Graham's Magazine, Vol. XLI, No. 4, October 1852

by Various

A Calm at Sea.

1. Trade winds. These are permanent, following the same direction throughout the year. They are met with between the tropics, and a few degrees to the north and south of those limits. The well-known name applied to them is a phrase of doubtful origin, but probably derived from the facilities afforded to trade and commerce by their constant prevalence and generally uniform course, though Hakluyt speaks of the “wind blowing trade,” meaning a regular tread or track. The parallels of 28° north and south latitude mark the medium external limits of the trade winds, between which, with some variations, their direction is from the north-east, north of the equator, and from the south-east, on the other side of the line, hence called the north-east and south-east trades. They are separated from each other by the region of calms, in which a thick foggy air prevails, with frequent sudden and transient rains attended by thunder and lightning. This region, in the Atlantic, extends across the whole ocean from the coasts of Africa to those of America, but its position shifts, being sometimes entirely north of the equator, and but rarely reaching one or two degrees south; and hence it may be considered as belonging to the northern hemisphere. The region also varies in breadth from two and a half to ten degrees, but usually occupies a width of four or five. These variations are dependent upon the position of the sun, which has an influence likewise upon the strength, direction, and situation of the trade winds themselves. When the sun has a northern declination, and approaches the tropic of Cancer, the boundary line of the north-east trade wind extends to 32° north latitude, and the wind has a more easterly direction, but the parallel of 25° is its northern boundary, and the wind inclines more north when the sun is south of the equator, and approaches the tropic of Capricorn. At that season, the southern boundary of the south-east trade wind extends to 30° S. lat., and the whole ocean is swept by it between that line and about 1° N. lat. The general width of the south-east trade is about 9° greater than that of the north-east, the region of calms, as before stated, being almost wholly in the northern hemisphere. In the basin of the Atlantic, the zone of the trade winds becomes broader, and their direction more easterly, as the coast of America is approached, the breezes blowing to the very shore. This is not the case on the African side of the Atlantic, where, through a tract of sea extending from fifty to eighty miles off shore, these winds are not found at all, but contrary westerly breezes prevail. The irregularity is easily explained. Owing to the rarefaction which the air undergoes over the great hot desert of the Sahara, the colder air from the contiguous sea rushes in to supply the partial vacuum created, and keep up the equilibrium of the atmosphere, producing winds blowing toward the shore.

In the Pacific Ocean, a similar zone is occupied by permanent north and south-easterly breezes, or trade winds, though subject to a variety of interruptions. An instance of irregularity occurs along the coasts of Peru and Chili, where the general direction of the wind is south, and a steady south-easterly wind is only experienced at the distance of five or six hundred miles from the shore. The numerous shoals and islands which are found in the Pacific, prevent uniformity in the tropical movements of the atmosphere. That intelligent hydrographer Captain Horsburgh has observed, that where shoal coral banks shoot up out of the deep water in many places between the tropics, a decrease of the prevailing wind is frequently experienced; for when a steady wind is blowing over the surface of the deep water, no sooner does a ship get upon the verge of a shoal coral bank, than a sudden decrease of the wind is often perceived. This he supposes to be occasioned by the atmosphere over these banks being less rarefied by the increased evaporation than that over the deep water, and consequently not requiring so great a supply of air to restore the equilibrium as the circumjacent parts, which are more rarefied and heated. It would undoubtedly be the case, if the earth were entirely covered with a mantle of water of uniform depth, that the trade-winds would everywhere prevail, throughout a zone, bounded by the parallels of from 25° to 32° on each side of the equator. But the large masses of land, of uneven surface, which occur between the tropics, and the consequent inequalities of temperature, check the tendency of the intertropical atmosphere to a regular course, introduce derangement in its movements, so that it is only in the great open seas that the trade-winds are experienced. Still, it has been observed that, in some countries under and near the equator, constant easterly winds are found, which are no doubt identical in their cause with those that distinguish the equatorial regions of the ocean. They are met with on lands which exhibit extensive level plains, where nothing occurs to obstruct their passage and alter their direction. Thus, along the immense low tract drained by the Amazon an easterly wind prevails, by the assistance of which, the voyager is enabled to ascend rapidly against the strong current of the river. This wind blows from the estuary of the Amazon, where it is moderate, to its sources at the foot of the Andes, where it has gathered such strength, that Humboldt found it difficult to make head against it. The plain traversed by the lower course of the Orinoco has a similar easterly breeze, but of less force.

