Physical Geography of the Sea and its Meteorology/Chapter 6

by Matthew Fontaine Maury

CHAPTER VI.

§ 311-332.-RED FOGS AND SEA BREEZES.

311. The alternations of land and sea breezes.—The inhabitants of the sea-shore in tropical countries wait every morning with impatience the coming of the sea breeze. It usually sets in about ten o'clock. Then the sultry heat of the oppressive morning is dissipated, and there is a delightful freshness in the air which seems to give new life to all for their daily labours. About sunset there is again another calm. The sea breeze is now done, and in a short time the land breeze sets in. This alternation of the land and sea breeze—a wind from the sea by day and from the land by night—is so regular in intertropical countries, that they are looked for by the people with as much confidence as the rising and setting of the sun.

312. The sea breeze at Valparaiso.—In extra- tropical countries, especially those on the polar side of the trade-winds, this phenomenon is presented only in summer and fall, when the heat of the sun is sufficiently intense to produce the requisite degree of atmospherical rarefaction over the land. This depends in a measure, also, upon the character of the land upon which the sea breeze blows; for when the surface is arid and the soil barren, the heating power of the sun is exerted with most effect. In such cases the sea breeze amounts to a gale of wind. In the summer of the southern hemisphere the Seabreeze is more powerfully developed at Valparaiso than at any other place to which my services afloat have led me. Here regularly in the afternoon, at this season, the sea breeze blows furiously; pebbles are torn up from the walks and whirled about the streets; people seek shelter; the Almendral is deserted, business interrupted, and all communication from the shipping to the shore is cut off. Suddenly the winds and the sea, as if they had again heard the voice of rebuke, are hushed, and there is a great calm.

313. The contrast.—The lull that follows is delightful. The sky is without a cloud; the atmosphere is transparency itself; the Andes seem to draw near; the climate, always mild and soft, becomes now doubly sweet by the contrast. The evening invites abroad, and the population sally forth—the ladies in ball costume, for now there is not wind enough to disarrange the lightest curl. In the southern summer this change takes place day after day with the utmost regularity, and yet the calm always seems to surprise, and to come before one has time to realize that the furious sea wind could so soon be hushed. Presently the stars begin to peep out, timidly at first, as if to see whether the elements here below had ceased their strife, and if the scene on earth be such as they, from their bright spheres aloft, may shed their sweet influences upon. Sirius, or that blazing world Argus, may be the first watcher to send down a feeble ray; then follow another and another, all smiling meekly; but presently, in the short twilight of the latitude, the bright leaders of the starry host blaze forth in all their glory, and the sky is decked and spangled with superb brilliants. In the twinkling of an eye, and faster than the admiring gazer can tell, the stars seem to leap out from their hiding-places. By invisible hands, and in quick succession, the constellations are hung out; but first of all, and with dazzling glory, in the azure depths of space appears the Great Southern Cross. That shining symbol lends a holy grandeur to the scene, making it still more impressive. Alone in the night-watch, after the sea breeze has sunk to rest, I have stood on the deck under those beautiful skies gazing, admiring, rapt. I have seen there, above the horizon at once, and shining with a splendour unknown to these latitudes, every star of the first magnitude—save only six—that is contained in the catalogue of the 100 principal fixed stars of astronomers. There lies the city on the sea-shore wrapped in sleep. The sky looks solid, like a vault of steel set with diamonds. The stillness below is in harmony with the silence above, and one almost fears to speak, lest the harsh sound of the human voice, reverberating through those vaulted "chambers of the south," should wake up echo, and drown the music that fills the soul. On looking aloft, the first emotion gives birth to a homeward thought: bright and lovely as they are, those, to northern sons, are not the stars nor the skies of fatherland. Alpha Lyræ, with his pure white light, has gone from the zenith, and only appears for one short hour above the top of the northern hills. Polaris and the Great Bear have ceased to watch from their posts; they are away down below the horizon. But, glancing the eye above and around, you are dazzled with the splendours of the firmament. The moon and the planets stand out from it; they do not seem to touch the blue vault in which the stars are set. The Southern Cross is just about to culminate. Climbing up in the east are the Centaurs, Spica, Bootes, and Antares, with his lovely little companion, which only the best telescopes have power to unveil. These are all bright particular stars, differing from one another in colour as they do in glory. At the same time, the western sky is glorious with its brilliants too. Orion is there, just about to march down into the sea; but Canopus and Sirius, with Castor and his twin-brother, and Procyon, Argus, and Regulus—these are high up in their course; they look down with great splendour, smiling peacefully as they precede the Southern Cross on its western way. And yonder, farther still, away to the south, float the Magellanic clouds, and the "Coal Sacks"—those mysterious, dark spots in the sky, which seem as though it had been rent, and these were holes in the "azure robe of night," looking out in the starless, empty, black abyss beyond. One who has never watched the southern sky in the stillness of the night, after the sea breeze with its turmoil is done, can have no idea of its grandeur, beauty, and loveliness.

