Читать книгу The Sea and Its Living Wonders - G. Hartwig - Страница 41

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THE AËRIAL AND TERRESTRIAL MIGRATIONS OF THE WATERS.

Movements of the Waters through Evaporation.—Origin of Winds.—Trade-Winds.—Calms.—Monsoons.—Typhoons.—Tornadoes.—Water-Spouts.—The Formation of Atmospherical Precipitations.—Dew.—Its Origin.—Fog.—Clouds.—Rain.—Snow.—Hail Sources.—The Quantities of Water which the Rivers pour into the Ocean.—Glaciers and their Progress.—Icebergs.—Erratic Blocks.—Influence of Forests on the Formation and Retention of Atmospherical Precipitations.—Consequences of their excessive Destruction.—The Power of Man over Climate.—How has it been used as yet?

Neither storms nor ocean-currents, nor ebb and flood, however great their influence, cause such considerable movements of the waters, or force them to wander so restlessly from place to place as the silent and imperceptible action of the warming sunbeam. In every zone evaporation is constantly active in impregnating the atmosphere with moisture, but the chief seat of its power is evidently in the equatorial regions, where the vertical rays of the great parent of light and heat plunge, day after day, into the bosom of ocean, and perpetually saturate the burning air with aqueous vapours.

In this chapter I intend following these invisible agents of fertility and life, as they lightly ascend from the tropical seas, and accompanying them in their various transformations, until they once more return to the bosom of their great parent. A cursory view of the benefits they confer on the vegetable and animal world, as they wander over the surface of the land, will, I hope, agreeably occupy the reader, and serve to increase his admiration for that deep and dark blue ocean without which all organic life would soon be extinct upon earth.

I begin with a few words on the winged carriers of marine exhalations, the winds, which, although now and then detrimental or fatal to individuals by their violence, largely compensate for these local injuries, by the constant and inestimable benefits they confer on the whole body of mankind.

On taking a comprehensive view of their origin, we find that, like the oceanic currents, they are chiefly caused by the unequal influence of solar warmth upon the atmosphere under the line and at the poles. In the torrid zone, the air, rarefied by intense heat, ascends in perpendicular columns high above the surface of the earth, and there flows off towards the poles, in the same manner as in a vase filled with cold water and placed over the flame of a lamp, the warmed liquid rises from the bottom and spreads over the surface.

But cold air-currents must naturally come flowing in an opposite direction from the poles to the equator to fill up the void, as in the example I have cited, colder and consequently heavier water comes streaming down the sides of the vase to replace the liquid which is rising in the centre under the influence of heat.

Thus the unequal distribution of solar warmth over the surface of the earth evidently generates a constant circulation of air from the equator to the poles, and from the icy regions to the tropics, and by this means the purity of the atmosphere is chiefly maintained. The sun is not only the great fountain of warmth, he is also the universal ventilator; he not only calls forth animal life, but at the same time, by a simple and admirable mechanism, provides for its health by constantly renewing the air, which is essential to its existence.

If caloric were the sole agent which influences the direction of the winds, or if the earth were one uniform plain, the opposite air-currents I have mentioned would naturally flow straight to the north and south; but their course is modified or diverted in the same manner as that of the ocean-currents by the rotation of the globe. Thus, the cold air-current (polar-stream) which comes rushing upon us from the Arctic regions, is felt in our latitude as the biting east or north-east wind, so trying to our nerves and organs of respiration, while we enjoy the warm air-current from the tropics as the mild western or south-western breeze.

But besides the rotation of the earth, there are many other local influences by which the winds are deflected from their course, or by whose agency partial air-currents are called forth. Among these we particularly notice high chains of mountains, the unequal capacity of sea and land in absorbing and retaining heat, which gives rise to sea and land breezes; the increasing or diminishing power of the sun in different seasons by which the equilibrium of the air is modified in many countries, the difference of radiation from a sandy desert or a forest, electrical discharges from clouds, &c. &c.

Although subject to many of these local disturbances, the winds generally blow with an astonishing regularity in the tropical zone; while in our variable climate the polar and equatorial stream are engaged in a perpetual strife, now bringing us warmth and moisture from the south and west, now cold and dryness from the north and east.