We owe the discovery of the trade winds to Columbus, and this would have been prominently connected with his name, had it not been supplanted by the glory of a greater achievement, the revelation of a new world to the knowledge of mankind. The ancients were entirely unacquainted with these permanent breezes, and though maritime adventure had been largely prosecuted by the Portuguese at the instigation of Prince Henry, they had not penetrated into the region of the trades. Proceeding cautiously along the shores of Barbary, they had explored the coasts of Africa to Cape de Verde, rescued the Azore Islands from the “oblivious empire of the ocean,” and afterward, under Vasco di Gama, doubled the Cape of Good Hope; but these voyages carried them clear of the district of the north and south-east trade winds. But soon after leaving the Canaries in the Santa Maria, Columbus fell in with the former, which in the summer extend to the latitude of those islands, and—for the first time—a sail from the Old World swelled before the steady breath of the northern tropic. This circumstance, favorable to the success of his expedition, speedily excited the apprehensions of his crew, who found themselves borne, day after day, by a permanent breeze, farther from their native shores, and inferred the impossibility of returning, as they observed no change in its direction. Fortunately for his fame, and for the world, the great navigator firmly held on his course, reached the bounds of the before-supposed illimitable ocean, and re-crossed it in the region of the variables, to the north of the northern trade wind. Now, in passing from the Canaries to Cumana, on the north coast of South America, it is scarcely ever necessary to touch the sails of a ship; and with equal facility the passage is made across the Pacific, from Acapulco, on the west coast of Mexico, to the Philippine Islands. If a channel were cut through the Isthmus of Panama, the voyage to China would be remarkably facilitated by the trade winds of the Atlantic and Pacific Oceans; be more speedy, agreeable, and safe than the usual route by the Cape, the chief interruption to its uniformity occurring in the Caribbean Sea and the Gulf of Mexico, where the trade wind blows impetuously, the sea is stormy, and the sky gray and cloudy.

The theory respecting the origin of the trade winds, adopted by Doctor Dalton, Professor Daniell, and Sir John Herschel, was first proposed by George Hadley, the brother of the inventor of the quadrant, and embodies features of the previous theories of Halley and Galileo, who both grappled with this great geographical phenomenon. It is founded upon the rarefaction of the atmosphere of the torrid zone by the powerful heat to which that region is subject, in connection with the different velocities of the earth’s surface, in different degrees of latitude, in the diurnal rotation. Heat rarefies and expands a volume of air in a ratio equivalent to an addition of about seventy feet to the ordinary height of the atmosphere for every degree of thermometrical measurement. As the sun is always vertical at some place within the tropics, the average temperature of the earth’s surface in that region, bounded by the parallels of 23½° on each side of the equator, is much higher than in latitudes to the north and south; and the incumbent air acquiring this higher temperature, is thereby rarefied and expanded. The consequence is, that in obedience to hydrostatical laws, masses of air are continually buoyed up from the surface, or swelled round the torrid zone in the form of a protuberant belt, the upper strata flowing over, and running off in streams north and south toward the poles, where—having been cooled and condensed—they descend, and flow over the surface toward the equator, pouring in a perpetual current of air to supply the place of that buoyed up by the heat of the tropics. Thus, there is a constant current in the higher regions of the atmosphere, proceeding from the equator northward and southward to the poles; and, if the earth were at rest, there would be a constant wind in the lower regions of the atmosphere blowing directly from the poles to the equator, while in equatorial regions the two streamlets would meet, and neutralize each other’s influence. But the earth is not at rest! It is incessantly whirling upon its axis, the surface moving at a rate which varies according to the extent of the circumference. The velocity at the equator, where the circumference is the greatest, is about sixteen miles a minute; at 30° of latitude, which is below the most southerly point of Europe, it is about fourteen miles in the same time; and at 45°, or about the centre of France, it is about eleven miles. As the distance from the equator increases, north and south, the rate of the rotation thus becomes less, because the circle of the earth’s circumference diminishes in extent. Now a current of air flowing from the north or south polar regions, and setting toward the equator, will encounter as it proceeds an increased rotatory motion eastward, the direction of the earth’s axical revolution; and, not acquiring the new velocity at once, it will be left behind, and seem to deflect toward the west just in proportion as it does not keep up with the earth to the east. Hence, what would simply be a north or south wind but for the earth’s rotatory motion, becomes a north-east and south-east wind as it approaches those regions where, the velocity of the globe being so much greater than where it originated, it lags behind it in its easterly course. This is the exact path of the trade winds—breezes, with few exceptions, uniform in their direction, perpetual in their motion, and steady in their force—which wafted Columbus across the Atlantic, impelled the Portuguese from their southerly course, and bore them to the Brazils, and have since been important auxiliaries to the communication of the eastern with the western continent.

The existence of a current in the upper regions of the atmosphere counter to that below, assumed by the preceding theory, is not mere hypothesis. Clouds, though of rare occurrence in the district of the trade winds, have been observed to take a direction contrary to that which the surface-breezes would have given them. A circumstance remarkably in favor of the counter-current inferred from theory, occurred in the year 1812. There was then an eruption of the volcano of St. Vincent, one of the West India Islands, which covered the island of Barbadoes with a quantity of the ashes and volcanic matter ejected. The trade wind here blows with great power, and it is certain that the volcanic ashes would have been conveyed in a direction from Barbadoes, instead of toward it, by its action. To account for their transportation thither, it is necessary to suppose that the volcano ejected them to an elevation within reach of a superior stratum of air, blowing contrary to the course of the inferior current. When Humboldt was upon the Peak of Teneriffe the west wind blew with such violence that he could scarcely stand, though the island below was under the influence of the ordinary north-east trade wind; and the remark has often been made, that in the elevated parts of the Canary Islands, a contrary wind has been experienced to that which has been prevailing over the general surface.

