Abbildungen der Seite
PDF
EPUB

much from the western point near this coast, except at the two solstices. During the harmattan, and sometimes at other seasons, the wind will occasionally blow, a few hours before day, off the land,' a circumstance perfectly well known to all commanders of ships in the Guinea trade, who are generally obliged to stand out from the coast, as near as they can to the westward of N. or S. according to their destination, to catch the perennial winds; and until they reach them, they are constantly bafiled by squalls and calms accompanied with violent thunder and lightning, and fre. quently tney meet with water-spouts.

On the opposite coast of America, for the same reason, the wind blows almost constantly towards the E. varying perhaps a few de. grees N. or S according to the nature and situation of the neigh. bouring coast, and also to the sun's place in the ecliptic; for on this coast there are likewise periodical winds, a sort of monsoons varying from the N.E, to the S.E.

In that part of Brazil, which extends from the latitude of five degrees S. to the tropic, the wet season begins in April, when the wind changes to the S.E. with fresh gales accompanied with thunder and lightuing. But in September, when the wind shifts to the N.E. it brings with it a clear sky and fair weather. There is no country on this continent within the tropics, where the heats are more tole. rable, or the air more salubrious than in this part of America; for it is not only frequently refreshed with breezes from the sea, but, being mountainous, it also abounds with lakes and rivers, which often overflow their banks, so that the climate of the inland coun. try is equally temperate with the sea.coast.

But in the middle of this ocean, between the two great continents, and a very few degrees E. and W. of that central meridian, the regular perennial winds constantly prevail, subject to some slight variations according to the situation of the sun.

Whilst he is near the equator, ships find great difficulty in passing the line, at which season they are sometimes becalmed until his declination increases to, seven or eight degrees; but when it amounts to fifteen degrees N. or S. they generally cross it with a fresh breeze, and particularly when he is near either solstice. The perennial

' wind in both hemispheres varies likewise at these times; for when the sun is in Can. cer, the S.E. perennial extends to four or five and even six degrees across the line to the northward, inclining more to the S. than the

E. On the contrary, when he is in Capricorn the N. E. perennial extends an equal number of degrees to the south of the equator, but it inclines more to the northward. All which facts clearly prove, that the lower current of air, being rarefied by the reflected heat of the sun, ascends, and the equilibrium is restored by a large body of dense air, which rushes forward in a right line, and with a strong current, to fill up the vacuum.

The early Portuguese and other European navigators, in attempt. ing to sail towards the Cape of Good Hope, were greatly obstructed in their voyages thither by not adverting to these circumstances : but the use of the compass was then very little known. Most of them therefore endeavoured to keep close to the west coast of Africa, by which means, being then also ignorant of the real geography of this continent, they expected to shorten their distance ; but thus situated, they necessarily encountered constant calms and tornadoes, and seldom performed their voyage out and home in less than two or three years: whilst some other adventurers, nearly about the same period, in trying to avoid these inconveniences, fell in with the American coast, and were likewise detained by the S. E. wind, which of course retarded their progress on that side, for they could not without great difficulty, and that only at particular seasons, make their way to the southward.

But the better informed modern navigators, profiting by long and dear-bought experience, have learned to keep nearly the mid-channel, where they are assisted by constant perennial winds, and where they may yet allow some room for leeway in the southern tropic, a precaution particularly necessary whilst the sun is near our summer solstice, for at that time the S. E. perennial wind in. clines very much towards the southward. Both outward and home. ward bound India ships pass the equator in the Atlantic in about 18 or 20 degrees W. By keeping this course they never fall in with the coast of America, either going to the Cape of Good Hope, or returning from it; and at the same time they avoid the calme on the coast of Africa.

After having passed the southern tropic three or four degrees ships which sail from Europe between February and May seldom find themselves more than 26 or 28 degrees W. ; which by the trenching away of the American coast to the westward in these la. titudes, is about half-way between the two continents. The winds TOL. IV,

P

in these latitudes, in the month of May, are generally found va. riable, as if alternately and equally attracted by both continents; but as vessels advance to the southward in the months of May and June, and approach towards Africa, the wind between the latitude of 28 and 35 degrees S. comes round to the westward, and generally blows fresh from the N.W. until they have passed the Cape of Good Hope.

The wind just beyond the bounds of the perennial coming from this quarter, seems in some degree to confirm Dr. Halley's theory of the superior eurrent of air in this situation forming a contrary current at the commencement of the temperate zone. This obser. vation must be confined to particular seasons withiu certain limits, and not be considered as invariably the case, even in the southern Atlantic; for in those same parallels of latitude, the winds are light and variable, coming often from the S. E. and veering occa. sionally to almost every point of the compass. To the eastward of the Cape the S.E. wind blows frequently during their winter with considerable violence for several days successively. But the southerly winds to the eastward of the Cape blow with most violence when the sun is in Capricorn, that is during their summer months ; for when the land on the extremity of the east coast of Africa is heated by the presence of the sun, the colder air from the antarctic circle, put in motion by the sun's melting the ice in those frozen regions, frequently rushes forward towards the land near the Cape of Good Hope with considerable force.

