Читать книгу The History of Sumatra - William Marsden - Страница 7
ОглавлениеLAND 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.
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.
MINERAL PRODUCTIONS.
The earth of Sumatra is rich in minerals and other fossil productions.
GOLD.
No country has been more famous in all ages for gold, and, though the sources from whence it is drawn may be supposed in some measure exhausted by the avarice and industry of ages, yet at this day the quantity procured is very considerable, and doubtless might be much increased were the simple labour of the gatherer assisted by a knowledge of the arts of mineralogy.
COPPER, IRON, TIN, SULPHUR.
There are also mines of copper, iron, and tin. Sulphur is gathered in large quantities about the numerous volcanoes.
SALTPETRE.
Saltpetre the natives procure by a process of their own from the earth which is found impregnated with it; chiefly in extensive caves that have been, from the beginning of time, the haunt of a certain species of birds, of whose dung the soil is formed.
COAL.
Coal, mostly washed down by the floods, is collected in several parts, particularly at Kataun, Ayer-rammi, and Bencoolen. It is light and not esteemed very good; but I am informed that this is the case with all coal found near the surface of the earth, and, as the veins are observed to run in an inclined direction until the pits have some depth, the fossil must be of an indifferent quality. The little island of Pisang, near the foot of Mount Pugong, was supposed to be chiefly a bed of rock crystal, but upon examination of specimens taken from thence they proved to be calcareous spar.
HOT SPRINGS.
Mineral and hot springs have been discovered in many districts. In taste the waters mostly resemble those of Harrowgate, being nauseous to the palate.
EARTH OIL.
The oleum terrae, or earth oil, used chiefly as a preservative against the destructive ravages of the white-ants, is collected at Ipu and elsewhere.*
(*Footnote. The fountain of Naphtha or liquid balsam found at Pedir, so much celebrated by the Portuguese writers, is doubtless this oleum terrae, or meniak tanah, as it is called by the Malays.)
SOFT ROCK.
There is scarcely any species of hard rock to be met with in the low parts of the island near the seashore. Besides the ledges of coral, which are covered by the tide, that which generally prevails is the napal, as it is called by the inhabitants, forming the basis of the red cliffs, and not infrequently the beds of the rivers. Though this napal has the appearance of rock it possesses in fact so little solidity that it is difficult to pronounce whether it be a soft stone or only an indurated clay. The surface of it becomes smooth and glossy by a slight attrition, and to the touch resembles soap, which is its most striking characteristic; but it is not soluble in water and makes no effervescence with acids. Its colour is either grey, brown, or red, according to the nature of the earth that prevails in its composition. The red napal has by much the smallest proportion of sand, and seems to possess all the qualities of the steatite or soap-earth found in Cornwall and other countries. The specimens of stone which I brought from the hills in the neighbourhood of Bencoolen were pronounced by some mineralogists, to whom I showed them at the time, to be granite; but upon more particular examination they appear to be a species of trap, consisting principally of feldspar and hornblende, of a greyish colour and nearly similar to the mountain stone of North Wales.
PETRIFACTION.
Where the encroachments of the sea have undermined the land the cliffs are left abrupt and naked, in some places to a very considerable height. In these many curious fossils are discovered, such as petrified wood, and seashells of various sorts. Hypotheses on this subject have been so ably supported and so powerfully attacked that I shall not presume to intrude myself in the lists. I shall only observe that, being so near the sea, many would hesitate to allow such discoveries to be of any weight in proving a violent alteration to have taken place in the surface of the terraqueous globe; whilst, on the other hand, it is unaccountable how, in the common course of natural events, such extraneous matter should come to be lodged in strata at the height perhaps of fifty feet above the level of the water, and as many below the surface of the land.
COLOURED EARTHS.
Here are likewise found various species of earths which might be applied to valuable purposes, as painters' colours, and otherwise. The most common are the yellow and red, probably ochres, and the white, which answers the description of the milenum of the ancients.
VOLCANOES.
