The Lieutenant and Commander Part 3

I am not sure that, in the whole range of this extensive subject, there could be picked out an instance more in point to what has just been said, than these interesting phenomena of the Trade-winds. To sailors of every age and rank, and especially to naval officers, an acquaintance with the laws which regulate these extraordinary aerial currents must be of great importance. For a commander may be ordered, at a moment's warning, either to carry his own s.h.i.+p, or to lead a squadron, or to guard a convoy, from the northern to the southern hemisphere, or perhaps from the West to the East Indies. If, however, he have not previously made a tropical voyage or two, or have not studied the subject in its genuine theoretical spirit, as well as in the log-books of his predecessors, he may expect to find himself most wofully embarra.s.sed, both on entering and on leaving the Trades.

Independently of all such public objects concerned in these inquiries, there appears to exist a very general interest in the Trade-winds, sufficiently strong to engage the attention even of unprofessional persons. These vast currents of air, which sweep round and round the globe, in huge strips of more than twelve hundred miles in width, are in a manner forced on every one's notice, from contributing to that boundless interchange of the productions of distant regions by which modern times are so agreeably distinguished from the old.

The great Monsoons, again, of the Indian and China oceans play almost as important a part in this grand nautical drama along the coasts of those remote countries. These great phenomena will be found to obey precisely the same laws as their less fluctuating brethren the mighty Trades; and hence springs one of the chief delights of science when its study is conducted in a proper spirit. If the pursuit of truth be engaged in with sincerity, phenomena apparently the most opposite in character, for example, winds in different parts of the earth, but in the same lat.i.tude, blowing in totally different directions at the same season of the year, will always prove in the end ill.u.s.trative of one another, and of their common theory.

FOOTNOTES:

[2] On the renewal of their Charter, in 1833, the East India Company ceased to be traders, and these n.o.ble s.h.i.+ps no longer sail under the Company's flag.

CHAPTER VI.

THE TRADE WINDS.

There are few things more curious in the history of human knowledge than the establishment of extensive errors as to matters of fact, and the perverse tenacity with which they retain their hold on the public mind. In some cases it would almost seem that the pleasure which springs from genuine philosophical inquiry is subordinate to that which arises from the indolent process of taking things for granted.

This applies peculiarly to the phenomena of the Trade-winds, respecting which many erroneous ideas are generally entertained. To professional men these fallacies are calculated to prove extremely mischievous; while even to persons not directly connected with the sea, the existence of error may often be injurious: and, although it is not very easy to explain these things in a popular way, I shall attempt to give a description of the facts as they really exist.

The main characteristics may easily be described.

The great belt of the earth's surface, nearly three thousand miles in width, lying between the tropics (from 23-1/2 north to 23-1/2 south lat.i.tude), is the chief region of the Trade-winds; though in some parts of the world they extend to the lat.i.tude of 28 both north and south of the equator; while at other places well within the tropics, and even close to the line, totally different winds prevail. It is only in the open parts of the Pacific and Atlantic oceans that the true Trade-winds blow. In the Indian and China seas, and in many other portions of the great tropical belt, periodical winds, called Monsoons, are found. These s.h.i.+fting Trades exact the closest study from the practical navigator, in consequence of their extensive variety and seeming complication. But they are not less deserving the attention of merely curious inquirers, from the beautiful manner in which these modifications of the regular breezes obey the same general laws which direct the grand phenomena of the Trades. Indeed, the most extensive observation serves only to link the whole into one harmonious chain or series of explanations, exhibiting the uniformity as well as the exquisite adaptability of Nature, even in those departments called "inconstant," where she is supposed to be most capricious.

The only general a.s.sertion that can safely be made with respect to the Trade-winds is, that they blow more or less from the eastern half of the compa.s.s towards the western. On the north side of the equator, the north-east Trade-winds blow; and on the south side, the south-east Trade-winds. These two names have undoubtedly contributed to mystify the subject by naturally suggesting to the imagination currents of air blowing respectively from the north-east and the south-east, or at an angle of 45 with the meridian. And I have even seen sailors (old sailors too) quite surprised, and rather provoked, when they have encountered very different winds in those parts of their voyage, where, being misled by the force of names alone, they had taught themselves to expect a regular breeze from a particular quarter. But, in point of fact, the Trade-winds do very seldom blow directly from north-east and south-east; neither are they uniform in their direction on the same spot at different seasons of the year, nor is their strength uniform from month to month. I may add, that the equatorial limits, or bounding lines, of the trades, are not steadily confined to the same lat.i.tude. In short, so far from these winds being perfectly fixed in direction, force, and position, they are subject to very considerable mutations, dependent on the position of the sun. Their vast nautical value, in fact, as well as philosophical curiosity, turns mainly on their uniformity, which, in spite of all the fluctuations alluded to, gives them a very distinctive character.