314. Land and sea breezes along the shores of intertropical countries.—Within the tropics, however, the land and sea breezes are more gentle, and, though the night scenes there are not so suggestive as those just described, yet they are exceedingly delightful and altogether lovely. The oppressive heat of the sun and the climate of the sea-shore is mitigated and made both refreshing and healthful by the alternation of those winds which invariably come from the coolest place—the sea, which is the cooler by day, and the land, which is the cooler by night. About ten in the morning the heat of the sun has played upon the land with sufficient intensity to raise its temperature above that of the water. A portion of this heat, being imparted to the superincumbent air, causes it to rise, when the air, first from the beach, then from the sea, to the distance of several miles, begins to flow in with a most delightful and invigorating freshness.

315. Cause of land and sea breezes.—When a five is kindled on the hearth, we may, if we will observe the moats floating in the room, see that those nearest to the chimney are the first to feel the draught and to obey it—they are drawn into the blaze. The circle of inflowing air is gradually enlarged, until it is scarcely perceived in the remote parts of the room. Now the land is the hearth, the ray's of the sun the fire, and the sea, with its cool and calm air, the room; and thus we have at our firesides the sea breeze in miniature. "When the sun goes down the fire ceases; then the dry land commences to give off its surplus heat by radiation, so that by dew-fall it and the air above it are cooled below the sea temperature. The atmosphere on the land thus becomes heavier than on the sea, and, consequently, there is a wind seaward which we call the land breeze.