There are a number of volcano mountains in this, as in almost all the other islands of the eastern Archipelago. They are called in the Malay language gunong-api, or more correctly, gunong ber-api. Lava has been seen to flow from a considerable one near Priamang; but I have never heard of its causing any other damage than the burning of woods. This however may be owing to the thinness of population, which does not render it necessary for the inhabitants to settle in a situation that exposes them to danger of this kind. The only volcano I had an opportunity of observing opened in the side of a mountain, about twenty miles inland of Bencoolen, one-fourth way from its top, as nearly as I can judge. It scarcely ever failed to emit smoke; but the column was only visible for two or three hours in the morning, seldom rising and preserving its form, above the upper edge of the hill, which is not of a conical shape but extending with a gradual slope.
EARTHQUAKES.
The high trees with which the country thereabout is covered, prevent the crater from being discernible at a distance; and this proves that the spot is not considerably raised or otherwise affected by the earthquakes which are very frequently felt there. Sometimes it has emitted smoke upon these occasions, and in other instances not. Yet during a smart earthquake which happened a few years before my arrival it was remarked to send forth flame, which it is rarely known to do.* The apprehension of the European inhabitants however is rather more excited when it continues any length of time without a tendency to an eruption, as they conceive it to be the vent by which the inflammable matter escapes that would otherwise produce these commotions of the earth. Comparatively with the descriptions I have read of earthquakes in South America, Calabria, and other countries, those which happen in Sumatra are generally very slight; and the usual manner of building renders them but little formidable to the natives.
(*Footnote. Some gentlemen who deny the fact of its having at any time emitted flame, conjecture that what exhibits the appearance of smoke is more probably vapour arising from a considerable hot spring. The natives speak of it as a volcano.)
REMARKABLE EFFECTS OF AN EARTHQUAKE.
The most severe that I have known was chiefly experienced in the district of Manna in the year 1770. A village was destroyed by the houses falling down and taking fire, and several lives were lost.* The ground was in one place rent a quarter of a mile, the width of two fathoms, and depth of four or five. A bituminous matter is described to have swelled over the sides of the cavity, and the earth for a long time after the shocks was observed to contract and dilate alternately. Many parts of the hills far inland could be distinguished to have given way, and a consequence of this was that during three weeks Manna River was so much impregnated with particles of clay that the natives could not bathe in it. At this time was formed near to the mouth of Padang Guchi, a neighbouring river south of the former, a large plain, seven miles long and half a mile broad; where there had been before only a narrow beach. The quantity of earth brought down on this occasion was so considerable that the hill upon which the English resident's house stands appears, from indubitable marks, less elevated by fifteen feet than it was before the event.
(*Footnote. I am informed that in 1763 an entire village was swallowed up by an earthquake in Pulo Nias, one of the islands which lie off the western coast of Sumatra. In July or August of the same year a severe one was felt in Bengal.)
Earthquakes have been remarked by some to happen usually upon sudden changes of weather, and particularly after violent heats; but I do not vouch this upon my own experience, which has been pretty ample. They are preceded by a low rumbling noise like distant thunder. The domestic cattle and fowls are sensible of the preternatural motion, and seem much alarmed; the latter making the cry they are wont to do on the approach of birds of prey. Houses situated on a low sandy soil are least affected, and those which stand on distinct hills suffer most from the shocks because the further removed from the centre of motion the greater the agitation; and the loose contexture of the one foundation, making less resistance than the solidity of the other, subjects the building to less violence. Ships at anchor in the road, though several miles distant from the shore, are strongly sensible of the concussion.
NEW LAND FORMED.
Besides the new land formed by the convulsions above described, the sea by a gradual recess in some parts produces the same effect. Many instances of this kind, of no considerable extent however have been observed within the memory of persons now living. But it would seem to me that that large tract of land called Pulo Point, forming the bay of the name, near to Silebar, with much of the adjacent country has thus been left by the withdrawing or thrown up by the motion of the sea. Perhaps the point may have been at first an island (from whence its appellation of Pulo) and the parts more inland gradually united to it.* Various circumstances tend to corroborate such an opinion, and to evince the probability that this was not an original portion of the main but new, half-formed land. All the swamps and marshy grounds that lie within the beach, and near the extremity there are little else, are known, in consequence of repeated surveys, to be lower than the level of high-water; the bank of sand alone preventing an inundation. The country is not only quite free from hills or inequalities of any kind, but has scarcely a visible slope. Silebar River, which empties itself into Pulo Bay, is totally unlike those in other parts of the island. The motion of its stream is hardly perceptible; it is never affected by floods; its course is marked out, not by banks covered with ancient and venerable woods but by rows of mangroves and other aquatics springing from the ooze, and perfectly regular. Some miles from the mouth it opens into a beautiful and extensive lake, diversified with small islands, flat, and verdant with rushes only. The point of Pulo is covered with the arau tree (casuarina) or bastard-pine, as some have called it, which never grows but in the seasand and rises fast.
