During this last named period, when the north-east monsoon is blowing, viz. from October to May, the sun is acting with its greatest energy on the regions about the equator, and the seas lying between it and the southern tropic, while the countries formerly mentioned (Arabia, India, and China), lying under the northern tropic, become comparatively cool. The air over these regions becomes relatively more dense than the rarefied air near the line; consequently the cool air rushes to the southward to interchange places with that which has been heated; and as the cool air comes from slower-moving to quicker-moving parallels of latitude, that is, from the tropical to the equatorial regions, the north-easterly monsoon is produced, very much resembling in its effect, as it strictly does in its cause, the ordinary trade-wind of the Atlantic and Pacific oceans.
This is a very general view of what may be called the great monsoons of India; but there are many variations in different places, all of which are so readily explained by the foregoing theory, that they form by no means the least interesting branch of the subject, or the least satisfactory of its illustrations.
One of the most extensive of these varieties, though of a less general and sweeping character than those which blow in the Arabian sea and bay of Bengal, is found in a very remote part of the world. "From October to April this north-west monsoon prevails between the north-east part of Madagascar and the west coast of New Holland; and it is generally confined between the equator and 10° or 11° south latitude, but subject to irregularities." This westerly wind is evidently produced by the air drawn actually from the equator towards the slower moving latitudes of the earth, by the rarefaction of the air to the southward when the sun is near the tropic of Capricorn. "The south-east monsoon predominates from April to October in the space last mentioned, and in some places reaches to the equator." In this case, the slow moving air near the southern tropic is brought, as in the ordinary case of the south-east Trade wind, to the quick-moving parts of the earth's surface.
The following remark of Horsburgh's, in describing the monsoons, is extremely valuable, and assists to explain Hadley's theory of these matters:—"The parts where the north-west and the south-east monsoons prevail with greatest strength and regularity are in the Java sea, and from thence eastward to Timor, amongst the Molucca and Banda islands, and onward to New Guinea;" for it will be obvious to any one who inspects the globe, on reading this passage, that there occurs in the neighbourhood of the spots alluded to a powerful cause for the strength and regularity of the monsoons. The enormous island, or continent, as it might almost be called, of Australia, may well be supposed to act the part of a heater from October to April, when the sun is so nearly over it. During that period the equatorial air is drawn to the south, along the intermediate seas, amongst the Moluccas and other Spice islands, so as to produce a strong and steady north-westerly monsoon. Of course, the opposite effect will be produced when the sun retreats to the north, and leaves Australia to cool.
These instances are quite enough, I should imagine, to satisfy ordinary curiosity on this point; but professional men ought not to be contented till they have investigated all branches of this important topic; including that elegant and very useful episode, the land and sea breezes of all hot climates, and Horsburgh's East India Directory, which I have quoted above so frequently, is by far the best authority with which I am acquainted on these subjects. At the same time, I must not omit to do justice to a beautifully-written and accurate Essay on Winds and Currents, by that Prince of all Voyagers, Old Dampier; who, with means far more circumscribed than most of his successors, has contrived to arrange and condense his information in such a way as not only to render it available to practical men, but to make it intelligible and interesting to every class of readers.[4]
FOOTNOTES:
[3] It is necessary to note here that these questions have been examined since Captain Hall wrote, by Commander Maury, late secretary to the American navy, in the true analytical spirit, and immense progress made in our knowledge of these winds by the mass of practical observations on the subject made by practical navigators, and published under his directions.—ED.
[4] The principle of "Great Circle Sailing," which now guides the navigator to the Indian Ocean, must be studied in connection with this chapter. "For every degree the ship changes her longitude south of the Line she sails a shorter distance along the great circle than on any other curve; for on the parallel of 60° thirty miles corresponds to a distance of sixty at the equator."—ROBERTSON'S Theory of Great Circle Sailing: Bell and Daldy.