All mariners and passengers have spoken with delight of the region of the trade winds, not only on account of the favoring gale, but its genial influence, the transparent atmosphere, the splendid sunsets, and the brilliancy of the unclouded heavens, day and night. Columbus, in recording his first voyage into their territory, compares the air, soft and refreshing without being cool, to that of the pure and balmy April mornings he had experienced in Andalusia, wanting but the song of the nightingale and the sight of the groves, to complete the fancy that he was sailing along the Guadalquivir, “It is marvelous,” observes Las Casas, “the suavity which we experience when half way toward these Indies; and the more the ships approach the lands so much more do they perceive the temperance and softness of the air, the clearness of the sky, and the amenity and fragrance sent forth from the groves and forests; much more certainly than in April in Andalusia.” Humboldt lingers with pleasure, upon his first acquaintance with the tropical regions at sea, upon the mildness of the climate and the beauty of the southern sky, gradually opening new constellations to the view, stars contemplated from infancy progressively sinking and finally disappearing below the horizon, an unknown firmament unfolding its aspect, and scattered nebulæ rivaling in splendor the milky way. “A traveler,” he states, “has no need of being a botanist, to recognize the torrid zone, on the mere aspect of its vegetation; and without having acquired any notions of astronomy, without any acquaintance with the celestial charts of Flamstead and De la Caille, he feels he is not in Europe, when he sees the immense constellation of the Ship, or the phosphorescent clouds of Magellan, arise on the horizon. We pass those latitudes, as if we were descending a river, and we might deem it no hazardous undertaking, if we made the voyage in an open boat.” Mr. Bailey, in his Four Years in the West Indies, relates an adventure, nearly answering to that here referred to. The master of one of the small fishing smacks that ply along the coast of Scotland, who had no other knowledge of navigation than that which enabled him to keep his dead reckoning, and to take the sun with his quadrant at noon-day, having heard that sugar was a very profitable cargo, determined, by way of speculation, upon a trip to St. Vincent, to bring a few hogsheads of the commodity on his own account into the Scottish market. Accordingly, he freighted his vessel; made sail; crossed the Bay of Biscay in a gale; got into the trade winds, and scudded before them, at the rate of seven knots an hour, trusting to his dead reckoning all the way. He spoke no vessel during the whole voyage; and never once saw land until on the morning of the thirty-fifth day, when he descried St. Vincent’s right ahead, and running down, under a light breeze, along the windward coast of the island, came to anchor. The private signal of the little vessel was unknown to any of the merchants, and it immediately attracted notice. The natives were perfectly astonished—they had never heard of such a feat before; and deemed it quite impossible that a mere fishing smack, worked only by four men, and commanded by an ignorant master, should plow the billows of the Atlantic, and reach the West Indies in safety—yet so it was. This relation justifies the title given by the Spaniards to the zone where the trade winds are constant, el Golpo de las Damas, the Sea of the Ladies, on account of the ease with which it may be navigated, the uniform temperature prevalent night and day, and its pacific aspect.

Commencement of the Monsoon.

2. Monsoons. These are periodical winds, which sweep the northern part of the Indian Ocean, changing their direction after an interval of about six months, and hence the term Monsoon, the Anglicised form of the Persic mousum, or the Malay moossin, signifying a season, referring to their periodicity. Avoiding all minute detail, we shall merely give the range, direction, and duration of these singular, yet highly useful currents, and that in a very general way. From 3° south of the equator to the northern shores of the Indian Ocean, including the Arabian Sea, the Bay of Bengal, and the Chinese Sea, a south-west wind blows from April to October, and then a north-east wind sets in, and prevails through the next half year, from October to April. From 3° to 10° south of the equator a south-east wind blows from April to October, and a north-west during the succeeding six months. Without attending to local variations, these are the general phenomena. There is a south-west wind prevailing north of the equator from April to October, and southward of this, through a certain space, at the same season, a south-east wind. There is a north-east wind north of the equator from October to April, and coincidently, a north-west wind between 3° and 10° south of the line. The western boundary of the region of the monsoons is the African shore; its eastern limit is supposed to be about the meridian of 136° east longitude, which cuts the island of New Guinea; its northern confine is near the parallel of 27° north latitude, which intersects the Loo Choo islands; its southern extremity has been already stated. The monsoons are much stronger than the trade winds, and may be called gales, but they are by no means of uniform force, either as it respects themselves or each other, the same monsoon occasionally blowing with such violence that ships are obliged to reef their sails. It must not be imagined that these winds are confined to the ocean. They extend over the whole of Hindûstan to the Himalaya, the north-east monsoon bringing copious rains to its eastern shores, and the south-west monsoon performing the same office for its western coast.