Dr. Young and Hadley, the great authorities on the subject, are both wrong in their conclusions.[3] Where Hadley obtains his "experience"

he does not tell; but certain it is, that no sailor who ever crossed the equinoctial line could possibly have furnished him with two of his princ.i.p.al statements. The Trades are not strongest near the equator, as he states, nor when they reach that district do they blow along it, or in a parallel direction, but almost always at right angles to it.

If the earth had no motion on its axis, but were surrounded as at present with an atmosphere, and if the sun moved round and round it exactly above the equator, without varying his declination, the following effects would ensue: That portion of the earth lying, say thirty degrees, on each side of the equator, being more exposed to the action of the sun than those further from it, would become much warmer; while the superinc.u.mbent air, being greatly heated by the contact, would expand, or become specifically lighter, and would consequently rise. The adjacent air, both on the north and south, being cooler, and, of course, heavier, would rush in to supply the place of the heated air. This air coming from the regions beyond the tropics would, in its turn, be heated, and rise on reaching the warmer equatorial regions, giving place to a fresh supply, which, it is easy to see, must be furnished by the descent of that portion of air formerly heated at the equator, raised into the cold regions of the sky, and forced into a regular circuit by fresh elevations of heated air. All these and many other interesting results are clearly developed in Daniell's Meteorological Essays, a book which every one at all interested in such inquiries will find it advantageous to study. The first edition of this work was published in 1823, some years after these speculations had been forced upon my notice by a long course of service between the tropics.

It will be understood, that, as long as we imagine the globe at rest while this circulation is going on, the course of the lower air along the surface would be directly towards the equator, from due north in one hemisphere, and from due south in the other; while in the upper regions the currents would follow the opposite directions, and stream towards the poles. But the instant we conceive the earth put into rotatory motion from west to east, a change would take place in the course of these aerial currents, both above and below. It must be recollected that a volume of air, when once put in motion, will move on, like any other body, by the mere force of its own momentum, till that motion is destroyed by its friction against the substances along or through which it is impelled. Any one who has observed the ring of smoke sometimes projected from the mouth of a cannon will be sensible that this is true.

It may likewise be of use, before going further, to consider, that, if the globe, instead of being unequally heated; were equally heated at all parts, from pole to pole, and being surrounded by an atmosphere, were then made to revolve on its axis, it would carry the atmosphere round with it exactly at the rate at which it was itself going. That portion of the air in contact with the equator would move about 1000 miles in one hour, while that in lat.i.tude 90 would be as motionless as the poles themselves.

From this it will be seen, that, while the equator moves at the rate of 1000 miles an hour, the district about the lat.i.tude 30 moves only 860, or 140 miles slower. The average whirling velocity of the earth's easterly motion, in the s.p.a.ce between the equator and lat.i.tude 30, may be stated at 950 miles an hour; and that of the belt lying between 30 and 40, at about 800 miles.

In the hypothetical case, above suggested, of the whole surface being equally heated, and consequently the whole atmosphere at the same temperature, there would be a universal calm, whatever might be the rotatory motion impressed upon the earth. If, however, we next suppose, what really is the case, that the air over the tropical region is more heated than that which is farther from the equator, this rarefied air will instantly ascend, and occupy a place above the colder and denser air, which will flow in from the belts lying beyond the tropics.

When the comparatively slow-moving air of the temperate zone, lying beyond the tropics, first comes in contact with those quicker-moving parts of the earth forming the tropical edges of the torrid zone, the apparent motion of the air from the east, caused by the relative difference of the rotatory velocity between the air and earth, is great, compared to the other motion of the air, caused by its being drawn directly towards the equator, to supply the place of the heated portions raised into the sky. Consequently, at the tropical borders of both Trades the wind is found to blow very nearly from the east point.

