The history of Sumatra

by William Marsden

On the west coast of Sumatra, southward of the equinoctial, the south-east monsoon or dry season begins about May and slackens in September: the north-west monsoon begins about November, and the hard rains cease about March. The monsoons for the most part commence and leave off gradually there; the months of April and May, October and November generally affording weather and winds variable and uncertain.

CAUSE OF THE MONSOONS.

The causes of these periodical winds have been investigated by several able naturalists, whose systems, however, do not entirely correspond either in the principles laid down or in their application to the effects known to be produced in different parts of the globe. I shall summarily mention what appear to be the most evident, or probable at least, among the general laws, or inferences, which have been deduced from the examination of this subject. If the sea were perfectly uninterrupted and free from the irregular influence of lands, a perpetual easterly wind would prevail in all that space comprehended between the twenty-eighth or thirteenth degrees of north and south latitude. This is primarily occasioned by the diurnal revolution of the earth upon its axis from west to east; but whether through the operation of the sun, proceeding westward, upon the atmospheric fluid, or the rapidity of revolution of the solid body, which leaves behind it that fluid with which it is surrounded, and thereby causes it virtually to recede in a contrary direction; or whether these principles cooperate, or unequally oppose each other, as has been ingeniously contended, I shall not take upon me to decide. It is sufficient to say that such an effect appears to be the first general law of the tropical winds. Whatever may be the degree of the sun's influence upon the atmosphere in his transient diurnal course, it cannot be doubted but that, in regard to his station in the path of the ecliptic, his power is considerable. Towards that region of the air which is rarefied by the more immediate presence of the heat, the colder and denser parts will naturally flow. Consequently from about, and a few degrees beyond, the tropics, on either side, the air tends towards the equator; and, combining with the general eastern current before mentioned, produces (or would, if the surface were uniform) a north-east wind in the northern division, and a south-east in the southern; varying in the extent of its course as the sun happens to be more or less remote at the time. These are denominated the trade-winds, and are the subject of the second general observation. It is evident that, with respect to the middle space between the tropics, those parts which at one season of the year lie to the northward of the sun, are, during another, to the southward of him; and of course that an alteration of the effects last described must take place, according to the relative situation of the luminary; or in other words, that the principle which causes at one time a north-east wind to prevail at any particular spot in those latitudes must, when the circumstances are changed, occasion a south-east wind. Such may be esteemed the outline of the periodical winds, which undoubtedly depend upon the alternate course of the sun northwards and southwards; and this I state as the third general law. But although this may be conformable with experience in extensive oceans, yet, in the vicinity of continents and great islands, deviations are remarked that almost seem to overturn the principle. Along the western coast of Africa and in some parts of the Indian seas, the periodical winds, or monsoons as they are termed in the latter, blow from the west-north-west and south-west, according to the situation, extent, and nature of the nearest lands; the effect of which upon the incumbent atmosphere, when heated by the sun at those seasons in which he is vertical, is prodigious, and possibly superior to that of any other cause which contributes to the production or direction of wind. To trace the operation of this irregular principle through the several winds prevalent in India, and their periodical failures and changes, would prove an intricate but, I conceive, by no means an impossible task.* It is foreign however to my present purpose, and I shall only observe that the north-east monsoon is changed, on the western coast of Sumatra, to north-west or west-north-west by the influence of the land. During the south-east monsoon the wind is found to blow there, between that point and south. Whilst the sun continues near the equator the winds are variable, nor is their direction fixed till he has advanced several degrees towards the tropic: and this is the cause of the monsoons usually setting in, as I have observed, about May and November, instead of the equinoctial months.

(*Footnote. It has been attempted, and with much ingenious reasoning, by Mr. Semeyns in the third volume of the Haerlem Transactions which have but lately fallen into my hands.)

LAND AND SEA BREEZES.

Thus much is sufficient with regard to the periodical winds. I shall proceed to give an account of those distinguished by the appellation of land and sea breezes, which require from me a minuter investigation, both because, as being more local, they more especially belong to my subject, and that their nature has hitherto been less particularly treated of by naturalists.

