Kitabı oku: «The Ocean and its Wonders», sayfa 4
Next year he sets forth again but merchants are not always punctual. The lading cannot be completed in time, or adverse winds render the setting sail unadvisable. At length, after a month or six weeks’ delay, he proceeds on his voyage, and finds belt number one perhaps much the same as last year. He congratulates himself on his good fortune, and notes his observations; but in belt number two, the wind is somewhat modified, owing to its being later in the season,—it is rather against him. In number three it is right in his teeth, whereas last year it was quite in his favour. In number four, which we will suppose is the trade-wind belt (of which more hereafter), he finds the wind still easterly. Here, then, is the groundwork of confusion in our sailor’s mind. He has not the remotest idea that in belt number one the wind blows chiefly, but not always, in one particular direction; that in number four it blows invariably in one way; and that in number three it is regularly irregular. In fact, he does not know that such belts exist at all, and his opportunities of observing are not sufficiently frequent or prolonged to enable him to ascertain anything with certainty.
Now, when we remember that in this imperfect experience of his he is still further misled by his frequently encountering local vicissitudes—such as storms and calms resulting from local and temporary causes—we see how confusion becomes worse confounded. No doubt he does gather some few crumbs of knowledge; but he is called on, perhaps, to change his scene of action. Another ship is given to him, another route entered on, and he ceases altogether to prosecute his inquiries in the old region. Or old age comes on; and even although he may have been beginning to have a few faint glimmerings as to laws and systems in his mind, he has not the power to make much of these. He dies; his knowledge is, to a very large extent, lost, and his log-books disappear, as all such books do, nobody knows or cares where.
Now this state of things has been changing during the last few years. Log-books are collected in thousands. The experiences of many men, in reference to the same spots in the same years, months, and even hours, are gathered, collated, and compared; and the result is, that although there are conflicting elements and contradictory appearances, order has been discovered in the midst of apparent confusion, and scientific men have been enabled to pierce through the chaos of littlenesses by which the world’s vision has been hitherto obscured, and to lay bare many of those grand progressions of nature which move unvaryingly with stately step through space and time, as the river, with all its minor eddies and counter-currents, flows with unvarying regularity to the ocean.
Chapter Six
Trade-winds—Storms—Their Effects—Monsoons—Their Value—Land and Sea Breezes—Experiments—Hurricanes—Those of 1801—Rotatory Storms—Their Terrible Effects—China Seas—Hurricane in 1837—Whirlwinds—Weight of Atmosphere—Value of Atmospheric Circulation—Height of Atmosphere
Before proceeding to speak of the power and the dreadful effects of wind, it is necessary to say a word or two about the trade-winds.
It is supposed that the “trades” derived their name from the fact of their being favourable to navigation, and, therefore, to trade. They consist of two belts of wind, one on each side of the equator, which blow always in the same direction.
In the last chapter it was explained that the heated atmosphere at the equator rises, and that the cooler atmosphere from the poles rushes in to supply its place. That which rushes from the south pole is, of course, a south wind, that from the north pole a north wind; but, owing to the Earth’s motion on its axis from west to east, the one becomes a north-east, the other a south-east wind. These are the north-east and the South-east “trades.” They blow regularly—sometimes gently, sometimes fiercely—all the year round. Between the two is a belt of calms and changeable breezes, varying from 150 to 500 miles broad—according to the time of the year—where there are frequent and violent squalls, of very short duration, accompanied with heavy rains. This region is called by seamen the “doldrums,” and considerable trouble and difficulty do ships experience in crossing it.
It has already been explained that about latitude 30 degrees, the upper current of wind from the south descends. At the same point the upper current from the north also descends. They cut through each other, and the point where these two cut each other is the northern limit of the north-east trade-winds. The same explanation holds in regard to the southern limit of the south-east trades.
In the accompanying diagram the arrows within the circle point out the direction of the north-east and the south-east “trades” between the tropics of cancer and capricorn, and also the counter currents to the north and south of these, while the arrows around the circle show how counter currents meet and rise, or descend, and produce the calm belts.
We have hitherto enlarged chiefly on the grand currents of the atmosphere, and on those modifying causes and effects which are perpetual. Let us now turn to the consideration of those winds which are produced by local causes, and the effects of which are partial.