(*Footnote. Since I formed this conjecture I have been told that such a tradition of no very ancient date prevails amongst the inhabitants.)
ENCROACHMENT OF THE SEA.
None such are found toward Sungei-Lamo and the rest of the shore northward of Marlborough Point, where, on the contrary, you perceive the effects of continual depredations by the ocean. The old forest trees are there yearly undermined and, falling, obstruct the traveller; whilst about Pulo the arau-trees are continually springing up faster than they can be cut down or otherwise destroyed. Nature will not readily be forced from her course. The last time I visited that part there was a beautiful rising grove of these trees, establishing a possession in their proper soil. The country, as well immediately here about as to a considerable distance inland, is an entire bed of sand without any mixture of clay or mould, which I know to have been in vain sought for many miles up the neighbouring rivers. To the northward of Padang there is a plain which has evidently been, in former times, a bay. Traces of a shelving beach are there distinguishable at the distance of one hundred and fifty yards from the present boundary of the sea.
But upon what hypothesis can it be accounted for that the sea should commit depredations on the northern coast, of which there are the most evident tokens as high up at least as Ipu, and probably to Indrapura, where the shelter of the neighbouring islands may put a stop to them, and that it should restore the land to the southward in the manner I have described? I am aware that according to the general motion of the tides from east to west this coast ought to receive a continual accession proportioned to the loss which others, exposed to the direction of this motion, must and do sustain; and it is likely that it does gain upon the whole. But the nature of my work obliges me to be more attentive to effects than causes, and to record facts though they should clash with systems the most just in theory, and most respectable in point of authority.
ISLANDS NEAR THE WEST COAST.
The chain of islands which lie parallel with the west coast of Sumatra may probably have once formed a part of the main and been separated from it, either by some violent effort of nature, or the gradual attrition of the sea. I should scarcely introduce the mention of this apparently vague surmise but that a circumstance presents itself on the coast which affords some stronger colour of proof than can be usually obtained in such instances. In many places, and particularly about Pally, we observe detached pieces of land standing singly, as islands, at the distance of one or two hundred yards from the shore, which were headlands of points running out into the sea within the remembrance of the inhabitants. The tops continue covered with trees or shrubs; but the sides are bare, abrupt, and perpendicular. The progress of insulation here is obvious and incontrovertible, and why may not larger islands, at a greater distance, have been formed in the revolution of ages by the same accidents? The probability is heightened by the direction of the islands Nias, Batu, Mantawei, Pagi, Mego, etc., the similarity of the rock, soil, and productions, and the regularity of soundings between them and the main, whilst without them the depth is unfathomable.
CORAL ROCKS.
Where the shore is flat or shelving the coast of Sumatra, as of all other tropical islands, is defended from the attacks of the sea by a reef or ledge of coral rock on which the surfs exert their violence without further effect than that of keeping its surface even, and reducing to powder those beautiful excrescences and ramifications which have been so much the object of the naturalist's curiosity, and which some ingenious men who have analysed them contend to be the work of insects. The coral powder is in particular places accumulated on the shore in great quantities, and appears, when not closely inspected, like a fine white sand.
SURF.
The surf (a word not to be found, I believe, in our dictionaries) is used in India, and by navigators in general, to express a peculiar swell and breaking of the sea upon the shore; the phenomena of which not having been hitherto much adverted to by writers I shall be the more circumstantial in my description of them.
The surf forms sometimes but a single range along the shore. At other times there is a succession of two, three, four, or more, behind each other, extending perhaps half a mile out to sea. The number of ranges is generally in proportion to the height and violence of the surf.
The surf begins to assume its form at some distance from the place where it breaks, gradually accumulating as it moves forward till it gains a height, in common, of fifteen to twenty feet,* when it overhangs at top and falls like a cascade, nearly perpendicular, involving itself as it descends. The noise made by the fall is prodigious, and during the stillness of the night may be heard many miles up the country.