316. Lieut. [Marin H. Jansen|Jansen] on the land and sea breezes in the Indian Archipelago.—"A long residence in the Indian Archipelago, and, consequently, in that part of the world where the investigations of the Observatory at Washington have not extended, has given me," says Jansen,^[1] in his Appendix to the Physical Geography of the Sea, "the opportunity of studying the phenomena which there occur in the atmosphere, and to these phenomena my attention was, in the first place, directed. I was involuntarily led from one research to another, and it is the result of these investigations to which I would modestly give a place at the conclusion of Maury's Physical Geography of the Sea, with the hope that these first-fruits of the log-books of the Netherlands may be speedily followed by more and better. Upon the northern coast of Java, the phenomenon of daily land and sea breezes is finely developed. There, as the gorgeous 'eye of day' rises almost perpendicularly from the sea with fiery ardour, in a cloudless sky, it is greeted by the volcanoes with a column of white smoke, which, ascending from the conical summits high in the firmament above, forms a crown, or assumes the shape of an immense bouquet,"^[2] that they seem to offer to the dawn; then the joyful land breeze plays over the flood, which, in the torrid zone, furnishes, with its fresh breath, so much enjoyment to the inhabitants of that sultry belt of the earth, for, by means of it, everything is refreshed and beautified. Then, under the influence of the glorious accompaniments of the break of day, the silence of the night is awakened, and we hear commencing everywhere the morning hymn of mute nature, whose gesticulation is so expressive and sublime. All that lives feels the necessity of pouring"; forth, each in its way, and in various tones and accents, from the depths of inspiration, a song of praise. The air, still filled with the freshness of the evening dew, bears aloft the enraptured song, as, mingled with the jubilee tones which the contemplation of nature everywhere forces from the soul, it gushes forth in deep earnestness to convey the daily thank-offering over the sea, over hill and dale.^[3]As the sun ascends the sky, the azure vault is bathed in dazzling light; now the land breeze, wearied with play, goes to rest. Here and there it still plays over the water, as if it could not sleep; but finally becoming exhausted, it sinks to repose in the stillness of the calm. But not so with the atmosphere: it sparkles, and glitters, and twinkles, becoming clear under the increasing heat, while the gentle swelling of the now polished waves reflects, like a thousand mirrors, the rays of light which dance and leap to the tremulous but vertical movements of the atmosphere. Like pleasant visions of the night, that pass before the mind in sleep, so do sweet phantoms hover about the land breeze as it slumbers upon the sea. The shore seems to approach and to display all its charms to the mariner in the offing. All objects become distinct and more clearly delineated, ^[4] In the very fine mist of the morning, a noise—for example, the firing of cannon—at a short distance is scarcely heard, while at midday, with the sea-breeze, it penetrates for miles with great distinctness.—Jansen. while, upon the sea, small fishing-boats loom up like large vessels,^[5] The seaman, drifting along the coast, and misled by the increasing clearness and mirage, believes that he has been driven by a current towards the land; he casts the lead, and looks anxiously out for the sea breeze, in order to escape from what he believes to be threatening danger. The planks burn under his feet; in vain he spreads the awning to shelter himself from the broiling sun. Its rays are oppressive; repose does not refresh; motion is not agreeable. The inhabitants of the deep, awakened by the clear light of day, prepare themselves for labour. Corals, and thousands of Crustacea, await, perhaps impatiently, the coming of the sea breeze, which shall cause evaporation to take place more rapidly, and thus provide them with a bountiful store of building material for their picturesque and artfully constructed dwellings: these they know how to paint and to polish in the depths of the sea more beautifully than can be accomplished by any human art. Like them, also, the plants of the sea are dependent upon the winds, upon the clouds, and upon the sunshine: for upon these depend the vapour and the rains which feed the streams that bring nourishment for them into the sea.^[6]When the sun reaches the zenith, and his stern eye, with burning glare, is turned more and more upon the Java Sea, the air seems to fall into a magnetic sleep; yet even as the magnetizer exercises his will upon his subject, and the latter, with uncertain and changeable gestures, gradually puts himself in motion, and sleeping obeys that will, so also we see the slow efforts of the sea breeze to repress the vertical movements of the air, and to obey the will which calls it to the land. This vertical movement appears to be not easily overcome by the horizontal which we call wind. Yonder, far out upon the sea, arises and disappears alternately a darker tint upon the otherwise shining sea-carpet; finally that tint remains and approaches; that is the long-wished-for sea breeze: and yet it is sometimes one, yes, even two hours before the darker tint is permanent, before the sea breeze has regularly set in. Now small white clouds begin to rise above the horizon; to the experienced seaman they are a prelude to a fresh sea breeze. We welcome the first breath from the sea; it is cooling, but it soon ceases; presently it is succeeded by other grateful puffs of air, which continue longer; presently they settle down into the regular sea breeze, with its cooling and refreshing breath. The sun declines, and the sea wind—that is, the common trade-wind or monsoon which is drawn towards the land—is awakened. It blows right earnestly, as if it would perform its daily task with the greatest possible ado. The air, itself refreshed upon the deep, becomes gray from the vapour which envelops the promontories in mist, and curtains the inland with dark clouds. The land is discernible only by the darker tint which it gives to the mist; but the distance cannot be estimated. The sailor thinks himself farther from shore than he really is, and steers on his course carelessly", while the capricious wind lashes the waters, and makes a short and broken sea, from the white caps of which light curls are torn, with sportive hand, to float away like parti-coloured streamers in the sunbeam. In the meanwhile clouds appear now and then high in the air, yet it is too misty to see far. The sun approaches the horizon. Far over the land the clouds continue to heap up; already the thunder is heard among the distant hills; the thunder-bolts reverberate from hill-side to hill-side, while through the mist the sheets of lightning are seen.^[7] Finally, the 'king of day' sinks to rest; now the mist gradually disappears; and as soon as the wind has laid down the lash, the sea, which, chafing and fretting, had with curled mane resisted its violence, begins to go down also. Presently both wind and waves are hushed, and all again is still. Above the sea, the air is clearer or slightly clouded; above the land, it is thick, dark, and swollen. To the feelings, this stillness is pleasant. The sea breeze, the driving brine, that has made a salt-pan of the face, the short, restless sea, the dampness—all have grown wearisome, and welcome is the calm. There is, however, a somewhat of dimness in the air, an uncertain but threatening appearance. Presently, from the dark mass of clouds, which hastens the change of day into night, the thunder-storm peals forth. The rain falls in torrents in the mountains, and the clouds gradually overspread the whole sky. But for the wind, which again springs up, it would be alarming to the sailor, who is helpless in a calm. What change will take place in the air? The experienced seaman, who has to work against the trade-wind or against the monsoon, is off the coast, in order to take advantage of the land breeze (the destroyer of the trade) so soon as it shall come. He rejoices when the air is released from the land and the breeze comes, at first feebly, but afterward growing stronger, as usual during the whole night. If the land breeze meets with a squall, then it is brief, and becomes feeble and uncertain. We sometimes find then the permanent sea breeze close to the coast, which otherwise remains twenty or more English miles from it. One is not always certain to get the land breeze at the fixed time. It sometimes suffers itself to be waited for; sometimes it tarries the whole night long. During the greatest part of the rainy season, the land breeze in the Java Sea cannot be depended upon. This is readily explained according to the theory which ascribes the origin of the sea and land breezes to the heating of the soil by day, and the cooling by means of radiation by night; for, during the rainy season, the clouds extend over land and sea, interrupting the sun's rays by day and the radiation of heat by night, thus preventing the variations of temperature; and from these variations, according to this theory, the land and sea breezes arise. Yet there are other tropical regions where the land and sea breezes, even in the rainy season, regularly succeed each other."