Thus, in the Atlantic and Pacific Ocean we find the trade-winds perpetually blowing from the east, the north-east trade-wind between 9° and 27° N. lat., and the south-east trade-wind between 3° N. lat. and 25° S. lat. It was by their assistance that Columbus was enabled to discover America, and that the wretched barks of Magellan traversed the wide deserts of the Pacific from end to end.

Between these two regions of the trade-winds lies the dreaded zone or girdle of the equatorial calms (doldrums), where long calms alternate with dreadful storms, and the sultry air weighs heavily upon the spirits.

"Down dropt the breeze, the sails dropt down,

'Twas sad as sad could be;

And we did speak, only to break

The silence of the sea.

"Day after day, day after day,

We stuck, nor breath, nor motion,

As idle as a painted ship

Upon a painted ocean."

On their polar limits, the trade-wind zones are again girdled with calm belts, the horse latitudes, whose mean breadth is from ten to twelve degrees. The boundaries of these alternating regions of winds and calms are not invariably the same, on the contrary, they are perpetually moving to the north or south, according to the position of the sun.

From 40° N. lat. to the pole, westerly winds begin to be prevalent, and in the Atlantic Ocean their proportion to the easterly winds is as two to one.

In the Northern Indian Ocean and in the Chinese Sea we also find the trade-wind, which is there called the north-east monsoon; here, however, it only blows from October to April, as during the summer terrestrial influences prevail which completely divert it from its course.

From the wide plains of central Asia glowing with the rays of a perpetually unclouded sun, the rarefied air rises into the higher regions. Other columns of air rush from the equator to fill up the void, and cause the trade-wind to vary its course, and change into the south-western monsoons of the Indian Ocean, which blow from May to September. The regularly alternating monsoons materially contributed to the early development of navigation in the Indian seas, and conducted the Greeks and Romans as far as Ceylon, Malacca, and the Gulf of Siam. Similar monsoons, or deflections from the ordinary course of the trade-winds, occur also in the Mexican Gulf, in the Gulf of Guinea, and in that part of the Pacific which borders on Central America, through the influence of the heated plains of Africa, Utah, Texas, and New Mexico.

The passage from one monsoon to the other is of course only gradual, since the land also is only gradually heated and cooled. Thus at the change of the monsoon, an atmospheric war of several weeks' continuance occurs, during which the trade-wind and the monsoon measure their strength, and calms alternate with dreadful storms (typhoons, cyclones, tornadoes).

According to the researches and observations of Franklin, Cooper, Redfield, Reid, &c. &c., these storms are great rotatory winds, that move along a curved line in increasing circles. In the northern hemisphere, the rotatory movement follows a direction contrary to that of the hands of a clock; while the opposite takes place in the southern hemisphere. The knowledge of the laws which regulate the movements of storms is of great importance to the mariner, since it points out to him the direction he has to give his ship to gain the external limits of the tornado, and thus to remove it from danger.

Water-spouts are formed by two winds blowing in opposite directions, and raising or sucking up the water in their vortex. They generally form a double cone; the superior part with its apex downwards, consisting of a dense cloud, while the inferior cone, the apex of which is turned upwards, consists of water, which is thus sometimes raised to a height of several hundred feet.

Water-spouts seldom last longer than half-an-hour. Their course and movements are irregular; straight forwards; in zig-zag lines; alternately rising and falling; stationary; slow; or progressing with the rapidity of thirty miles an hour. The rotatory movement is also variable; its power is often very great, but sometimes water-spouts pass over small vessels without injuring them. They are more frequent near the coast than on the high seas; and are more commonly seen in warm climates. They seem to occur particularly in regions where calms frequently alternate with storms, which is not to be wondered at, since they owe their origin to miniature storms or whirlwinds.

How do the aqueous vapours with which evaporation impregnates the atmosphere, again descend upon the surface of the earth?

Everybody knows that when in summer a bottle filled with cold water is brought into the room, it soon gets covered with thick dew-drops, which presently trickle down its sides, although it was perfectly dry on entering. Whence does this moisture come from? Not from the inside of the bottle as ignorant people might imagine, but from the surrounding atmosphere; in consequence of the capacity of the air to absorb and retain moisture, increasing or diminishing, as its temperature grows warmer or colder.

