« ZurückWeiter »
therefore, at first will be light and variable, and give rise to the Prodromi, but will increase as the sun advances, so that this northerly current of air from the Alps, and westerley current of air from the Atlantic Ocean, continuing to move at right angles, and nearly with equal velocity from the same distance, will at length unite and produce a N. W. wind on the western part of Italy, which is the part whence the summer Etesiæ are said to blow.
In this manner it appears to me we may account for the periodi. cal winds which are known to prevail both in Greece and Italy, and throughout every branch of the Mediterranean. But in the autumn all these winds become variable, sometimes blowing from the sea towards the coast, and at other times in a contrary direc. tion. These frequent changes may probably be attributed to the sudden alteration in the temperature of the sea and land; for as the sun regularly declines towards the equinoctial, the earth, both on the continent of Europe to the northward, and of Africa to the southward, gradually cools again, subject for some time to slight variations, either on the earth or the sea, which must necessarily produce variable winds in the Mediterranean, untill some weeks after the autumnal equinox. The western branch of the Mediter. ranean, situated between the Alpine regions to the N. and the con. tinent of Africa to the S. must at all seasons be subject to violent changes of both wind and weather, particularly in the months of March and September, about the two equinoxes, when the sudden variations of heat and cold are greater than at any other time of the year. But as during the winter season the sea will generally be warmer than the land on either side, a current of air will move sometimes towards the Mediterranean from the continent of Eu. rope, and nearly at the same time perhaps from that of Africa, which, late in autumn and throughout the winter, will produce opposite currents of air in every part of the sea, particularly near the respective coasts; and to these opposite currents of air may probably be impoted also the sudden storms, accompanied with heavy showers of rain, that frequently occur on the African side of the Mediterranean, during the early part of winter. The gusts of wind at this time, though violent, are generally of short dura. tion.
[Id. It is to this periodical wind that Lucretius ascribes one, and
apparently the chief cause of the rise and exundation of the Nile. The passage occurs lib. vi. 712.
Nilus in æstatem crescit, campisque redundat,
The Nile now calls us, pride of Egypt's plains ;
3. Tropical Land-wind. The island of Ceylon, which lies to the southward of the Coro. mandel coast, and where the peninsula becomes extremely narrow, partakes of both monsoons, but priocipally of the S. W. The wind immediately on the coast, at the commencement of this monsoon, takes nearly the same direction as the coast itself. From the latitude of 9 to 13 degrees, the coast lies nearly N. N. E. and S. S. W. and from the latitude of 15 degrees to the head of the gulf called Balascore Roads, it runs almost N. E. and S. W. The S. W. monsoon therefore on this coast blows at first along shore, from which cause it is called the Long Shore Wind. The nature of the soil on the coast probably contributes to give it this direction ; for the soil being, in some respects, like the Gulf of Guinea on the coast of Africa, low and sandy, the air near the earth must consequently be much rarefied under almost a vertical sun, and the denser air, coming across the Indian Ocean or the Gulf of Sind, will follow that direction on the coast to fill up the vacuum. But these winds continue only to the end of May or the beginning of June, when the sun being neer the summer solstice, the hot land wind on the coast of Coromandel commend
ences, and continues about six weeks. To understand the causes of this sudden change, we must again advert to the geography of the country, and consider the state of the atmosphere at this period on the two coasts.
The southern part of the peninsula, from the latitude of 16 de. grees to Cape Comorin, may be divided longitudinally into three parts, beginning at Madras, which is situated in the longitude of 80° 28' 45" E. About two degrees to the westward of that meri, dian is a range of mountains, forming the eastern boundary of the Valley of Baramaul, where the high land of Mysore commences, commonly called the Ballagat, or country above the Passes. This high or table land of Mysore rises at least 2,000 feet above the coast of Coromandel, and runs through the peninsula from N. to S. nearly in the longitude of 784 degrees. Two degrees farther to the westward is another range of mountains, which may be considered as the boundary of the Malabar coast; and the country situated between these two meridians, from 76 to 78 degrees, is properly the country of Mysore. With this sketch of the map of the country before us, and with a recollection of the first principle of this hypothesis, it will not be difficult to account for the hot land wind prevailing in the Carnatic during the months of May and June.