317. Sanitary influences of land and sea breezes.—One of the causes which make the west coast of Africa so very unhealthy when compared with places in corresponding latitudes on the opposite side of the Atlantic, as in Brazil, is no doubt owing to the difference in the land and sea breezes on the two sides. On the coast of Africa the land breeze is "universally scorching hot." ^[8] There the land breeze is the trade-wind. It has traversed the continent, sucking up by the way disease and pestilence from the dank places of the interior. Reeking with miasm, it reaches the coast. Peru is also within the trade-wind region, and the winds reach the west coast of South America, as they do the west coast of Africa, by an overland path; but, in the former case, instead of sweeping over dank places, they come cool and fresh from the pure snows of the Andes. Between this range and the coast, instead of marshes and a jungle, there is a desert—a rainless country, upon which the rays of the sun play with sufficient force not only to counteract the trade-wind power and produce a calm, but to turn the scale, and draw the air back from the sea, and so cause the sea breeze to blow regularly.

318. Influences which regulate their strength.—On the coast of Africa, on the contrary, a rank vegetable growth screens the soil from the scorching rays of the sun, and the rarefaction is not every day sufficient to do more than counteract the trade-wind force and produce a calm. The same intensity of ray, however, playing upon the intertropical vegetation of a lee-shore, is so much force added to the sea breeze; and hence, in Brazil, the sea breeze is fresh, and strong, and healthful; the land breeze feeble, and therefore not so sickly. Thus we perceive that the strength as well as regularity of the land and sea breezes not only depend upon the topography of a place, but also upon its situation with regard to the prevailing winds; and also that a given difference of temperature between land and water, though it may be sufficient to produce the phenomena of land and sea breezes at one place, will not be adequate to the same effect at another; and the reason is perfectly philosophical.

