CHAPTER XXI. The Potomac sails from Canton-The Bashee Straits-The northeast tradewind-Cause and description of the tradewinds-Periodical winds-Monsoons-The sirocco, the simoon, land and seabreezes-Perennial and variable winds-An East India voyage; its natural facilities-Period for commencing one-Reflections arising from the subject-Arrival at the Sandwich Islands. A FEW days after the return of our officers from Canton, the frigate having, in the meantime, been completely supplied with water, provisions, &c., got under way at Lintin Bay, and proceeded to her former anchorage off Macao, from whence she proceeded to sea on the following day, with a light breeze and pleasant weather. The breeze freshened as she cleared the land, and long before nightfall, the sky had become overcast with thick black clouds. The Potomac was now dashing along at the rate of eleven knots, with her yards nearly square, when a light was discovered almost directly ahead, and in the next moment the frigate was alongside of a large vessel, moving with almost equal rapidity towards the bay we had just left. The thickness of the atmosphere had concealed the two vessels from each other until their proximity had become somewhat perilous to both, for a few feet more would have brought them in contact; and such was the velocity with which they passed each other, that the hurried inquiry of "What vessel is that?" was lost to the ear of all but the interrogator. Onward flew the stranger, and with equal speed forward dashed the Potomac, as if ambitious of redeeming the time she had spent in the waters of the "Celestial Empire." Her course was laid for what is called the Bashee Passage, a well-known outlet from the Chinese Sea into the mighty Pacific, formed by a chain or cluster of numerous islands lying to the south of Formosa, about ninety miles east of Canton. The island which gives its name to the whole cluster, of which it is the most eastern, is of a circular form, six miles in diameter, and has a town on it of the same name. Its productions are plantains, bananas, pine apples, sugarcane, potatoes, yams, and cotton, with numerous goats and hogs. Most of the islands in the cluster are considerably elevated, and vessels can run between them with safety if the weather be clear; as it is supposed that there are no hidden dangers around them, though breakers are visible at a considerable distance. The morning when these islands were expected to be seen from the deck of the Potomac, was too thick for that purpose, though the wind was blowing fresh. The frigate entered the passage at the rate of ten knots; when the wind suddenly came out ahead, and took the ship aback. She was got off with difficulty, and filled away, when the wind had increased to almost a gale. On she dashed, without making either of the islands, though but a short distance from them, and entered the great Pacific with increasing velocity. The threatening appearances which had disfigured the atmosphere, softened in their aspect, and passed away, as the Potomac left the Chinese Sea behind her; and her sails in due time were filled with the welcome northeast tradewind, favoured by whose friendly influence the frigate now shaped her course across the broad Pacific, towards the well-known Sandwich Islands. As nothing of interest occurred to relieve the tedious monotony of this passage of fifty days, we cannot, perhaps, better beguile the time than by recording the result of our observations on the prevailing winds of this interesting region of the globe; especially as we have just attached some importance to the northeast tradewind, which was expected to accompany the Potomac on her long and lonely route of ten thousand miles! Those aerial currents which are called winds, are naturally distinguished into two kinds, constant and variable. The former, which are better known by the name of tradewinds, prevail, with little exception, between the tropics; and, like the great equatorial current of the ocean, circulate round the globe from east to west. They extend to about thirty degrees on each side of the equator, and blow with a steady breeze, almost the whole year, from an eastern to a western quarter of the heavens, where their course is not interrupted by land. Their direction, however, declines several degrees from due east and west, corresponding to the declination of the sun, either north or south. The cause of the tradewind, so remarkable on account of its general uniformity everywhere between the tropics, is principally owing to the joint influence of the superior temperature of the torrid zone, and the rotation of the earth on its own axis. The air at the surface of the earth being highly rarefied between the tropics, naturally ascends by its acquired levity into the higher regions of the atmosphere; while its place is supplied by the colder air rushing from the poles towards the equator. This newcomer soon becomes rarefied like its predecessor, and ascends in its turn, and is ultimately carried towards the two poles, to supply the deficiency caused by the continual stream from these points. An aerial current is thus established, constantly proceeding from the poles towards the equator, in the lower regions. Each of these polar currents, moving in its progress towards the equator, from zones where the earth's motion on its axis is slower, to others where it is more rapid, cannot have the same velocity eastward as the solid parts of the globe; and, therefore, the aerial particles of which it is composed, gradually acquire a relative motion in an opposite direction. The currents from the two hemispheres meeting near the equator, and verging, each of them, towards the west, the meridional motions are destroyed by being in