Thus when the cold bottle is introduced into the room, the warm sheet of air, which is in immediate contact with its surface, immediately cools, and being no longer able to retain all the moisture with which it was impregnated, is obliged to deposit it on the sides of the vessel. This familiar example suffices to explain the formation of dew, rain, hail, snow, hoar-frost, and all other atmospherical precipitations. They all result from the influence of some refrigerating cause upon the air; such as the passage of a warm current into a cooler region; the influx of a cold wind; a cold-radiating chain of high mountains; a forest, and so forth.

The very name of dew is refreshing, and calls forth a host of pleasing ideas, associated as it is with the memory of serene skies and sunny mornings. How beautiful are its diamonds glittering in all the colours of the rainbow, on verdant meads, or on the blushing petals of the rose. How suggestive of all that is lovely, pure, and innocent!

Poetry is of older date than prose, and bards have sung long before philosophers inquired. Thus, although the children of song from Homer and Theocritus to Byron and Wordsworth so frequently mention dew in their immortal strains, it is only in our time that its formation has been fully explained by Dr. Wells, who in a very ingenious and masterly essay on this subject, first proved that it results from the ground radiating or projecting heat into free space, and consequently becoming colder than the neighbouring air. During calm and clear nights, the upper surfaces of grass-blades, for instance, radiate their caloric into the serene sky, from which they receive none in return. The lower parts of the plant, being slow conductors of heat, can only transmit to them a small portion of terrestrial warmth, and their temperature consequently falling below that of the circumambient atmosphere, they condense its aqueous vapours. Clouds on the contrary compensate for the loss of heat the grass sustains from radiation, by reflecting or throwing back again upon the terrestrial surface, the caloric which would else have been dissipated in a clear sky, and this is the reason why dew does not fall, or but slightly falls during clouded nights. It is easy to conceive why none is formed in windy weather, as then the air in contact with the ground is constantly removed ere it has time to cool so far as to compel it to part with its moisture. We can also understand why dew is more abundant in autumn and spring than at any other season; as then very cold nights frequently follow upon warm days; and why it is most copious in the torrid zone, as in those sultry regions the air is more saturated with moisture than anywhere else, and the comparatively cold nights are almost constantly serene and calm. Hoar-frost is nothing but congealed dew, and owes its formation to the same causes.

When warmer air-currents are cooled by being transported into colder regions, or from any other refrigerating cause, a great part of their moisture generally condenses into small vesicles, but very little heavier than the surrounding atmosphere, which then becomes visible under the form of clouds, those great beautifiers of our changing skies, that frequently trace such picturesque, gorgeous, or singular groups and landscapes in the aërial regions. The inhabitants of countries where the heavens are monotonously serene, may well envy us the charms of a phenomenon which in some measure affords us compensation for so many disagreeable vicissitudes of the weather. Who that has admired at sunset the light clouds so beautifully fringed with silver and gold, or glowing with the richest purple, and loves to follow them in all their wonderful and fantastic transformations, will deny that they are the poesy and life of the skies, the awakeners of pleasing fancies and delightful reveries?

Thin wreaths of clouds have been observed, by travellers that have ascended the most elevated mountains, floating high above the peak of Chimborazo or Dhawalagiri, and thus shows us to what an amazing altitude the emanations of ocean are carried by the ascending air-current.

Sometimes when light clouds pass into a warmer atmosphere, they gradually dissolve and vanish; more frequently the accumulating moisture, too heavy to continue floating in the air, or condensed by electrical explosions, descends upon the earth in rain, which, with few exceptions, visits every part of the globe, either in its liquid form or congealed to snow or hail. But the quantity of rain which annually falls in different regions is very unequal, and strange to say, it is not most considerable in those countries whose climate enjoys an unenviable notoriety for its clouded atmosphere and the great number of its rainy days. In the tropical regions it is generally only about the time of the summer solstice that abundant showers of rain fall regularly every afternoon, while the rest of the year, the sky is uninterruptedly serene; but during the short period of the rainy season, a far greater quantity of water is precipitated upon the earth, than in the temperate zones.