And here we are induced to revert to the Gulf Stream, which has been already referred to as a local disturber of the regular flow of the atmosphere. This immense body of heated water, passing through cold regions of the sea, has the effect of causing the most violent storms. The hurricanes of the West Indies are among the most violent in the world. We have read of one so violent that it “forced the Gulf Stream back to its sources, and piled up the water in the Gulf to the height of thirty feet. A vessel named the Ledbnry Snow attempted to ride it out. When it abated, she found herself high up on the dry land, having let go her anchor among the tree-tops of Elliott’s quay! The Florida quays were inundated many feet; and it is said the scene presented in the Gulf Stream was never surpassed in awful sublimity on the ocean. The water thus dammed up rushed out with frightful velocity against the fury of the gale, producing a sea that beggared description.”
The monsoons of the Indian Ocean are among the most striking and regular of the locally-caused winds. Before touching on their causes, let us glance at their effects. They blow for nearly six months in one direction, and for the other six in the opposite direction. At the period of their changing, terrific gales are frequent—gales such as we, in our temperate regions, never dream of.
What is termed the rainy season in India is the result of the south-west monsoon, which for four months in the year deluges the regions within its influence with rain.
The commencement of the south-west monsoon is described as being sublime and awful beyond description. Before it comes, the whole country is pining under the influence of long-continued drought and heat; the ground is parched and rent; scarcely a blade of verdure is to be seen except in the beds of rivers, where the last pools of water seem about to evaporate, and leave the land under the dominion of perpetual sterility. Man and beast pant for fresh air and cool water; but no cool breeze comes. A blast, as if from the mouth of a furnace, greets the burning cheek; no blessed drops descend; the sky is clear as a mirror, without a single cloud to mitigate the intensity of the sun’s withering rays. At last, on some happy morning, small clouds are seen on the horizon. They may be no bigger than a man’s hand, but they are blessed harbingers of rain. To those who know not what is coming, there seems at first no improvement on the previous sultry calms. There is a sense of suffocating heat in the atmosphere; a thin haze creeps over the sky, but it scarcely affects the broad glare of the sun.
At length the sky begins to change. The horizon becomes black. Great masses of dark clouds rise out of the sea. Fitful gusts of wind begin to blow, and as suddenly to cease; and these signs of coming tempest keep dallying with each other, as if to tantalise the expectant creation. The lower part of the sky becomes deep red, the gathering clouds spread over the heavens, and a deep gloom is cast upon the earth and sea.
And now the storm breaks forth. The violent gusts swell into a continuous, furious gale. Rain falls, not in drops, but in broad sheets. The black sea is crested with white foam, which is quickly swept up and mingled with the waters above; while those below heave up their billows, and rage and roar in unison with the tempest. On the land everything seems about to be uprooted and hurled to destruction. The tall straight cocoa-nut trees are bent over till they almost lie along the ground; the sand and dry earth are whirled up in eddying clouds, and everything movable is torn up and swept away.
To add to the dire uproar, thunder now peals from the skies in loud, continuous roars, and in sharp angry crashes, while lightning plays about in broad sheets all over the sky, the one following so close on the other as to give the impression of perpetual flashes and an unintermitting roar; the whole scene presenting an aspect so awful, that sinful man might well suppose the season of the Earth’s probation had passed away, and that the Almighty were about to hurl complete destruction upon his offending creatures.
But far other intentions are in the breast of Him who rides upon the storm. His object is to restore, not to destroy—to gladden, not to terrify. This tempestuous weather lasts for some days, but at the end of that time the change that comes over the face of nature seems little short of miraculous. In the words of Mr Elphinstone, who describes from personal observation—“The whole earth is covered with a sudden but luxuriant verdure, the rivers are full and tranquil, the air is pure and delicious, and the sky is varied and embellished with clouds.
“The effect of this change is visible on all the animal creation, and can only be imagined in Europe by supposing the depth of a dreary winter to start at once into all the freshness and brilliancy of spring. From that time the rain falls at intervals for about a month, when it comes on again with great violence; and in July the rains are at their height. During the third month they rather diminish, but are still heavy. In September they gradually abate, and are often suspended till near the end of the month, when they depart amid thunders and tempests, as they came.”
Such are the effects of the monsoons upon land and sea. Of course the terrific gales that usher them in and out could not be expected to pass without doing a good deal of damage, especially to shipping. But this is more than compensated by the facilities which they afford to navigation.
