MONSOON (in French mousson), is a term applied to the periodical winds of the Indian Ocean, and, according to 3larsden, is a corruption of the word mosseem, which, both in Arabic and Malay, signifies a year. These winds have from the earliest times attracted the attention of the navigator, Lia, by taking advantage of their regular blowing, an easy and speedy voyage may be calculated upon with certainty ; whilst, by contravening their force, the voyage is rendered laborious, slow, and uncertain.
The Indian Ocean is open towards the south, and no considerable island occurs between the meridian of Madagascar and that of the western shores of Australia, as far south as the sea has been navigated (70° S.); on the west is Africa, on the north Arabia, Beloochistan, and Hindustan, and on the east India without tho Ganges, the Indian Archipelago, and Australia. The monsoons, however, prevail also in the seas between Australia and China; and these seas are considered as a portion of the Indian Ocean.
The southern part of the Indian Ocean, or that which lies between the western coast of Australia and the island of Madagascar, has the regular south-east trade-wind, which extends southward to between 28° and 30° S. lat., but its northern border varies between 12° and 3° S. lat., approaching nearer to the equator when the sun is in the northern hemisphere. Cook found it, in February, between 10° and 12°; BABil Hall, in June, near Nicholson, in August, at 3i° ; and Carteret, in at some distance south of Java.
The monsoons do not begin immediately north of the northern border of the trade-wind, but are separated from it by a region which corresponds with the region of calms in the Atlantic. [Calais.) Though calms occur in this region of the Indian Ocean also, they arc not, regular, and are frequently interrupted by winds, which, when the sun is in the northern hemisphere, generally blow between south-west and north went, rind, in the other six months, between south-east and north-east. They are usually called the north-west and north-east monsoons, but not with propriety, as they do not coincide with the proper monsoons in time, nor do they seem to be connected with them in any way.
The region of the proper monsoons lies to the north of this region, and they blow with the greatest force and with most regularity between the eastern coast of Africa and Hindustan. When the sun is in the southern hemisphere a north-east wind, and when it is in the northern hemisphere a south-west wind blows over this sea. When the sun passes the equator the winds are variable, and accompanied with gales, hurricanes, thunder-storms, and calms ; but it is observed, that as soon as one of the monsoons ceases, the clouds in the upper regions of the atmosphere take the direction of the opposite monsoon, though it is not till three or four weeks afterwards that this monsoon becomes prevalent on the surface of the sea.
The north-east monsoon blows from November to March. It extends one or two degrees south of the equator. It becomes regular near the coasts of Africa sooner than in the middle of the sea, and near the equator sooner than in the vicinity of the coasts of Arabia. It blows with most regularity and force in the month of January, and especially in the most northern angle of the sea, between the island of Socotra and Bombay. This monsoon is not accompanied with rain on tho eastern coast of the sea, but it brings rain to the eastern coast of Africa, where the rainy season falls between the beginning of November and the end of March. It may be laid down as a certain fact that neither of the monsoons brings rain by itself, but only when it reaches a coast after having passed over a wide extent of sea.
The south-west monsoon does not extend south of the equator, but usually begins a short distance north of it. It blows from the latter end of April to the middle of October. Along the coast of Africa, where it rather proceeds from the south, it appears at the end of March ; but along the coast of Malabar, not before the middle of April : it ceases, however, sooner in the former than in the latter region. It is also observed that the south-west monsoon sets in sooner on the coast of Malabar than on that of Canara and Concan. At Anjengo (in 8° 30' N. lat.) it makes its appearance on the 12th of April, and at Bombay on the 15th of April, so that it proceeds a degree farther northward in the course of three days. It reaches the coast of Arabia still later. On the sea it is a serene wind of moderate force ; but when it approaches the coast of Hindustan, the atmosphere becomes overcharged with moisture, and the rain descends in torrents, but more abundantly on the southern than on the northern coast. Near Tellicherry, the annual quantity of rain amounts to 116 inches, but at Bombay it does not exceed 63 inches. It was formerly supposed that the south-west monsoon was arrested by the Western Ghauts, but a better acquaintance with the table-land of Deccan has shown that the greatest quantity of rain descends on it during the south-west monsoon, and with a wind blowing from south-west. It has latterly been observed that its effects extend to the coast of Coromandel, where, during its prevalence, a few showers fall. But the farther this monsoon advances into the continent, the smaller is the quantity of rain which it causes. Northward its effects extend over the table-land of 31a1wa, the low country along the course of the Indus, and even to the coast of Beloochistan, as far as the Straits of Ormuz.