Since the cool air of the temperate and comparatively slow-moving zones beyond the tropics is thus drawn towards the equator, and comes successively in contact with parallels of lat.i.tude moving faster and faster towards the east, there must be gradually imparted to it, by the increased friction, a considerable degree of the increased rotatory velocity belonging to the low lat.i.tudes it has now reached; that is to say, there will be less and less difference of velocity between the easterly motion of this temperate air and the easterly motion of the earth; and, consequently, the wind, as it approaches the equator, will appear to blow less and less directly from the eastward. But, while the earth's rotation within the tropics is thus acting on the slower-moving air which has travelled to it from beyond the tropics, with increased friction at every successive moment, there has been no such powerful counteracting influence in operation to diminish the meridional motion impressed on the air in question; for, although in proceeding from the tropics towards the equator, the wind might, at first sight, be supposed to have its speed somewhat lessened by friction along the earth's surface, the r.e.t.a.r.dation due to this cause, if there be any at all, must be inconsiderable, compared to that which affects the motion caused by the difference in the rotatory velocity of the earth at the different parallels. It must be recollected, also, that there is a constant demand for fresh air from the north and south, to occupy the place of the heated and rarefied air which is raised up in the torrid zone; and this demand being pretty equal, the motion it produces on the air in the direction of the meridian must likewise be uniform.

If it be admitted that all the easterly character of the Trade-winds is due to the difference of velocity between the rotation of the torrid zone of the earth from west to east, and that of the air impressed only with the slower rotatory motion to the east of the temperate zone, it will follow, that, if this difference of velocities between the earth and the air in contact with it be diminished or annihilated, the easterly character of these winds will be diminished or annihilated likewise. At the same time, there is no cause in operation, that I can discover, to alter the direction of the meridional motion, as it may be called, of the Trade-winds, or that by which they are impelled directly towards the equator.

At first starting from the temperate zone, on its voyage to the equator, the cold air of that slow-moving region is impressed with a rotatory velocity of only 800 miles per hour to the eastward, but it soon comes over parts of the earth moving more than 100 miles per hour faster to the eastward than itself. The difference of velocity in the earth's rotation between lat.i.tudes 30 and 20 is 74 miles an hour, while between 20 and 10 it is only 45 miles, and in the next ten degrees the difference in rate per hour is reduced to 15 miles.

The velocity with which the air drawn from beyond the tropics travels along the sea towards the equator is probably not above twenty miles an hour, a rate slow enough to allow time for the constantly-increasing friction of the earth's rotation to act upon it, and draw it more and more entirely to the east. By the time it has reached the equatorial regions, the friction of the earth's surface has operated long enough to carry the air completely along with it; and, of course, all relative motion being done away with, everything easterly in the character of the Trade-winds will be at an end.

But, although this constantly-increasing friction of the earth's rotation has thus annihilated all relative easterly motion between the air and earth, that air still retains its motion towards the equator; and accordingly we do find the Trade-winds, at their equatorial limits, blowing, not from the east, as Hadley, Dr. Young, and others, conceived, but directly from the north and from the south respectively. The strength and velocity of the Trades at these places is, in general, considerably diminished, chiefly, perhaps, by the air becoming heated, and rising up rather than flowing along; and also, no doubt, by the meeting of the two opposite currents of air--one from the north, the other from the south--which produces the intermediate s.p.a.ce called the Calms, or the Variables.

In strict conformity with these theoretical views, the clouds above the Trades are almost invariably observed to proceed in the contrary direction to the winds below. On the top of the Peak of Teneriffe I found a gentle breeze blowing from the south-westward, directly opposite to the course of the Trade-wind.

The more detailed circ.u.mstances usually met with in that part of a voyage to India which lies between 30 north and 30 south, and which I am about to describe, will now, I imagine, be readily understood.

Before setting out, however, I must strongly recommend any one wis.h.i.+ng to see these matters clearly, to have them fixed in his mind to useful purpose, to follow both the theoretical and the practical parts of this explanation with the a.s.sistance of a terrestrial globe.