In many parts of the world, especially in the Indian Ocean, merchants calculate with certainty on these periodical winds. They despatch their ships with, say, the north-east monsoon, transact business in distant lands, and receive them back, laden with foreign produce, by the south-west monsoon. If there were no monsoons, the voyage from Canton to England could not be accomplished in nearly so short a time as it is at present.
And now as to the cause of monsoons. They are, for the most part, deflected trade-winds. And they owe their deflection to the presence of large continents. If there were no land near the equator, the trade-winds would always blow in the same manner right round the world; but the great continents, with their intensely-heated surfaces, cause local disturbance of the trade-winds. When a trade-wind is turned out of its course, it is regarded as a monsoon. For instance, the summer sun, beating on the interior plains of Asia, creates such intense heat in the atmosphere that it is more than sufficient to neutralise the forces which cause the trade-winds to blow. They are, accordingly, arrested and turned back. The great general law of the trades is in this region temporarily suspended, and the monsoons are created.
It is thus that the heated plains of Africa and Central America produce the monsoons of the Atlantic, the Pacific, and the Gulf of Mexico.
We think it unnecessary to explain minutely the causes that produce variation in the monsoons. Every intelligent reader will readily conceive how the change of seasons and varied configuration as well as unequal arrangement of land and water, will reverse, alter, and modify the direction and strength of the monsoons.
Land and sea breezes are the next species of wind to which we would direct attention. They occur in tropical countries, and owe their existence to the fact that the land is much more easily affected by sudden changes of temperature than the sea. Thus, the land in warm regions is much heated by the sun’s rays during the day; the atmosphere over it becomes also heated, in virtue of which it rises: the cool atmosphere over the sea rushes in to supply its place, and forms the sea breeze: which occurs only during the day.
At night the converse of this takes place. Land heats and cools rapidly; water heats and cools slowly. After the sun sets, the cooling of the land goes on faster than that of the sea. In a short time the atmosphere over the land becomes cooler than that over the sea; it descends and flows off out to sea; thus forming the land breeze. It occurs only at night, and when the change from one to the other is taking place there is always a short period of calm. Land and sea breezes are of the greatest use in refreshing those regions which, without them, would be almost, if not altogether, uninhabitable.
In “The Tempest,” an interesting work on the origin and phenomena of wind, published by the Society for Promoting Christian Knowledge, a curious and simple experiment is described, whereby the existence of upper and under currents of air and the action of land and sea breezes may be clearly seen and understood. We quote the passage:—
“The existence of the upper and under currents of air which mark the phenomena of the trade-winds, and of land and sea breezes, may be beautifully illustrated in two adjoining rooms, in one of which a good fire is burning, while in the other there is none. If the door between the two rooms be thrown open, the cold air will enter the heated room in a strong current, or, in other words, as a violent wind. At the same time the heated air of the warm room ascends and passes the contrary way into the cold room, at the upper part of the same doorway; while in the middle of this opening, exactly between the two currents, the air appears to have little or no motion. The best way to show this experiment is to introduce the flame of a candle into the doorway between a hot and a cold room. If the flame be held near the bottom of the doorway, where the air is most dense, it will be strongly drawn towards the heated room; and if held near the top of the door it will be drawn towards the cold room with somewhat less force; while midway between the top and bottom the flame will be scarcely disturbed.
“There is also another pretty experiment which illustrates well the theory of land and sea breezes. Take a large dish, fill it with cold water, and in the middle of this put a water-plate or a saucer filled with warm water. The first will represent the ocean, and the latter an island made hot by the rays of the sun, and rarefying the air above it. Take a lighted wax candle and blow it out; and, if the air of the room be still, on applying it successively to every side of the saucer, the smoke will be seen moving towards the saucer and rising over it, thus indicating the course of the air from sea to land. On reversing the experiment, by filling the saucer with cold water (to represent the island at night) and the dish with warm water, the land breeze will be shown by holding the smoking wick over the edge of the saucer; the smoke will then be wafted to the warmer air over the dish.”
We have just tried the first of these experiments, with complete success. We would, however, recommend a piece of twisted brown paper, lighted and blown out, instead of a wax candle, because it gives out more smoke and is probably more obtainable on short notice. The experiment of the doorway, moreover, does not require that there should lie two rooms with a door between. We have found that the door of our study, which opens into a cold passage, serves the purpose admirably.