In this island, as well as all other countries between the tropics of any considerable extent, the wind uniformly blows from the sea to the land for a certain number of hours in the four and twenty, and then changes and blows for about as many from the land to the sea; excepting only when the monsoon rages with remarkable violence, and even at such time the wind rarely fails to incline a few points, in compliance with the efforts of the subordinate clause, which has not power, under these circumstances, to produce an entire change. On the west coast of Sumatra the sea-breeze usually sets in, after an hour or two of calm, about ten in the forenoon, and continues till near six in the evening. About seven the land-breeze comes off, and prevails through the night till towards eight in the morning, when it gradually dies away.

CAUSE OF THE LAND AND SEA-BREEZES.

These depend upon the same general principle that causes and regulates all other wind. Heat acting upon air rarefies it, by which it becomes specifically lighter, and mounts upward. The denser parts of the atmosphere which surround that so rarefied, rush into the vacuity from their superior weight; endeavouring, as the laws of gravity require, to restore the equilibrium. Thus in the round buildings where the manufactory of glass is carried on, the heat of the furnace in the centre being intense, a violent current of air may be perceived to force its way in, through doors or crevices, on opposite sides of the house. As the general winds are caused by the DIRECT influence of the sun's rays upon the atmosphere, that particular deviation of the current distinguished by the name of land and sea breezes is caused by the influence of his REFLECTED rays, returned from the earth or sea on which they strike. The surface of the earth is more suddenly heated by the rays of the sun than that of the sea, from its greater density and state of rest; consequently it reflects those rays sooner and with more power: but, owing also to its density, the heat is more superficial than that imbibed by the sea, which becomes more intimately warmed by its transparency and by its motion, continually presenting a fresh surface to the sun. I shall now endeavour to apply these principles. By the time the rising sun has ascended to the height of thirty or forty degrees above the horizon the earth has acquired, and reflected on the body of air situated over it, a degree of heat sufficient to rarefy it and destroy its equilibrium; in consequence of which the body of air above the sea, not being equally, or scarcely at all, rarefied, rushes towards the land and the same causes operating so long as the sun continues above the horizon, a constant sea-breeze, or current of air from sea to land, prevails during that time. From about an hour before sunset the surface of the earth begins to lose the heat it has acquired from the more perpendicular rays. That influence of course ceases, and a calm succeeds. The warmth imparted to the sea, not so violent as that of the land but more deeply imbibed, and consequently more permanent, now acts in turn, and by the rarefaction it causes draws towards its region the land air, grown cooler, more dense, and heavier, which continues thus to flow back till the earth, by a renovation of its heat in the morning, once more obtains the ascendancy. Such is the general rule, conformable with experience, and founded, as it seems to me, in the laws of motion and the nature of things. The following observations will serve to corroborate what I have advanced, and to throw additional light on the subject for the information and guidance of any future investigator.

The periodical winds which are supposed to blow during six months from the north-west and as many from the south-east rarely observe this regularity, except in the very heart of the monsoon; inclining, almost at all times, several points to seaward, and not unfrequently blowing from the south-west or in a line perpendicular to the coast. This must be attributed to the influence of that principle which causes the land and sea winds proving on these occasions more powerful than the principle of the periodical winds; which two seem here to act at right angles with each other; and as the influence of either is prevalent the winds draw towards a course perpendicular to or parallel with the line of the coast. Excepting when a squall or other sudden alteration of weather, to which these climates are particularly liable, produces an irregularity, the tendency of the land-wind at night has almost ever a correspondence with the sea-wind of the preceding or following day; not blowing in a direction immediately opposite to it (which would be the case if the former were, as some writers have supposed, merely the effect of the accumulation and redundance of the latter, without any positive cause) but forming an equal and contiguous angle, of which the coast is the common side. Thus, if the coast be conceived to run north and south, the same influence, or combination of influences, which produces a sea-wind at north-west produces a land-wind at north-east; or adapting the case to Sumatra, which lies north-west and south-east, a sea-wind at south is preceded or followed by a land-wind at east. This remark must not be taken in too strict a sense, but only as the result of general observation. If the land-wind, in the course of the night, should draw round from east to north it would be looked upon as an infallible prognostic of a west or north-west wind the next day. On this principle it is that the natives foretell the direction of the wind by the noise of the surf at night, which if heard from the northward is esteemed the forerunner of a northerly wind, and vice versa. The quarter from which the noise is heard depends upon the course of the land-wind, which brings the sound with it, and drowns it to leeward--the land-wind has a correspondence with the next day's sea-wind--and thus the divination is accounted for.