Both monsoons occur in the Bay of Bengal, but there they are some what modified and less regular. The north-east monsoon does not set in regularly before the beginning of December, when it blows from north-north-east and east-north-east. In January and February it ia irregular, and frequently turns to the south-east; and in March it ceases entirely in the centre of the sea, and blows faintly in the north eastern parts, with long intervals of calms. Though the north-east monsoon only becomes regular in December, it is preceded by heavy squalls and winds in the southern parts, especially along the coast of Coromandel ; and it is during these squalls that this part of Hindustan is fertilised by abundant rains, which however are less plentiful than those brought to the coast of Malabar by the south-west monsoon. The mean annual quantity of rain at Madras does not exceed 46 inches, The south-west monsoon is more regular in its strength, but not in its direction, as it blows frequently from the south, and even from south east, especially near the mouth of the Ganges. Little rain falls during this wind on the west coast of the Gulf of.Bengal; but rain is abundant in the countries which enclose it on the north, in which direction it extends to the foot and declivities of the Himalaya mountains, and as far as the place where the Ganges issues from the range; but here too the quantity decreases as it advances inland. The mean annual quantity of rain at Calcutta is nearly 72 inches, and at Benares 464 inches. The quantity of rain which, during this monsoon, descends on the eastern coast of the bay, is still larger, especially towards its northern recess, in Chittagong and Aracan, where, according to a rough estimate, founded on partial observations, it amounts annually to 200 inches.
The monsoons are subject to much greater variation to the east of the Bay of Bengal. Ifi the Chinese Sea and the Sooloo Sea the wind is south-south-east when the sun is in the northern hemisphere, but it does not blow with regular force, being sometimes interrupted by high winds. It brings rain to all the countries which surround the sea on the north and east. In the southern provinces of China, in the Philippine Islands, And in the Sooloo Islands, the rainy season com mences in the beginning of May, and terminates in September and October. The rains, though rather abundant, when compared with those which fall in the countries without the tropics, are much inferior in quantity to the rains in Bengal and Chittagong. It is remarkable that these rains extend to the countries surrounding the Gulf of Tonkin. The monsoon blows in these seas from north-north-west when the sun is in the southern hemisphere ; but it is still less regular in its force, though more so than in the Bay of Bengal. It brings rain to the countries on the west and south of the sea, and especially to the coast of Cochin China Proper and the northern shores of Borneo, where the rains commence at the end of October and continue until March, They are more abundant than those which fall during the opposite monsoon on the northern and eastern shores of the Chinese Sea.
Along the equator, and about one or two degrees from it, it appears that the winds are subject to frequent changes ; and it is also observed that the islands situated within these limits have frequent showers nearly every day in the year.
In the Java Sea and the southern parts of the straits which lead from that sea to the Chinese Sea, the -wind generally blows from the west when the sun is in the southern hemisphere, and from the east when it is in the northern but it turns frequently to the north-west and south-east, which is also the case with the winds in the sea south of Java, and between the Lesser Sunda Islands and Australia. The westerly winds bring rain to these islands, but not in such abun dance, as the south-west monsoon carries them to Malabar. During the easterly winds the air is generally dry, but sometimes there are heavy showers. In these islands neither season has that decided per manent character which distinguishes it on continent of India, and the rains are far from being so violent. But the northern coast of Australia partakes more of the chai-acter of the season of the continent of India. The rains indeed do not seem to come down such violence ; not a drop falls during June, July, August, and September which is also sometimes the case in Java and the Lesser Sunda Islands.