Most s.h.i.+ps touch at Madeira, either to take in a stock of wine, to get fruit and vegetables, or to form a pleasant break in the early and most disagreeable part of the voyage. Some s.h.i.+ps pa.s.s barely in sight of the high mountain which rises above the town of Funchal, and satisfy themselves with taking sights for verifying the rates of their chronometers when on the meridian of the island; while others tantalise their pa.s.sengers still more by sweeping through the roads, without anchoring, or communicating with the sh.o.r.e. The captains by such s.h.i.+ps are pretty deeply, if not very loudly, abused by all hands, pa.s.sengers especially, who are perhaps the most dissatisfied, because the most idle, of mortals. Shortly after leaving Madeira, which is in 32-1/2 north lat.i.tude, a s.h.i.+p may expect to meet the Trades; but she cannot calculate with any certainty upon catching them till she arrives at the parallel of 28. On first reaching the Trade-wind it will be found to blow very nearly from due east, and with this a course is easily steered past or amongst the Canaries, and thence for the Cape de Verdes. Some navigators pa.s.s within this group, others keep so far out as barely to make San Antonio; and this, I think, is considered the best route. As the s.h.i.+p proceeds to the southward, the wind draws gradually round from the east to north-east, and eventually to north-north-east, and even to north, at the southern margin of the north-east Trade-wind.

The position of this margin or southern edge, which in technical language is called the equatorial limit of the Trade, varies considerably with the season of the year. From December to May inclusive it frequently reaches as far as the 3rd degree of north lat.i.tude, though it ranges about 5 and 6 north. From June to November it is s.h.i.+fted back as far, sometimes, as 13 north, but it seldom extends as far south as 8 north. Subjects which are treated of in a series of tables showing the equatorial limits of both Trade-winds, deduced by the late Captain James Horsburgh, hydrographer of the East India Company, from the observations of 238 s.h.i.+ps. These tables show very clearly the effect of the absence or presence of the sun in s.h.i.+fting the limits of the Trades, drawing them after him, as it were. The presence of the sun in either hemisphere obstructs considerably the regularity and strength of the Trade-winds in that hemisphere, and _vice versa_.

The great difficulty experienced in making the outward-bound voyage commences after the s.h.i.+p has been deserted by the north-east Trade, for she has then to fight her way to the southward across the region of Calms and Variables. But as these Variables blow generally from the southward and westward, from a cause afterwards to be explained, it is obvious enough why this part of the homeward voyage is always more easily made than the outward pa.s.sage. These southerly breezes, which are met with in the Variables, blow at times with considerable force, and greatly perplex the young navigator, who, trusting perhaps to some of the erroneous published accounts, not unnaturally reckons upon meeting the regular Trade-wind, blowing, as he supposes, from the east near the equator, not from the south; still less is he prepared or pleased to find it blowing from the south-westward.

This troublesome range, intervening between the two Trades, varies in width from 150 to more than 500 miles. It is widest in September, and narrowest in December or January. I now speak more particularly of what happens in the Atlantic. In the wide Pacific, far from land, fewer modifying circ.u.mstances interfere with the regular course of the phenomena, than in the comparatively narrow sea formed by the opposite shoulders of Africa and South America.

Calms, also, are met with in this intermediate region, or purgatory of the outward-bound voyage, and occasionally violent tornados or squalls, which in a moment tear away every rag of canvas from a s.h.i.+p's yards. For several hours at a time, also, rain falls down in absolute torrents. Even when the weather clears up, and a fresh breeze comes, it is generally from the southward, directly in the outward-bound navigator's teeth. He must have patience, however, and strive to make the most of it by keeping on that tack by which most southing is to be gained. It is now, I believe, generally held to be the best practice to place the s.h.i.+p between 18 and 23 of west longitude on losing the north-east Trade; and likewise to endeavour, if possible, to cross the equator somewhere between these two longitudes. Before reaching the line, however, the navigator will almost always be met by the south-east Trade-wind. From January to May he may expect to meet it in 1 or 2 north lat.i.tude; but in summer and autumn he will find the northern or equatorial limit of the south-east Trade a degree or two still further to the northwards of the lines.

On first encountering the south-east Trade an outward-bound s.h.i.+p is obliged to steer much more to the westward than she wishes to do, in consequence of the wind blowing so directly towards the equator, and not along it, as some of the books will insist on, in spite of Nature.

