The lieutenant and commander

by Basil Hall

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 ship is to get far enough south to ensure this fair wind. Beyond the latitude 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 passages 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 passage, 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 passage-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 latitude along the upper regions of the atmosphere towards the poles it becomes cooled, and is ready to descend again, between the latitudes 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 latitude, 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 latitudes of 30° and 60°.

In all that has been said above it has been assumed 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 circumstances 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 latitude 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 latitudes, 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 superincumbent air than the ocean which lies between it and the equator. Accordingly, the air over Mexico, between the latitudes of 10° and 30°, is more heated than that which lies over the sea between the line and latitude 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 shores 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 shifting character is the circumstance 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 shifts 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 passages 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 retard 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 shifts.

Experienced sailors, in like circumstances, acquainted with the varieties of winds prevailing in those seas, would speedily get their vessels out of this scrape, 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 mass 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 rushing 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 Japan 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 space 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 shift exactly at the same period."