319. Land breezes from the west coast of Africa scorching hot.—It is easier to obstruct and turn back the current in a sluggish than in a rapid stream. So, also, in turning a current of air first upon the land, then upon the sea—very slight alternations of temperature would suffice for this on those coasts where calms would prevail were it not for the land and sea breezes, as for instance, in and about the region of equatorial calms; there the air is in a state of rest, and will obey the slightest call in any direction; not so in regions where the trades blow over the land, and are strong. It requires, under such circumstances, a considerable degree of rarefaction to check them and produce a calm, and a still farther rarefaction to turn them back, and convert them into a regular sea breeze. Hence the scorching land breeze (§ 317) on the west coast of Africa: the heat there may not have been intense enough to produce the degree of rarefaction required to check and turn back the south-east trades. In that part of the world, their natural course is from the land to the sea, and therefore, if this view be correct, the sea breeze should be more feeble than the land breeze, neither should it last so long. 320. Land breeze in Brazil and Cuba.—But on the opposite side—on the coast of Brazil, as at Pernambuco, for instance—where the trade-wind comes from the sea, we should have this condition of things reversed, and the sea breeze will prevail for most of the time—then it is the land breeze which is feeble and of short duration: it is rarely felt. Again, the land and sea breezes in Cuba, and along the Gulf shores of the United States, will be more regular in their alternations than they are along the shores of Brazil or South Africa, and for the simple reason that the Gulf shores lie nearly parallel with the prevailing direction of the winds. In Rio de Janeiro, the sea breeze is the regular trade-wind made fresher by the daily action of the sun on the land. It is worthy of remark, also, that, for the reason stated by Jansen, the land and sea breezes in the winter time are almost unknown in countries of severe cold, though in the summer the alternation of wind from land to sea, and sea to land, may be well marked.

321. Night scenes when sailing with the land breeze.—"Happy he," remarks Jansen, "who, in the Java Sea at evening, seeking the land breeze off the coast, finds it there, after the salt-bearing, roaring sea wind, and can, in the magnificent nights of the tropics, breathe the refreshing land breeze, ofttimes laden with delicious odours.^[9] The veil of clouds, either after a squall, with or without rain, or after the coming of the land breeze, is speedily withdrawn, and leaves the sky clearer during the night, only now and then flecked with dark clouds floating over from the land. Without these floating clouds the land breeze is feeble. When the clouds float away from the sea, the land breeze does not go far out from the coast, or is wholly replaced by the sea breeze, or, rather, by the trade-wind. If the land breeze continues, then the stars loom forth, as if to free themselves from the dark vault of the heavens, but their light does not wholly vanquish its deep blue, which causes the Coal-sacks to come out more distinctly near the Southern Cross, as it smiles consolingly upon us, while Scorpio, the emblem of the tropical climate, stands like a warning in the heavens. The starlight, which is reflected by the mirrored waters, causes the nights to vie in clearness with the early twilight in high latitudes. "Numerous shooting stars weary the eye, although they break the monotony of the sparkling firmament. Their unceasing motion in the unfathomable ocean affords a great contrast to the seeming quiet of the gently-flowing, aerial current of the land breeze. But at times, when, 30° or 40° above the horizon, a fire-ball arises which suddenly illumines the whole horizon, appearing to the eye the size of the fist, and fading away as suddenly as it appeared, falling into fiery nodules, then we perceive that, in the apparent calm of nature, various forces are constantly active, in order to cause, even in the invisible air, such combinations and combustions, the appearance of which amazes the crews of ships. When the slender keel glides quickly over the mirrored waters upon the wings of the wind, it cuts for itself a sparkling way, and disturbs in their sleep the monsters of the deep, which whirl and dart quicker than an eight-knot ship; sweeping and turning around their disturber, they suddenly clothe the dark surface of the water in brilliancy. Again, when we go beyond the limits of the land breeze, and come into the continuous trade-wind, we occasionally see from the low-moving, round black clouds (unless it thunders), light blue sparks collected upon the extreme points of the iron belaying-pins, etc.;^[10] then the crew appear to fear a new danger, against which courage is unavailing, and which the mind can find no power to endure. The fervent, fiery nature inspires the traveller with deep awe. They who, under the beating of the storm and terrible violence of the ocean, look danger courageously in the face, feel, in the presence of these phenomena, insignificant, feeble, anxious. Then they perceive the mighty power of the Creator over the works of his creation. And how can the uncertain, the undetermined sensations arise which are produced by the clear yet sad light of the moon? she who has always great tears in her eyes, while the stars look sweetly at her, as if they loved to trust her and to share her affliction.^[11] In the latter part of the night the land breeze sinks to sleep, for it seldom continues to blow with strength, but is always fickle and capricious. With the break of day it again awakes, to sport a while, and then gradually dies away as the sun rises. The time at which it becomes calm after the land and sea breezes is indefinite, and the calms are of unequal duration. Generally, those which precede the sea breeze are rather longer than those which precede the land breeze. The temperature of the land, the direction of the coast-line with respect to the prevailing direction of the trade-wind in which the land is situated, the clearness of the atmosphere, the position of the sun, perhaps also that of the moon, the surface over which the sea breeze blows, possibly also the degree of moisture and the electrical state of the air, the heights of the mountains, their extent, and their distance from the coast, all have influence thereon. Local observations in regard to these can afford much light, as well as determine the distance at which the land breeze blows from the coast, and beyond which the regular trade-wind or monsoon continues uninterruptedly to blow. The direction of land and sea winds must also be determined by local observations, for the idea is incorrect that they should always blow perpendicularly to the coast-line. Scarcely has one left the Java Sea—which is, as it were, an inland sea between Sumatra, Borneo, Java, and the archipelago of small islands between both of the last named—than, in the blue waters of the easterly part of the East Indian Archipelago, nature assumes a bolder aspect, more in harmony with the great depth of the ocean. The beauty of the Java Sea, and the delightful phenomena which air and ocean display, have here ceased. The scene becomes more earnest. The coasts of the eastern islands rise boldly out of the water, far in whose depths they have planted their feet. The south-east wind, which blows upon the southern coasts of the chain of islands, is sometimes violent, always strong through the straits which separate them from each other, and this appears to be more and more the case as we go eastward. Here, also, upon the northern coast, we find land breezes, yet the trade-wind often blows so violently that they have not sufficient power to force it beyond the coast. Owing to the obstruction which the chain of islands presents to the south-east trade-wind, it happens that it blows with violence away over the mountains, apparently as the land breeze does upon the north coast;^[12] yet this wind, which only rises when it blows hard from the south-east upon the south coast, is easily distinguished from the gentle land breeze. The regularity of the land and sea breezes in the Java Sea and upon the coasts of the northern range of islands, Banca, Borneo, Celebes, etc., during the east monsoon, must in part be ascribed to the hindrances which the south-east trade-wind meets in the islands which lie directly in its way—in part to the inclination towards the east monsoon which the trade-wind undergoes after it has come within the archipelago—and, finally, to its abatement as it approaches the equator. The causes which produce the land breezes thus appear collectively not sufficiently powerful to be able to turn back a strong trade-wind in the ocean."