(*Footnote. It may be presumed that in this estimation of its height I was considerably deceived.)
Though in the rising and formation of the surf the water seems to have a quick progressive motion towards the land, yet a light body on the surface is not carried forward, but, on the contrary, if the tide is ebbing, will recede from the shore; from which it would follow that the motion is only propagated in the water, like sound in air, and not the mass of water protruded. A similar species of motion is observed on shaking at one end a long cord held moderately slack, which is expressed by the word undulation. I have sometimes remarked however that a body which sinks deep and takes hold of the water appears to move towards shore with the course of the surf, as is perceptible in a boat landing which seems to shoot swiftly forward on the top of the swell; though probably it is only after having reached the summit, and may owe its velocity to its own weight in the descent.
Countries where the surfs prevail require boats of a peculiar construction, and the art of managing them demands the experience of a man's life. All European boats are more or less unfit, and seldom fail to occasion the sacrifice of the people on board them, in the imprudent attempts that are sometimes made to land with them on the open coast. The natives of Coromandel are remarkably expert in the management of their craft; but it is to be observed that the intervals between the breaking of the surfs are usually on that coast much longer than on the coast of Sumatra.
The force of the surf is extremely great. I have known it to overset a country vessel in such a manner that the top of the mast has stuck in the sand, and the lower end made its appearance through her bottom. Pieces of cloth have been taken up from a wreck, twisted and rent by its involved motion. In some places the surfs are usually greater at high, and in others at low, water; but I believe they are uniformly more violent during the spring-tides.
CONSIDERATIONS RESPECTING THE CAUSE OF THE SURF.
I shall proceed to inquire into the efficient cause of the surfs. The winds have doubtless a strong relation to them. If the air was in all places of equal density, and not liable to any motion, I suppose the water would also remain perfectly at rest and its surface even; abstracting from the general course of the tides and the partial irregularities occasioned by the influx of rivers. The current of the air impels the water and causes a swell, which is the regular rising and subsiding of the waves. This rise and fall is similar to the vibrations of a pendulum and subject to like laws. When a wave is at its height it descends by the force of gravity, and the momentum acquired in descending impels the neighbouring particles, which in their turn rise and impel others, and thus form a succession of waves. This is the case in the open sea; but when the swell approaches the shore and the depth of water is not in proportion to the size of the swell the subsiding wave, instead of pressing on a body of water, which might rise in equal quantity, presses on the ground, whose reaction causes it to rush on in that manner which we call a surf. Some think that the peculiar form of it may be plainly accounted for from the shallowness and shelving of the beach. When a swell draws near to such a beach the lower parts of the water, meeting first with obstruction from the bottom, stand still, whilst the higher parts respectively move onward, by which a rolling and involved motion is produced that is augmented by the return of the preceding swell. I object that this solution is founded on the supposition of an actual progressive motion of the body of water in forming a surf; and, that certainly not being the fact, it seems deficient. The only real progression of the water is occasioned by the perpendicular fall, after the breaking of the surf, when from its weight it foams on to a greater or less distance in proportion to the height from which it fell and the slope of the shore.
That the surfs are not, like common waves, the immediate effect of the wind, is evident from this, that the highest and most violent often happen when there is the least wind and vice versa. And sometimes the surfs will continue with an equal degree of violence during a variety of weather. On the west coast of Sumatra the highest are experienced during the south-east monsoon, which is never attended with such gales of wind as the north-west. The motion of the surf is not observed to follow the course of the wind, but often the contrary; and when it blows hard from the land the spray of the sea may be seen to fly in a direction opposite to the body of it, though the wind has been for many hours in the same point.
Are the surfs the effect of gales of wind at sea, which do not happen to extend to the shore but cause a violent agitation throughout a considerable tract of the waters, which motion, communicating with less distant parts, and meeting at length with resistance from the shore, occasions the sea to swell and break in the manner described? To this I object that there seems no regular correspondence between their magnitude and the apparent agitation of the water without them: that gales of wind, except at particular periods, are very unfrequent in the Indian seas, where the navigation is well known to be remarkably safe, whilst the surfs are almost continual; and that gales are not found to produce this effect in other extensive oceans. The west coast of Ireland borders a sea nearly as extensive and much more wild than the coast of Sumatra, and yet there, though when it blows hard the swell on the shore is high and dangerous, is there nothing that resembles the surfs of India.
