-like though it at first appears, is nevertheless palpable and strong enough to guide us along the “circuits of the wind” till we enter “the chambers of the south.”
165. The frequency of the fall of “rain dust” between the parallels of 17º and 25º north, and in the vicinity of the Cape Verde Islands, is remarked upon with emphasis by the microscopist. It is worthy of remark, because, in connection with the investigations at the Observatory, it is significant.
166. The latitudinal limits of the northern edge of the northeast trade-winds are variable. In the spring they are nearest to the equator, extending sometimes at this season not farther from the equator than the parallel of 15º north.
167. The breadth of the calms of Cancer is also variable; so also are their limits. The extreme vibration of this zone is between the parallels of 170 and 38º north, according to the season of the year. According to the hypothesis (§ 42) suggested by my researches,
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this is the zone in which the upper currents of atmosphere that ascended in the equatorial calms, and flowed off to the northward and eastward, are supposed to descend. This, therefore, is the zone in which the atmosphere that bears the “rain dust,” or "African sand," descends to the surface; and this, therefore, is the zone, it might be supposed, which would be the most liable to showers of this “dust.” This is the zone in which the Cape Verde Islands are situated; they are in the direction which theory gives to the upper current of air from the Oronoco and Amazon with its “rain dust,” and they are in the region of the most frequent showers of “rain dust,” all of which are in striking conformity with this theory as to the circulation of the atmosphere. It is true that, in the present state of our information, we can not tell why this “rain dust” should not be gradually precipitated from this upper current, and descend into the stratum of trade winds, as it passes from the equator to higher northern latitudes; neither can we tell why the vapor which the same winds carry along should not, in like manner, be precipitated on the way; nor why we should have a thunder-storm, a gale of wind, or the display of any other atmospherical phenomenon to-morrow, and not to-day: all that we can say is, that the conditions of to-day are not such as the phenomenon requires for its own development.
168. Therefore, though we can not tell why the sea dust should not fall always in the same place, we may nevertheless suppose that it is not always in the atmosphere, for the storms that take it up occur only occasionally, and that when up, and in passing the same parallels, it does not always meet with the conditions — electrical and others — favorable to its descent, and that these conditions might occur now in this place, now in that. But that the fall does occur always in the same atmospherical vein or general direction, my investigations would suggest, and Ehrenberg’s researches prove.
169. Judging by the fall of sea or rain dust, we may suppose that the currents in the upper regions of the atmosphere are remarkable for their general regularity, as well as for their general direction and sharpness of limits, so to speak. We may imagine that certain electrical conditions are necessary +o a shower of “sea dust” as well as to a thunder-storm; and that the interval between the time of the equinoctial disturbances
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in the atmosphere and the occurrence of these showers, though it does not enable us to determine the true rate of motion in the general system of atmospherical circulation, yet it assures us that it is not less on the average than a certain rate. I do not offer these remarks as an, explanation with which we ought to rest satisfied, provided other proof can be obtained; I rather offer them in the true philosophical spirit of the distinguished microscopist himself, simply as affording, as far as they are entitled to be called an explanation, that explanation which is most in conformity with the facts before us, and which is suggested by the results of a novel and beautiful system of philosophical research.
170. Thus, though we have tallied the air, and put labels on the wind, to “tell whence it cometh and whither it goeth,” yet there evidently is an agent concerned in the circulation of the atmosphere whose functions are manifest, but whose presence has never yet been clearly recognized.
171. When the air which the northeast trade-winds bring down meets in the equatorial calms that which the southeast trade winds convey, and the two rise up together, what is it that makes them cross? where is the power that guides that from the north over to the south, and that from the south up to the north? The conjectures in the next chapter as to “ the relation between magnetism and the circulation of the atmosphere” may perhaps throw some light upon the answer to this question.