The history of Sumatra

by William Marsden

The effect of the sea-wind is not perceived to the distance of more than three or four leagues from the shore in common, and for the most part it is fainter in proportion to the distance. When it first sets in it does not commence at the remoter extremity of its limits but very near the shore, and gradually extends itself farther to sea, as the day advances; probably taking the longer or shorter course as the day is more or less hot. I have frequently observed the sails of ships at the distance of four, six, or eight miles, quite becalmed, whilst a fresh sea-breeze was at the time blowing upon the shore. In an hour afterwards they have felt its effect.*

(*Footnote. This observation as well as many others I have made on the subject I find corroborated in the Treatise before quoted from the Haerlem Transactions which I had not seen when the present work was first published.)

Passing along the beach about six o'clock in the evening when the sea-breeze is making its final efforts, I have perceived it to blow with a considerable degree of warmth, owing to the heat the sea had by that time acquired, which would soon begin to divert the current of air towards it when it had first overcome the vis inertiae that preserves motion in a body after the impelling power has ceased to operate. I have likewise been sensible of a degree of warmth on passing, within two hours after sunset, to leeward of a lake of fresh water; which proves the assertion of water imbibing a more permanent heat than earth. In the daytime the breeze would be rendered cool in crossing the same lake.

Approaching an island situated at a distance from any other land, I was struck with the appearance of the clouds about nine in the morning which then formed a perfect circle round it, the middle being a clear azure, and resembled what the painters call a glory. This I account for from the reflected rays of the sun rarefying the atmosphere immediately over the island, and equally in all parts, which caused a conflux of the neighbouring air, and with in the circumjacent clouds. These last, tending uniformly to the centre, compressed each other at a certain distance from it, and, like the stones in an arch of masonry, prevented each other's nearer approach. That island, however, does not experience the vicissitude of land and sea breezes, being too small, and too lofty, and situated in a latitude where the trade or perpetual winds prevail in their utmost force. In sandy countries, the effect of the sun's rays penetrating deeply, a more permanent heat is produced, the consequence of which should be the longer continuance of the sea-breeze in the evening; and agreeably to this supposition I have been informed that on the coast of Coromandel it seldom dies away before ten at night. I shall only add on this subject that the land-wind on Sumatra is cold, chilly, and damp; an exposure to it is therefore dangerous to the health, and sleeping in it almost certain death.

SOIL.

The soil of the western side of Sumatra may be spoken of generally as a stiff, reddish clay, covered with a stratum or layer of black mould, of no considerable depth. From this there springs a strong and perpetual verdure of rank grass, brushwood, or timber-trees, according as the country has remained a longer or shorter time undisturbed by the consequences of population, which, being in most places extremely thin, it follows that a great proportion of the island, and especially to the southward, is an impervious forest.

UNEVENNESS OF SURFACE.

Along the western coast of the island the low country, or space of land which extends from the seashore to the foot of the mountains, is intersected and rendered uneven to a surprising degree by swamps whose irregular and winding course may in some places be traced in a continual chain for many miles till they discharge themselves either into the sea, some neighbouring lake, or the fens that are so commonly found near the banks of the larger rivers and receive their overflowings in the rainy monsoons. The spots of land which these swamps encompass become so many islands and peninsulas, sometimes flat at top, and often mere ridges; having in some places a gentle declivity, and in others descending almost perpendicularly to the depth of a hundred feet. In few parts of the country of Bencoolen, or of the northern districts adjacent to it, could a tolerably level space of four hundred yards square be marked out. I have often, from an elevated situation, where a wider range was subjected to the eye, surveyed with admiration the uncommon face which nature assumes, and made inquiries and attended to conjectures on the causes of these inequalities. Some choose to attribute them to the successive concussions of earthquakes through a course of centuries. But they do not seem to be the effect of such a cause. There are no abrupt fissures; the hollows and swellings are for the most part smooth and regularly sloping so as to exhibit not unfrequently the appearance of an amphitheatre, and they are clothed with verdure from the summit to the edge of the swamp. From this latter circumstance it is also evident that they are not, as others suppose, occasioned by the falls of heavy rains that deluge the country for one half of the year; which is likewise to be inferred from many of them having no apparent outlet and commencing where no torrent could be conceived to operate. The most summary way of accounting for this extraordinary unevenness of surface were to conclude that, in the original construction of our globe, Sumatra was thus formed by the same hand which spread out the sandy plains of Arabia, and raised up the alps and Andes beyond the region of the clouds. But this is a mode of solution which, if generally adopted, would become an insuperable bar to all progress in natural knowledge by damping curiosity and restraining research. Nature, we know from sufficient experience, is not only turned from her original course by the industry of man, but also sometimes checks and crosses her own career. What has happened in some instances it is not unfair to suppose may happen in others; nor is it presumption to trace the intermediate causes of events which are themselves derived from one first, universal, and eternal principle.

CAUSES OF THIS INEQUALITY.

To me it would seem that the springs of water with which these parts of the island abound in an uncommon degree operate directly, though obscurely, to the producing this irregularity of the surface of the earth. They derive their number and an extraordinary portion of activity from the loftiness of the ranges of mountains that occupy the interior country, and intercept and collect the floating vapours. Precipitated into rain at such a hight, the water acquires in its descent through the fissures or pores of these mountains a considerable force which exerts itself in every direction, lateral and perpendicular, to procure a vent. The existence of these copious springs is proved in the facility with which wells are everywhere sunk; requiring no choice of ground but as it may respect the convenience of the proprietor; all situations, whether high or low, being prodigal of this valuable element. Where the approaches of the sea have rendered the cliffs abrupt, innumerable rills, or rather a continued moisture, is seen to ooze through and trickle down the steep. Where on the contrary the sea has retired and thrown up banks of sand in its retreat I have remarked the streams of water, at a certain level and commonly between the boundaries of the tide, effecting their passage through the loose and feeble barrier opposed to them. In short, every part of the low country is pregnant with springs that labour for the birth; and these continual struggles, this violent activity of subterraneous waters, must gradually undermine the plains above. The earth is imperceptibly excavated, the surface settles in, and hence the inequalities we speak of. The operation is slow but unremitting, and, I conceive, fully capable of the effect.