PROBABLE CAUSE OF THE SURF.
These, so general in the tropical latitudes, are, upon the most probable hypothesis I have been able to form, after long observation and much thought and inquiry, the consequence of the trade or perpetual winds which prevail at a distance from shore between the parallels of thirty degrees north and south, whose uniform and invariable action causes a long and constant swell, that exists even in the calmest weather, about the line, towards which its direction tends from either side. This swell or libration of the sea is so prodigiously long, and the sensible effect of its height, of course, so much diminished, that it is not often attended to; the gradual slope engrossing almost the whole horizon when the eye is not very much elevated above its surface: but persons who have sailed in those parts may recollect that, even when the sea is apparently the most still and level, a boat or other object at a distance from the ship will be hidden from the sight of one looking towards it from the lower deck for the space of minutes together. This swell, when a squall happens or the wind freshens up, will for a time have other subsidiary waves on the extent of its surface, breaking often in a direction contrary to it, and which will again subside as a calm returns without having produced on it any perceptible effect. Sumatra, though not continually exposed to the south-east trade-wind, is not so distant but that its influence may be presumed to extend to it, and accordingly at Pulo Pisang, near the southern extremity of the island, a constant southerly sea is observed even after a hard north-west wind. This incessant and powerful swell rolling in from an ocean, open even to the pole, seems an agent adequate to the prodigious effects produced on the coast; whilst its very size contributes to its being overlooked. It reconciles almost all the difficulties which the phenomena seem to present, and in particular it accounts for the decrease of the surf during the north-west monsoon, the local wind then counteracting the operation of the general one; and it is corroborated by an observation I have made that the surfs on the Sumatran coast ever begin to break at their southern extreme, the motion of the swell not being perpendicular to the direction of the shore. This manner of explaining their origin seems to carry much reason with it; but there occurs to me one objection which I cannot get over, and which a regard to truth obliges me to state. The trade-winds are remarkably steady and uniform, and the swell generated by them is the same. The surfs are much the reverse, seldom persevering for two days in the same degree of violence; often mountains high in the morning and nearly subsided by night. How comes a uniform cause to produce effects so unsteady, unless by the intervention of secondary causes, whose nature and operation we are unacquainted with?
It is clear to me that the surfs as above described are peculiar to those climates which lie within the remoter limits of the trade-winds, though in higher latitudes large swells and irregular breakings of the sea are to be met with after boisterous weather. Possibly the following causes may be judged to conspire, with that I have already specified, towards occasioning this distinction. The former region being exposed to the immediate influence of the two great luminaries, the water, from their direct impulse, is liable to more violent agitation than nearer the poles where their power is felt only by indirect communication. The equatorial parts of the earth performing their diurnal revolution with greater velocity than the rest, a larger circle being described in the same time, the waters thereabout, from the stronger centrifugal force, may be supposed to feel less restraint from the sluggish principle of matter; to have less gravity; and therefore to be more obedient to external impulses of every kind, whether from the winds or any other cause.
TIDES.
The spring-tides on the west coast of Sumatra are estimated to rise in general no more than four feet, owing to its open, unconfined situation, which prevents any accumulation of the tide, as is the case in narrow seas. It is always high-water there when the moon is in the horizon, and consequently at six o'clock nearly, on the days of conjunction and opposition throughout the year, in parts not far remote from the equator.* This, according to Newton's theory, is about three hours later than the uninterrupted course of nature, owing to the obvious impediment the waters meet with in revolving from the eastward.
(*Footnote. Owing to this uniformity it becomes an easy matter for the natives to ascertain the height of the tide at any hour that the moon is visible. Whilst she appears to ascend the water falls and vice versa; the lowest of the ebb happening when she is in her meridian. The vulgar rule for calculating the tides is rendered also to Europeans more simple and practical from the same cause. There only needs to add together the epact, number of the month, and day of the month; the sum of which, if under thirty, gives the moon's age--the excess, if over. Allow forty-eight minutes for each day or, which is the same, take four-fifths of the age, and it will give you the number of hours after six o'clock at which high-water happens. A readiness at this calculation is particularly useful in a country where the sea-beach is the general road for travelling.)