Were we treating chiefly of the atmosphere in this work, it would be necessary that we should enlarge on all the varieties of winds, with their causes, effects, and numerous modifications. But our main subject is the Ocean. The atmosphere, although it could not with justice have been altogether passed over, must hold a secondary place here; therefore we will conclude our remarks on it with a brief reference to hurricanes.
It has been ascertained that most of the great storms that sweep with devastating fury over the land and sea are not, as was supposed, rectilinear in their motion, but circular. They are, in fact, enormous whirlwinds, sometimes upwards of one hundred and fifty miles in diameter; and they not only whirl round their own centres, but advance steadily forward through space.
In the year 1831, a memorable and dreadful series of storms passed over some of the India Islands, and caused terrible havoc, especially in the island of Barbadoes. The peculiarity of these hurricanes was that they ravaged the different islands at different dates, and were therefore supposed to be different storms. Such, however, was not the case. It was one mighty cyclone, or circular storm,—a gigantic whirlwind,—which traversed that region at the rate of about sixteen miles an hour. It was not its progressive, but its rotatory motion, that constituted its terrible power. On the 10th of August it reached Barbadoes; on the 11th, the islands of Saint Vincent and Saint Lucia; on the 12th it touched the southern coast of Porto Rico; on the 13th it swept over part of Cuba; on the 14th it encountered Havanna; on the 17th it reached the northern shores of the Gulf of Mexico and travelled on to New Orleans, where it raged till the 18th. It thus, in six days, passed, as a whirlwind of destruction, over two thousand three hundred miles of land and sea. It was finally dissipated amid heavy rains.
The effect of a hurricane is well described by Washington Irving. “About mid-day,” he says, “a furious gale sprang up from the east, driving before it dense volumes of cloud and vapour. Encountering another tempest from the west, it appeared as if a violent conflict ensued. The clouds were rent by incessant flashes, or rather streams, of lightning. At one time they were piled up high in the sky, at another they descended to the earth, filling the air with a baleful darkness, more impenetrable than the obscurity of midnight. Wherever the hurricane passed, whole tracts of forest were shivered and stripped of their leaves and branches; and trees of gigantic size, which resisted the blast, were torn up by the roots and hurled to a great distance. Groves were torn from the mountain-precipices, and vast masses of earth and rock precipitated into the valleys with terrific noise, choking the course of the rivers.
“The fearful sounds in the air and on the earth, the pealing thunder, the vivid lightning, the howling of the wind, the crash of falling trees and rocks, filled every one with affright, and many thought that the end of the world was at hand. Some fled to caverns for safety, for their frail houses were blown down, and the air was filled with the trunks and branches of trees, and even with fragments of rocks, carried along by the fury of the tempest. When the hurricane reached the harbour, it whirled the ships round as they lay at anchor, snapped their cables, and sunk three of them to the bottom with all who were on board. Others were driven about, dashed against each other, and tossed mere wrecks upon the shore by the swelling surges of the sea, which in some places rolled for three or four miles upon the land. This tempest lasted for three hours.”
The China seas are the most frequently visited by severe tempests, or typhoons; yet of all vessels, the Chinese junks, as they are called, seem to be least adapted by their build for encountering such storms.
A terrible hurricane burst upon the China seas in the month of January 1837, as we learn from the “United Service Journal” of that year. An English vessel was exposed to it. The sea, rising in mountains around and over the ship’s sides, hurled her rapidly on her passage homeward, when suddenly a wreck was discovered to the westward. The order to shorten sail was given, and promptly obeyed; and when they neared the wreck they found her to be a Chinese junk without mast or rudder—a helpless log on the breast of that boiling sea.
There were many Chinamen on deck vehemently imploring assistance. The exhibition of their joy on beholding the approach of the stranger was of the wildest and most extravagant nature; but it was doomed to be suddenly turned to despair, as the violence of the storm drove the ship past the wreck. It became necessary to put her on the other tack, a manoeuvre which the poor creatures construed into abandonment, and the air rang with the most agonising shrieks of misery. But hope was again raised, when a boat was lowered and a rope thrown on board for the purpose of towing the junk to the ship. This intention was frustrated by the windlass breaking. At sight of this one man, in a paroxysm of despair, jumped overboard after the rope; but he missed it. Being a good swimmer, he tried to reach the boat; but his feeble power could avail him nothing in the midst of such raging elements: he speedily sank to rise no more.