The sun's declination in the month of May is between 15 and 22 degrees N.; he will therefore before the end of this month have been vertical over all these countries, and consequently have pro. duced a considerable degree of heat in the Carnatic ; but at the same time the double range of mountains to the westward will have arrested the clouds brought thither by the S. W. monsoon, and made them precipitate their contents both on the Malabar coast and in the Mysore country. The principal point of rarefaction then, at this season, will be the Carnatic, which may, as usual, be considered as the heated room, and the nearest cold body of
air will come from the table land of Mysore to restore the equilibrium,
In the Carnatic, during the months of May and June, the ther. mometer of Farenheit in the shade is generally at 90 or even 100 degrees and upwards, whilst near the mountains the same kind of thermometer will not be more than 70 or 80 degrees at the utmost. The current of the air then will move from the mountains across the Carnatic towards the coast of Coromandel, and of course pro. duce the hot land winds, but they are severely felt only on the east side of the Carnatic, at a distance from the mountains : at Amboor, and even at Vellore, which are situated near them, those winds are neither extremely hot, nor of long duration; and in the narrow part of the peninsula, in the beautiful little province of Coimbatore, although so far to the southward, in consequence of their vicinity to the hills, the inhabitants are never incommoded by land winds.
This rarefaction in the Caspatic, and the current of air which comes from the Ballagat Mountains, and blows from the W. to the E. to fill up the vacuum, are sufficiently strong inland to counteract the effects of the monsoon in this part of the peninsula; but the westerly wind soon loses its effect on coming to the coast, for it never extends above one or two leagues out to sea, where the S. W. monsoon blows incessantly at this season of the year.
But within a mouth after the summer solstice, the current of the S. W. monsoon begins to slacken, when the regular land and sea winds again commence upon the coast of Coromandel, and continue with slight variations for a month or six weeks. Towards the end of August, as the sun approaches the line, the heat in Asia and the cold in Africa begin to abate; consequently the monsoon daily becomes more faint, and like the slack water between the flood and ebb tides, the air in the Gulf of Bengal has little motion : frequently it moves about in eddies, and after it has fluctuated be. tween the two monsoons for three weeks, sometimes almost a month, being attended with squalls from different quarters, the N. E. wind at length prevails, and like the change of tides, moves at first with considerable rapidity. But the tremendous gales, or rather hurricanes, which sometimes blow in the gulf at this season, and bear down every thing before them, seldom happen precisely at the begioniog of the monsoon, nor does it appear that they are the
effect of a current of air like the monsoon, blowing constantly from the same quarter for several months, but rather resemble whirlwinds, which proceed 'principally from some sudden change in the upper regions of the atmosphere, and which, though extremely violent, are merely local and temporary. But before we conclude the account of the S. W. monsoon in Hindustan, it may be proper to observe, that this monsoon brings the violent raios into the pro. vinces of Bengal and Bahar, which generally begin at Calcutta about the middle of June, two months after their commencement to the southward of the gulf.
[Capper. 4. Khumseen. The Arabian and Persian gulfs are not only separated by Arabia, but the major part of the former is within the tropic, whilst the nortberu part of it, like the whole of the Gulf of Persia, from Muscat to Bossora, is situated beyond the tropic. In comparing the winds of these gulfs, therefore, we must make a distinction between the northern and southern division of the Arabian Gulf. From the entrance of the Straights of Babelmandel to the city of Yambo, the S.W. monsoon prevails at the same time as it does in the Gulf of Sind, that is from April to September. But from the 15th of May to the beginning of August, the S.W. monsoon is ex. tended, or rather elongated, from Yambo to Suez, notwithstand. ing the latter is almost eight degrees beyond the tropic. This wind is called by the Arabs the Khumseen (afty), being supposed by them to precede the overflowing of the Nile about fifty days *.
The Khumseen wind blows in the northern part of the Arabian Gulf, as far as the sea.coast of the Delta.
It is very well known that the soil of Upper, and even of a part of Lower Egypt, on one side of the Arabian Gulf, and of Arabia Petrea and Arabia Deserta on the other, consists chiefly of rocks and sands. As the sun approaches towards the solstice, and from very obvious causes, for a month or six weeks aftewards, the atmosphere over those countries must be excessively rarefied ; whilst this rarefaction continues to the northward, the air to the N. after the commencement of the rains, being infinitely more cold and dense, will be impelled forward towards the N. to re.
* The reader will hence observe that the Khumseen is synonymous with the prodromi, or breezes that precede and introduce the Etesian wind. EDITOR