The direction of the monsoons in the vicinity of the land is frequently changed by the direction of the coast, especially when • mountain range extends along the shores. The most remarkable instance of this phenomenon is observed on the south-western coast of the island of Sumatra. The south-west monsoon is felt at Achecn Head, its most northern point ; but, being opposed by the range of high mountains running along the south-western coast, it is changed into a north-west wind, which blows as far south as 1" N. lat. South of the equator the wind is not south-west, but south-east, and may be considered as the trade-wind, which, as already observed, extends as far north as 8' S. lat. when the sun is in the northern hemisphere. During this period that part of the island which lies north of the equator has its rainy season, whilst the southern districts have dry weather. When the sun is in the northern hemisphere the southern portion has abundant rains, with frequent thunder-storms ; and the northern enjoys a serene sky. In that season of the year the wind blows in the southern part from north west, and is that wind which is generally called the north-west monsoon ; but the northern districta are under the influence of the north-east monsoon It is not easy to explain the origin of these periodical winds. It is
admitted on all hands that they are only a modification of the trade winds, produced by the peculiar form of the countries lying within and around the Indian Ocean. This modification, it is said, is produced by the difference of temperature to which the high tablelands of Asia and Africa are subject during the two great divisions of the year. Wuen the sun is in the northern hemisphere the heat causes such a rarefaction of the atmosphere on the table-land of Asia as to make the air flow rapidly from the colder region near the equator to that quarter ; and this is the south-west monsoon. When, on the contrary, the sun, during its stay in the southern hemisphere, heats the air on the table land of southern Africa, the contrary effect takes place, and that is the north-east monsoon. This explanation however is hardly satisfactory. It is a fact that on the high table lands the air is always in a state of rarefaction, compared with that of low countries, and that the summer heat is never such as to cause a degree of rarefaction sufficient to produce a motion in the air from the lower countries towards the table lands. Besides this, the Himalaya mountains, with their immensely high masses, lie in the line in which the south-west monsoon blows, and its effects are not observed to be sensible in the higher part of that range. The table-lands of Beloochistan snd Arabia cannot be con sidered as affecting the direction of the wind, for in that case the wind would rather blow from the south-east than from the south-west. We think therefore that the cause of this modification of the trade-wind must be sought for in less remote localities. As fur the north-east monsoon, we are inclined to consider it entirely as a continuation of the north-east trade-wind, which is only interrupted by the two penin sulas stretching southward into the Indian Ocean ; and this interrup tion is the cause why It is commonly less constant and regular than the trade.wind itself. The question therefore is only why this trade-wind is interrupted by a wind blowing in an opposite direction when the sun is in the northern hemisphere. in this part of the year the trade wind in the northern hernia here retires thirteen degrees from the equator. If this fart is applied to the Indian Ocean, only the most northent reeesaes of the three gulfs, the Arabian Sea, the Bay of Bengal, and the Chinese Sea, would fall within its limits, and the remainder would be within the region of calms. In such a state of indir'erence a less powerful agency may produce a great effect. The south-east trade-wind, which, when the sun is on the north of the equator, extends to the vicinity of the equator, is prevented by the elevated tableland of Africa from proceedher in its direction, and is therefore diverted from its course. It follows the whiffing of the coast to the north-east ; but as the coast of Africa, as well as that of Arabia, is skirted by very high mountains, it finds no way to escape in a western direction. It would however probably not acquires that degree of constancy and force by which it is characterised, if it did not blow towards a country in which a considerable rarefaction of the air is by the sun a approaching to the northern tropic. This is the Indira Desert, called Thurr, in which the heat In summer rises to an excessive degree. on account of its small elevation above the sea, its sandy soil, and the almost complete want of vegetation. The rarefaction produced by this heat hive* strength and constancy to the south-west monis. on, and caries it to the very base of the HxnmlaTe mount tins, though the desert itself does not partake of the fertilising rain', which this monsoon brings to all the coast whose mountains oppose Re progress. This, we think, is sufficient to explain the south-west monsoon in the Arabian Sea, where it is most constant and The south-east trade-wind, net extending to the north of the equator, cannot be considered as contributing to produce the south-west mou s-sun in the Bay of Bengal ; and this wind therefore seems to owe its existence merely to the rarefaction of the air produced by the summer beat In the wide plain of the Oangisi; but an this plain Is partly covered with trees (Sunticrbund and Terail and nearly everywhere with vegetation, the effect of the heat on the temocniture of the air is lees regular; and thus it may be explained that the south-west monsoon in this POIL is less regular and constant. The vegetation which covers the Gangetic Plain is probably also the reason why the rains brought by the monsoon are distributed over its whole extent, whilst it passes over the dry sands of the Thurr almost without letting a single fall.
The origin of the south monsoon in the Chinese Sea is more difficult to explain. The great plain of Slain and Cautboja, the mountain.nmge of Cochin China, which extends along the shores of the sea front south to north, and the plains of Tonquin, probably contribute largely to it ; but we are too imperfectly acquainted with the extent and nature of these plains to be able to form a correct opinion on this point. That this south monsoon extends to the island of Formosa can hardly be adduced as an objection to this explanation, when it is considered that the north-east trade-wind retires to the most northern corner of the Chinese Sea, and is there very feeble and irregular.
As for the monsoons of the Java Sea and of the seas between the Lesser Sundt Islands and Australia, they seem to owe their origin principally to the changes of temperature which occur in the countries lying along the northern coast of Australia. [Wm.) MONSTItANS DE DRO1T. [Peerrox OF Moue ]