So that if she be a dull sailer she may have some difficulty in weathering the coast of Brazil about Cape St. Roque. As she proceeds onwards, however, and makes a little more southing, the wind will haul more and more round from the south to the south-east, then east-south-east, and eventually to east at the southern limit of the Trade-wind. An inexperienced sailor, on first entering the south-east Trade, is very apt to be too solicitous about making southing, and hugs the wind much too close; whereas he ought rather to keep his s.h.i.+p off a little, give her a fathom or two of the fore and main sheets, and take a small pull of the weather topsail and top-gallant braces, to ensure making good way through the water. Indeed, many officers go so far as to recommend flanking across the south-east Trade with a fore-topmast studding-sail set. Although, I think, there can be no doubt of the soundness of this advice, I confess that it does require no inconsiderable degree of faith to adopt a course, which, apparently, takes the s.h.i.+p not directly away from her object, but very much out of the straight road. In this respect, it may be remarked that the scale of navigation on every Indian voyage is so great, and the importance of getting into those parallels where favourable breezes are certain to be met with, of so much more consequence than the gain of mere distance, that two or three hundred miles to the right or left, or even twice that s.p.a.ce, is often not to be regarded.

Accordingly, in cutting or flanking across the south-east Trade-wind, the object, it should be remembered, is not to shorten the distance, but to reach those lat.i.tudes where strong westerly gales are to be met with, by help of which five hundred or a thousand miles of lost distance are speedily made up, and the rest of the pa.s.sage secured.

In those regions lying beyond the southern tropic westerly winds prevail during the greater part of the year, exactly as we find on this side of the northern tropic. In the southern hemisphere, and far from the land, the wind may be said to blow from the westward almost as steadily as the Trades do from the eastward. The great object, therefore, for an outward-bound s.h.i.+p is to get far enough south to ensure this fair wind. Beyond the lat.i.tude of 30, and as far as 40, this purpose will generally be answered.

We are sufficiently familiar in England with the fact of westerly winds prevailing in the Atlantic. From a list of the pa.s.sages made by the New York sailing packets across the Atlantic, during a period of six years, it is shown that the average length of the voyage from Liverpool to America, that is, towards the west, was forty days; while the average length of the homeward pa.s.sage, or that from west to east, was only twenty-three days. And it may fix these facts more strongly in the recollection, to mention that the pa.s.sage-money from England to America (in the days of sailing packets) was five guineas more than that paid on the return voyage.

This prevalence of westerly winds beyond the tropics is readily explained by the same reasoning which has been applied to the Trades blowing within them. The swift moving air of the torrid zone, on being rarefied and raised up, flows along towards the poles, and in a direction from the equator, above the cooler and slower-moving air, which, as I have already described, is drawn along the surface of the earth from the temperate regions beyond the tropics. When the rarefied equatorial air has travelled some thirty or forty degrees of lat.i.tude along the upper regions of the atmosphere towards the poles it becomes cooled, and is ready to descend again, between the lat.i.tudes of 30 and 60, to supply the place of the lower air, drawn off towards the equator by the Trade-winds. But this partially-cooled air falls on a part of the earth's surface which is moving much more slowly towards the east, in its diurnal rotation, than the air which has descended upon it, and which is still impressed with a great proportion of its eastern velocity due to the equatorial parallels of lat.i.tude, where it was heated and raised up. The necessary consequence of this is, to produce a rapid motion in the air from the west over the earth's surface; and this, combined with the other motion of the same portion of air, or that which has driven it from the equatorial regions, produces this remarkable prevalence of south-westerly winds in the northern hemisphere, and north-westerly winds in the southern hemisphere, in those districts lying between the lat.i.tudes of 30 and 60.

In all that has been said above it has been a.s.sumed that the quickest-moving or equatorial belt of the earth is also the hottest, and consequently that over which the air has the greatest tendency to rise. But, although this is generally true, it is not, by any means, universally so. The variations, however, which are observed to occur in those places where the circ.u.mstances form an exception to the general rule, tend strongly to confirm the theory of Hadley. The monsoons of India, as I shall presently show, are examples of this; but the most striking instance with which I am personally acquainted occurs in the Pacific Ocean, between the Bay of Panama and the Peninsula of California, from lat.i.tude 8 to 22 north. If the huge continent of Mexico were taken away, and only sea left in its place, there can be no doubt but the ordinary phenomena of the Trade-winds would be observable in that part of the Pacific above mentioned. Cool air would then be drawn from the slow moving parallels lying to the northward, towards the swift moving lat.i.tudes, near the equator, in order to supply the place of the rarefied air removed to the higher regions of the atmosphere, and, of course, north-easterly breezes would be produced. But when the sun comes over Mexico, that vast district of country is made to act the part of an enormous heater, and becomes a far more powerful cause of rarefaction to the superinc.u.mbent air than the ocean which lies between it and the equator. Accordingly, the air over Mexico, between the lat.i.tudes of 10 and 30, is more heated than that which lies over the sea between the line and lat.i.tude 20; and as the coolest, or least heated, that is, the most dense fluid, always rushes towards the place lately occupied by the hottest and most buoyant, the air from the equator will be drawn towards the coast of Mexico, the great local source of heat and rarefaction.