opposite directions; and they therefore advance together, with the remaining motion from the eastward, all round the globe. But there are certain situations, where the tradewind is not only interrupted in its general course towards the west, but is actually suspended, and succeeded by a wind blowing in an opposite direction, which continues with great regularity for several months. This is classed under the general term of periodical winds; and is called a monsoon, from the Persian word monsum, which signifies season. In the East Indies, there are two of these periodical winds, distinguished by the names of the northeast and the southwest monsoons; which may be said to change every six months, according to the sun's place in the ecliptic. In the same class are also included the sirocco, the simoon, land and seabreezes, and long-shore winds. The sirocco is a periodical wind, which prevails in Italy about Easter; and the simoon is a hot, suffocating wind, which prevails at certain seasons on the deserts of Arabia and Africa. Both of these winds are supposed to be highly charged with carbonic acid gas, mixed with some other noxious exhalations, unsuitable for respiration. Perennial winds are those which blow the whole year in one direction; while those which are called variable, are those which prevail in all parts of the world beyond the tropics. In the middle of the Atlantic Ocean, between the two great continents, Africa and America (and a very few degrees east and west of that central meridian), the regular perennial winds constantly prevail, subject only to slight variations from the position of the sun. While the sun is passing over the equator, it is often difficult for vessels to cross the line; and such is the influence of that luminary, that they are sometimes becalmed until his declination increases to seven or eight degrees. But when at fifteen degrees, particularly if near either of the tropics, they generally pass the line with a fresh breeze. At these times, the tradewind also varies. When the sun is in Cancer, the southeast perennial extends from four to six degrees across the line to the northward, and inclining more to the south than to the east; and the contrary takes place when the sun is in Capricorn. This is a natural consequence. The lower current of air being rarefied by the reflected heat of the sun, ascends, and the equilibrium is restored by a larger body of dense air, which rushes forward in a right line, and with a strong current, to fill up the vacuum. The ancient Portuguese, not being acquainted with these circumstances, thought to shorten the route of their East India voyages by hugging the continent of Africa; an error which inevitably exposed them to calms and storms, and sometimes prolonged their voyages to an extent of two and three years. Long and dear-bought experience, however, has since taught navigators to hold nearly a middle course between the two continents, in which they are favoured by constant tradewinds, and where they may allow some leeway in the southern tropics, a precaution necessary when the sun is in or near the tropic of Cancer; for at that time the southeast wind inclines very much to the south. Both outward and homeward bound ships, engaged in the India trade, should therefore cross the equator in about eighteen or twenty degrees of west longitude; by which means they will not fall in with the American continent, and, at the same time, will avoid the calms on the coast of Africa. Ships sailing from the United States or Europe, between the months of February and May, after having passed three or four degrees beyond the southern tropic, seldom find themselves more than 26° or 28° west; which, considering the trending away of the American coast, may still be considered about midway between the two continents. The wind, in these latitudes, in the month of May, will generally be found variable, as if equally attracted by land on each side; but, as vessels advance farther south, in the months of May and June, say between 28° and 35°, the wind hauls round more from the westward, and is generally fresh from the northwest until the passage around the Cape of Good Hope can be effected. East of the Cape, in the winter, the southeast winds frequently blow fresh for several days successively; but the southerly winds, in this quarter, blow with most violence when the sun is in the tropic of Capricorn. As the land on the southerly extremity of the African continent becomes warmed by the presence of the sun, the heavy body of cold air, from the antarctic seas, rushes north, with strong currents. Proceeding on a few degrees from the coast of Madagascar, the southeast tradewind blows at all seasons of the year as far nearly as the Island of Java, where it comes in reach of the monsoons. Within the Mosambique channel, owing to the proximity of the two great bodies of land, the winds partake of the nature of mon soons. The seasons in Hindostan are distinguished by the northeast and southwest monsoons; but farther to the eastward and southward of the line, and in the Gulf of Bengal, the monsoons blow from different quarters. The northeast becomes in those parts the northwest, and the southwest becomes the southeast. These changes are owing to local circumstances; the position of large bodies of land and water. In speaking of the Indian Ocean, we mean that portion of the globe contained between the Gulf of Sinde and Bengal on the north, to the tropic of Capricorn, south; and from the east of Africa, to the west shores of New-Holland: from that parallel of latitude to the south pole, containing the ocean between Africa and New-Holland, as the great Southern Ocean. In the Gulf of Bengal, the wind may be said to blow six months |