While on the island of Guadaloupe, the annual quantity of rain amounts to 274·2 French inches, and to 283·3 at Mahabuleshwar, on the western declivity of the Ghauts, which, as far as has hitherto been ascertained, is the place where most rain descends; only from 35 to 40 inches fall on the western coast of England, where the skies are chronically weeping.

It is a remarkable circumstance that the annual quantity of rain which falls in the same place remains about the same from year to year; so that by an admirable balancing of conflicting influences, nature seems to have provided for stability in a province which of all others might be supposed most open to the caprices of chance.

Having thus followed the exhalations of ocean to the end of what may be called the first stage of their journey, and seen them descend in a condensed form upon the surface of the dry land, I will now accompany them in their ulterior progress to the bosom of the seas. A great part of them have many transformations and changes to undergo ere they can accomplish their return; repeatedly rising in vapours from the solid earth, and falling in showers upon its surface; or circulating through the tissues of organic life: but after all these intermediate stages and delays, they ultimately find their way into rivulets or streams, which after many a meander restore them to the vast reservoir from which they arose.

The waters that descend upon solid rocks, or fall in large quantities upon abrupt declivities, immediately flow into the brooks or rivers; but when they gently and gradually alight upon a porous soil, they are absorbed by the earth, and, displacing in virtue of capillary attraction, and of their superior weight, the air which fills the interstices between its solid particles, sink deeper and deeper until they meet with a solid and impenetrable stratum. If this forms a hollow basin, they naturally settle in the cavity; whence they are slowly displaced by fresh accessions and evaporation; but if its deepest declivity lies somewhere near the surface, they gradually gush forth under the form of sources or springs, having unequal distances to perform before they can reach the orifice. If no fresh supply of water falls, ere the most distant particles have reached their journey's end, the source dries up: but if new atmospheric precipitations continually take place, the source is perennial, although naturally of unequal strength at different times.

The temperature of springs varies from icy coldness to boiling heat. Cold springs arise when the waters, by which they are fed, descend from high mountains or do not penetrate a great way into the bowels of the earth; but if the filtering waters reach a depth which is constantly of a higher temperature, they then gush forth in the form of warm or even boiling springs.

A crowd of agreeable associations attaches itself to the idea of sources and springs, for they are generally both pleasing and useful to man. How we long in summer for the refreshing waters of the cool fountain issuing from the mountain side, and murmuring through the woods. The lover of nature spends hours near some solitary spring, and forgets the flow of time, as he observes the bubbling and listens to the sweet music of its crystal waters. A luxuriant vegetation marks their progress, though all around be burnt up by the scorching sun. Along their margin many a wild flower blooms, and herbs and shrubs and trees rejoice in a more vivid green, and statelier growth. There also congregate such members of the finny race, as delight in cooler streams of untainted purity, and birds love to build their nests among the sheltering foliage. Thus a little world forms around the gushing spring, and shows on a diminutive scale, how all that lives and breathes depends upon the liquid element for its existence.

While the waters filter through the earth they naturally dissolve a variety of substances, and all springs are more or less mixed with extraneous particles. But many of them, particularly such as are of a higher temperature and consequently arise from deeper strata, contain either a larger quantity or so peculiar a combination of mineral substances as to acquire medicinal virtues of the highest order, and to become objects of importance to a large portion of mankind. Numberless invalids annually flock to the hygeian fountains which nature unceasingly pours forth from her mysterious laboratory, and are by them restored to the enjoyments of a pleasurable existence.

How truly wonderful is the chain of processes which first raises vapours from the deep, and eventually causes them to gush forth from the entrails of the earth, laden with blessings and enriched with treasures more inestimable than those the miner toils for!

Although a river generally has its source in mountainous regions, it must be remembered that all the waters that descend upon the territory of which it forms the lowest level, gradually find their way into its current. Thus, the monarch of all streams, the Amazon River, is the natural drain of a territory thirty times larger than England. Thousands of rivulets and brooks, fed by the waters which descend from the slopes of thousands of glens and valleys, or filter through the vast forest-plains that rise but a few feet above their surface, all contribute to swell the majesty of its current. Its sources are in reality wherever, on that vast extent of land, water descends and drains into any one of its innumerable affluents. When we hear that on an average the river of the Amazons alone restores every minute half a million of tons of water to the ocean, and then consider the countless number of streams all alike active, that are scattered over the globe, we may form a faint idea of the vast quantity of vapours which are constantly rising from the deep, and of the magnitude of these silent operations of nature. Yet such is the immensity of ocean, that supposing all the waters it constantly loses, never to return again into its bosom, it would require thousands of years of evaporation to exhaust the immensity of its reservoirs!