But as this equatorial air is of course impressed with a more rapid eastern velocity than those parts of the earth which form the southern sh.o.r.es of Mexico, a westerly wind must be produced by the relative difference in these two motions. At that particular season of the year when the sun is in high southern declination, Mexico is not exposed to his perpendicular rays. The equatorial regions are then more heated than Mexico, and accordingly we actually find north-easterly breezes nearly as they would be if Mexico were out of the way, and quite in accordance with our theory.

In like manner, in the Atlantic, when the sun is far to the north, the great deserts of the western angle, or shoulder of Africa, become as vehemently heated, or more so, perhaps, than Mexico, and this draws the air from the equator, so as to produce the south-westerly winds I have already spoken of in the troublesome range called the Variables.

Finally, the great monsoons of the Indian ocean and China sea contribute to establish this theory of Hadley, though I am not aware that he ever brought it to bear on these very interesting phenomena.

They are eminently deserving of such notice, however, from being periodical Trade-winds of the highest order of utility in one of the busiest commercial regions of the world. Their periodical or s.h.i.+fting character is the circ.u.mstance upon which their extensive utility in a great measure depends, amongst nations where the complicated science of navigation is but in a rude state. Myriads of vessels sail from their homes during one monsoon before the wind, or so nearly before it, that there is no great skill required in reaching all the ports at which they wish to touch; and when the wind s.h.i.+fts to the opposite quarter, they steer back again, in like manner, with a flowing sheet.

Thus, with an exceedingly small portion of nautical skill, they contrive to make their pa.s.sages by means of what we blue-jackets call "a soldier's wind, there and back again." It will sometimes happen that these rude navigators miscalculate their time, or meet with accidents to r.e.t.a.r.d them till the period of change has gone past, and then they have no resource but to wait for half-a-year till the monsoon s.h.i.+fts.

Experienced sailors, in like circ.u.mstances, acquainted with the varieties of winds prevailing in those seas, would speedily get their vessels out of this sc.r.a.pe, into which the lubberly Chinese junks sometimes fall. They might, and certainly would, lose time in making a roundabout of some two or three hundred miles in searching for a wind; but, if they really knew what they were about, they would be sure to catch it at last, and to turn it to their purpose.

From April to October, when the sun's rays fall with greatest effect on Arabia, India, and China, and the several interjacent seas to which these immense countries give their name, the air in contact with them, becoming heated, rises, and gives place to fresh supplies drawn from the equator. But this equatorial ma.s.s of air has had imparted to it by the earth's rotation a greater degree of velocity in the direction from west to east than belongs to the countries and seas just mentioned; and this additional velocity, combined with its motion from the equator, in rus.h.i.+ng to fill up the vacuum caused by the rarefaction of the air over those regions intersected by the tropic, causes the south-west monsoon. "This wind," says Horsburgh, "prevails from April to October, between the equator and the tropic of Cancer, and it reaches from the east coast of Africa to the coasts of India, China, and the Philippine Islands; its influence extends sometimes into the Pacific Ocean as far as the Marian Islands, on to longitude about 145 east, and it reaches as far north as the j.a.pan Islands."

The late Captain Horsburgh thus describes what takes place in the winter months:--"The north-east monsoon," he says, "prevails from October to May, throughout nearly the same s.p.a.ce that the south-west monsoon prevails in the opposite season mentioned above. But the monsoons are subject to great obstructions by land; and in contracted places, such as Malacca Strait, they are changed into variable winds.

Their limits are not everywhere the same, nor do they always s.h.i.+ft exactly at the same period."

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 lat.i.tude, 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 ill.u.s.trations.

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 lat.i.tude, but subject to irregularities." This westerly wind is evidently produced by the air drawn actually from the equator towards the slower moving lat.i.tudes of the earth, by the rarefaction of the air to the southward when the sun is near the tropic of Capricorn.