Bacon long since, and even Pliny before him, has in effect obserred, that, from the vicinity of lofty mountains covered with snow, the winds blow periodically when the snow begins to melt. Ubicunque siti sunt montes alti et nirales, ab ea parte plant venti stati ad tempus quo nives solvuntur.

The S.E. perennial wind blows constantly some few degrees to the eastward of Madagascar at all seasons of the year, as far nearly a's the island of Java, where it comes within the reach of the regular monsoon; and indeed between the island of Madagascar and the Bain land of Africa, commonly called the Mosambique Channel, the perennial winds are checked by the proximity of the two great bodies of land, and consequently partake of the nature of mon. soons.

[Capper on Winds and Monsoors.

SECTION IV.

Periodical Winds.

Tropical Sea.winds or Monsoons. Tue name as well as the nature of the monsoons is misunder. stood; the word is not derived from the name of a great mariner, but clearly from the Persian word mousum, meaning season. In tropical countries there are but two seasons : those in Hindustan are distinguised by the N.E. and S.W. monsoons. But farther to the eastward and southward of the line, and the gulf of Bengal, the monsoons blow from different quarters. The N.E. becomes in those parts the N.W. and the S.W. becomes the S.E. The causes of those changes and the original causes of the monsoons I shall here. after attempt to explain, but first I shall endeavour to point out some generally prevailing errors respecting the course and changes of them in different parts of Hindustan, derived in all probability from the early navigators to India. As neither ancient nor modern geographers have yet fixed, with any degree of precision, the names or boundaries of the different oceans, seas, and gulfs where the monsoo's prevail, to avoid further interruption and trouble I shall beg leave in this place to make a new division of them.

The gulf of Bengal is apparently so called on account of the rich and fertile province of that name, situated at the north, or head of it. In this gulf therefore no alteration is proposed. The S.W. boundaries of this gulf I shall fx at Dondre Head, on the island of Ceylon, latitude 5° 50' N, and longitude 80° 48' E. of Greenwich. And for the S.E, side, Acheen Head, latitude 5° 30' N. longitude 95°30' E. For the northern extremity, the well known city of Cal. cutta, latitude 22" 34' 45' N. longitude 88° 29 30' E. On the W. side of the peninsula, the coast of Malabar, with Cape Guardafui, on the coast of Africa, forms another considerable gulf, frequently called the Arabian, but generally the Indian Sea : but this latter in particular seems to be a name equally applicable, and often applied to the gulf of Bengal, and even to the seas to the eastward and southward, and consequently is very indefinite; whilst the Arabian Sea may be confounded with the Red Sea or Arabian Gulf. Adopting therefore in a great measure the plan of the oriental gea. graphers, I shall name this sea the Gulf of Sind. The river Indes giving the name to the first, and Hindustan divided by the Ganges to the second division. The river Indus will then be placed at tše head of one bay, and the Ganges at the head of the other ; Tatta, a considerable city, situated on the former, and Calcuita en the latter. Tatta, according to Major Rennel, is in the latitude of 24° 50' N. longitude, 67° 37' E. Cape Guardafvi to the S.W.Jatitude 12 degrees N. longitude 52' 30' E. and Cape Comoris to the S.E. in the latitude of 7° 56' N. longitude 78° 5' E.

From the southern extremity of these two gulfs to the tropical Capricorn, extending likewise eastward from the east coast of Africa to the west side of New Holland, I shall denominate the Indian Ocean, this being a considerable portion of the ocean leading to both gulfs in India, as well as to China and the eastero islands, including all India, both within and without the Ganges. From that parallel of latitude to the south pole, including that part of the ocean situated between the E of Africa and the W. of New Holland, I shall call the Great Southern Ocean. These new divisions may not, perhaps, be deemed in every respect strictly accurate, but they will answer our purpose, and therefore, without farther preface, we will now proceed to make some observations on the different monsoons and prevailing winds within these boundaries.

The winds in the gulf of Bengal are generally said to blow six months from the N. E. and the other six from the S.W. This is far from being precisely true respecting any part of India; it is, how. prer, sufficiently accurate for our present purpose, and therefore ! shall in part adopt this position as well as the common country name of monsoca; trusting, that in the course of this enquiry, I shall be able to account for the several deviations of the wind from the monsoon points, and at the same time in some measure to esplain the causes of them.

From the island of Ceylon to Balasore Roads, the N.E. monsood is said to begin, near the coast of Coromandel, early in October. But in fact between the two monsoons, the expiration of the one and the commencement of the other, the winds and currents are variable on this coast, partaking of both; frequently, however, calms prevail during the whole month of September, and even early

« ZurückWeiter »