322. Red fogs in the Mediterranean.—Seamen tell us of "red fogs" which they sometimes encounter, especially in the vicinity of the Cape de Verd 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,"^[13] 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 colour, and it sometimes comes down in such quantities as to obscure the sun, darken the horizon, and cover the sails and rigging with a thick coating of dust, though the vessel may be hundreds of miles from the land.

323. Red fogs near the equator.—Dr. Clymer, Fleet-surgeon of the African squadron, reports a red fog which was encountered in February, 1856. by the U. S. ship Jamestown. "We were," says he, "immersed in the dust-fog six days, entering it abruptly on the night of the 9th of February, in lat. 7° 30' N., and long. 15° W., and emerging from it (and at the same time from the zone of the equatorial calms into the north-east trades) on the 10th instant, in lat. 9° N., and long. 19° W. With these winds, we beat to Porto Praya (in lat. 14° 54' N. and long. 23° 30' W.), crossing a south-west current of nearly a mile an hour, arriving at Porto Praya on the 22nd of February. The red dust settled thickly on the sails, rigging, spars, and decks, from which it was easily collected. It was an impalpable powder, of a brick-dust or cinnamon colour. The atmosphere was so dusky that we could not have seen a ship at mid-day beyond a quarter of a mile."^[14]

324. Putting tallies on the wind.—Now the patient reader, who has had the heart to follow me in a preceding chapter (IV.) around with "the wind in his circuits," will perceive that evidence in detail is yet wanting to establish it as a fact that the north-east and south-east trades, after meeting and rising up in the equatorial calms, do cross over and take the paths represented by R S and F G, Plate I. Statements, and reasons, and arguments enough have already been made and adduced (§ 288) 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 plausible, positive proof that they are true cannot fail to be received with delight and satisfaction. Were it possible to take a portion of this air, which should represent, as it travels along with the south-east trades, the general course of atmospherical circulation, and to put a tally on it by which we could follow it in its circuits and always recognize 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 either marks or tallies may be put on 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 disbelief, 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 and those tallies again, 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 south-east trade-winds bring to the equator does rise up there and pass over into the northern hemisphere. 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.