Another rope, however, was secured to the junk, and by means of it the rest of the crew (eighteen in number) were saved. Their gratitude was boundless. They almost worshipped the officers, the crew, and the vessel, prostrating themselves and kissing the feet of the former, and the very planks of the latter.
Well-built ships, however, are not always able to withstand the violence of rotatory storms. Instances occur in which the tightest built and best manned ships are destroyed as suddenly as the clumsiest of ill-managed junks. Not many years ago, a vessel was proceeding prosperously on her voyage, when signs of a coming tempest induced the wary captain to reduce, and, finally, to take in all sail. But his precautions were in vain. The storm burst on the devoted ship, and in a few minutes the masts went over the side, and the hull lay a total wreck upon the sea.
These hurricanes or cyclones, although in reality whirlwinds, are so large that man’s eye cannot measure them, and it is only by scientific investigation that we have arrived at the knowledge of the fact. The whirlwind, properly so called, is a much smaller body of atmosphere. Sometimes we see miniature whirlwinds, even in our own temperate land, passing along a road in autumn, lifting the leaves and dust into the air and carrying them along in the form of a rotatory pillar. In other regions they exert a power quite equal to the tempest, though in a more limited space, overturning houses, uprooting trees, cutting a track twenty or thirty yards wide through the dense forest as thoroughly as if a thousand woodmen had been at work there for many years.
When whirlwinds pass from the land to the sea they create waterspouts; of which we shall have something to say in another chapter. Meanwhile, we think it may be interesting to give the following miscellaneous information regarding the atmosphere, gathered from the work of Dr Buist, who devoted much earnest study to the subject of atmospheric phenomena.
“The weight of the atmosphere is equal to that of a solid globe of lead sixty miles in diameter. Its principal elements are oxygen and nitrogen gases, with a vast quantity of water suspended in them in the shape of vapour; and, commingled with these, a quantity of carbon in the shape of fixed air, sufficient to restore from its mass many-fold the coal that now exists in the world. Water is not compressible or elastic; it may be solidified into ice or vaporised into steam: but the air is elastic and compressible. It may be condensed to any extent by pressure, or expanded to an infinite degree of tenuity by pressure being removed from it. It is not liable to undergo any changes in constitution beyond these, by any of the ordinary influences by which it is affected.”
If the heating and cooling process—which we have described as being carried on between the equator and the poles—were to cease, we should have a furious hurricane rushing perpetually round the globe at the rate of one thousand miles an hour,—ten times the speed of the most violent tornado that has ever carried devastation over the surface of the earth.
The air, heated and dried as it sweeps over the arid surface of the soil, drinks up by day myriads of tons of moisture from the sea,—so much, indeed, that, were none restored to it, the surface of the ocean would be depressed eight or ten feet annually.
We do not certainly know the height of the atmosphere. It is said that its upper surface cannot lie nearer to us than fifty, and can scarcely be further off than five hundred, miles. “It surrounds us on all sides, yet we cannot see it; it presses on us with a weight of fifteen pounds on every square inch of the surface of our bodies—in other words, we are at all times sustaining a load of between seventy and one hundred tons of it on our persons—yet we do not feel it! Softer than the finest down, more impalpable than the lightest gossamer, it leaves the cobweb undisturbed, and, at times, scarcely stirs the most delicate flower that feeds on the dew it supplies; yet it bears the fleets of nations on its wings round the world, and crushes the most refractory substances with its weight. It bends the rays of the sun from their path to give us the aurora of the morning and the twilight of evening. It disperses and refracts their various tints to beautify the approach and the retreat of the orb of day. But for the atmosphere, sunshine would burst on us in a moment and fail us in the twinkling of an eye, removing us in an instant from midnight darkness to the blaze of noon.”
We have written a good deal on this subject, yet the thousandth part has not been told of even the grand and more obvious operations of the atmosphere, much less the actions and results of its minor and invisible processes. Were we to descend with philosophers into the minuter laboratories of the world, and consider the permeating, ramifying, subtle part the atmosphere plays in the innumerable transformations that are perpetually going on around and within us, we should be constrained to feel more deeply than we have ever yet felt, that the works of the Creator are indeed wonderful beyond all expression or conception.