325. They tell of a crossing at the calm belts.—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 south-east trade-wind region of South America. Professor Ehrenberg has examined specimens of sea-dust from the Cape de Verds 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 spot. 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 north-east 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 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 he 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 (§ 295) that the rain or calm belt between the trades travels up and down the earth from north to south and back again, making the rainy season wherever it goes. The reason of this will be explained in another place. 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. At the time of the vernal equinox, the valley of the Lower Orinoco is then in its dry season—everything is parched up with the 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;^[15] 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 the bed of 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? 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. May not, therefore, the whirlwinds which accompany the vernal equinox, and 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?

326. Humboldt's description of the dust-whirlwinds of the Orinoco.—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-coloured 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 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 liorses stretching out their long necks and snuffing the wind, if haply a moist current may betray the neighbourhood 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! . . . Hardly has the surface of the earth received the refreshing moisture, when the previously barren steppe begins to exhale sweet odours, and to clothe itself with killingias, 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."

327. Are the great deserts centres of circulation?—The arid plains and deserts, as well as high mountain ranges, have, it may well be supposed, an influence upon the movements of the great aerial ocean, as shoals and other obstructions have upon the channels of circulation in the sea. The deserts of Asia, for instance, produce (§ 299) a disturbance upon the grand system of atmospherical circulation, which, in summer and autumn, is felt in Europe, in Liberia, and away out upon the Indian Ocean, as far as the parallel of the 10th degree of south latitude. There is an indraught from all these regions towards these deserts. These indraughts are known as monsoons at sea; on the land, as the prevailing winds of the season. Imagine the area within which this indraught is felt, and let us ask a question or two, hoping for answers. The air which the indraught brings into the desert places, and which, being heated, rises up there, whither does it go? It rises up in a column a few miles high and many in circumference, we know, and we can imagine that it is like a shaft many times thicker than it is tall; but how is it crowned? Is it crowned like the stem of a mushroom, with an efflorescence or ebullition of heated air flaring over and spreading out in all directions, and then gradually thinning out as an upper current, extending even unto the verge of the area whence the indraught is drawn? If so, does it then descend and return to the desert plains as an indraught again? Then these desert places would constitute centres of circulation for the monsoon period; and if they were such centres, whence would these winds get the vapour for their rains in Europe and Asia? Or, instead of the mushroom shape, and the flare at the top in all directions from centre to circumference, does the uprising column, like one of those submarine fountains which are said to be in the Gulf Stream off the coast of Florida, bubble up and join in with the flow of the upper current? The right answers and explanations to these questions would add greatly to our knowledge concerning the general circulation of the atmosphere. It may be in the power of observation and the microscope, or of the magnetic telegraph, to give light here. Let us hope.

328. The colour of "sea-dust."—The colour 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 colour" 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 dirty-red colour; but the threads of this cocoon, as seen singly in the diaphragm, were of a golden colour: there would seem, therefore, no difficulty in reconciling the difference between the colours of the rain-dust when viewed in little piles by the microscopist, and when seen attenuated and floating in the wind by the great traveller.

329. A clew leading into the chambers of the south.—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 through the "circuits of the wind" even unto "the chambers of the south." The frequency of the fall of "rain dust" between the parallels of 17° and 25° north, and in the vicinity of the Cape Verd 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. The latitudinal limits of the northern edge of the north-east 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 15° north. 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 17° and 38° north, according to the season of the year.

330. Red fogs do not always occur at the same place, but they occur on a north-east and south-west range.—According to the hypothesis (§ 210) suggested by my researches, this is the region 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 region in which the atmosphere that bears the "rain dust," or "African sand," descends to the surface; and this, therefore, is the region, it might be supposed, which would be the most liable to showers of this "dust." This is the region in which the Cape Verd Islands are situated; they are in the direction which, theory gives to the upper current of air from the Orinoco and Amazon with its "rain dust," and they are in the region of the most frequent showers of "rain dust": all of which, though they do not absolutely prove, are nevertheless strikingly in conformity with this theory as to the circulation of the atmosphere.

331. Condition requisite to the production of a sea fog.—It is true that, in the present state of our information, we cannot 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 vapour 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. Therefore, though we cannot tell why the "sea-dust" should not always fall 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, any more than the vapour from a given part of the sea, always meet with the conditions—electrical and others—favourable to its descent, and that these conditions, as with the vapour, may 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. 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 to a shower of "sea-dust" as well as to a thunder-storm; and that the interval between the time of the equinoctial disturbances 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 assures us that it is not less on the average than a certain rate. We cannot pretend to prescribe the conditions requisite for bringing the dust-cloud down to the earth. The radiation from the smoke-dust—as the particles of visible smoke may be called—has the effect of loading each little atom of smoke with dew, ca,using 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.

332. What is the agent that guides the air across the calm belts?—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. It is not, however, my province, or that of any other philosopher, to dictate belief. Any one may found hypotheses if he will state his facts and the reasoning by which he derives the conclusions which constitute the hypothesis. Having done this, he should patiently wait for time, farther research, and the judgment of his peers, to expand, confirm, or reject the doctrine which he may have conceived it his duty to proclaim. 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, When the air which the north-east trade-winds bring down, meets in the equatorial calms that which the south-east trade-winds convey, and the two streams 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.

1. I have been assisted in my investigations into these phenomena of the sea by many thinking minds; among those whose debtor I am stands first and foremost the clear head and warm heart of a foreign officer, Lieutenant Marin Jansen, of the Dutch Navy, whom I am proud to call my friend. He has served many years in the East Indies, and has enriched my humble contributions to the "Physical Geography of the Sea" with contributions from the store-house of his knowledge, set off and presented in many fine pictures, and has appended them to a translation of the first edition of this work in the Dutch language. He has added a chapter on the land and sea breezes; another on the changing of the monsoons in the first Indian Archipelago: he has also extended his remarks to the north-west monsoon, to hurricanes, the south-cast trades of the South Atlantic, and to winds and currents generally.
2. Upon the coast of Java I saw daily, during the east monsoon, such a column of smoke ascending at sunrise from Bromo, Lamongan, and Smiro. Probably there is no wind above.—Jansen.
3. In the very fine mist of the morning, a noise—for example, the firing of cannon—at a short distance is scarcely heard, while at midday, with the sea-breeze, it penetrates for miles with great distinctness.—Jansen.
4. The transparency of the atmosphere is so great that we can sometimes discover Venus in the sky in the middle of the day.—Jansen.
5. Especially in the rainy season the land looms very greatly; then we see mountains which are from 5000 to 6000 feet high at a distance of 80 or 100 English miles.
6. The archipelago of coral islands on the north side of the Straits of Sunda is remarkable. Before the salt water flowed from the Straits it was deprived of the solid matter of which the Thousand Islands are constructed. A similar group of islands is found between the Straits of Macassar and Balié.—Jansen.
7. At Buitenzorg, near Batavia, 40 English miles from the shore, five hundred feet above the sea, with high hills around, these thunder-storms occur between 4 p.m. and 8 p.m.
8. Jansen.
9. In the Roads of Batavia, however, they are not very agreeable.—Jansen.
10. I have seen this in a remarkable degree upon the south coast of Java; these sparks were then seen six feet above the deck, upon the frames of timber {koussen der blokken), in the implements, etc.—Jansen.
11. Some one has ventured the remark that at full moon, near the equator, more dew falls than at new moon, and to this are ascribed the moonheads (maan hoofden), which I have seen, however, but once during all the years which I have spent between the tropics.—Jansen.
12. Such is the case, among others, in the Strait of Madura, upon the heights of Bezoekio.
13. Prof. Ehrenberg calls it "Sea-dust."
14. See Sailing Directions, 8th ed., vol ii., p. 377.
15. Humboldt