It might be supposed that the waters which congeal on the sides of mountains covered with perennial snow, or fill Alpine valleys in the form of glaciers, were eternally fixed on earth—but there also we are deceived by delusive appearances of immobility. Every year the glacier slowly but restlessly makes a step forwards into the valley, and while its lower end dissolves, new supplies of snow constantly feed it from above. It has been calculated by Agassiz that the ice masses of the Aar glacier require 133 years to perform their descent from its summit to its inferior extremity—a distance of ten miles—so that their sojourn in that chilled valley far surpasses that of the oldest patriarch of the mountains. How great must be their delight when they at last are liberated from the spell which so long enchained them, and freely bound along on their way to Ocean! How they must shudder at the idea of once more returning to their desolate prison, and long for the perpetual warmth of spicy groves and tropical gardens!

In the colder regions of the earth, in Greenland or Spitzbergen, immense glaciers frequently fill the valleys that open on the sea, descend even beyond the water's edge, and, as they move along, their overhanging masses separate from their base and plunge into the deep with a crash louder than thunder. The icebergs that drift about the Arctic seas, and are annually conveyed by the currents into lower latitudes, are formed in this manner. Huge blocks of granite, detached by atmospherical vicissitudes from the higher mountains and precipitated on the surface of the glaciers, frequently float on the broad back of an iceberg far away from the spot where they seemed rooted for eternity. As their crystal support melts away in its progress to warmer climes, these rocky fragments, which have been appropriately named erratic blocks, fall to the bottom of the sea hundreds or even thousands of miles from the starting point of their journey. Thus the great bank of Newfoundland is covered with stones from distant Greenland, raised high in the air by volcanic power myriads of years ago, and now condemned to an equally long repose below the surface of ocean. When will they rise again above the waters, and what further changes will they have to undergo ere their compacted atoms resolve themselves into dust and assume new forms? But, however remote their dissolution, it will inevitably come, for Time is all-powerful, and has an eternity to work out his changes.

The large blocks of stone that so wonderfully migrate on the wandering iceberg form but a small and insignificant portion of the terrestrial spoils which are transported to ocean by the returning waters. Every river is more or less laden with earthy particles which its current carries onwards to the sea and deposits at its mouth. In course of time their accumulation, as I have already mentioned, forms large tracts of fertile territory encroaching upon the maritime domains.

I shall end with a few words on the influence of forests in attracting or retaining the atmospherical moisture, as it is a subject of great importance in the economy of nations, and shows us how much it is in the power of man to improve or to defeat the provisions of nature in his favour.

Forests always cool the neighbouring atmosphere, for their foliage offers an immense warmth-radiating surface, so that the vapours readily condense above them and descend in frequent showers. At the same time their roots loosen the soil, and the successive falling of their leaves forms a thick layer of humus, which has an uncommon power in attracting and retaining moisture. Their thick canopy of verdure also prevents the rays of the sun from penetrating to the ground, and absorbing its humidity. Thus the soil on which forests stand is constantly saturated with water, and becomes the parent of perennial sources and rills, that spread fertility and plenty far from the spot where they originated.

The rain-attractive influence of forests did not escape the attention of Columbus, who ascribed the frequent showers which refreshed and cooled the air, as he sailed along the coasts of Jamaica, to the vast extent and density of the woods that covered the mountains of that island. On this occasion he mentions in his journal that formerly rain had been equally abundant on Madeira, the Canaries, and the Azores, before their shady forests were felled or burnt by the improvident settlers.

The wanton destruction of woods has entailed barrenness on countries renowned in former times for their fertility. The mountains of Greece were covered with trees during the great epoch of her history, and the well-watered land bore abundant fruits, and sustained a numerous population. But man recklessly laid waste the sources of his prosperity. Along with the woods, many brooks and rivulets disappeared, and ceased to water the parched plains. The rain gradually washed the vegetable earth from the sides of the naked hills, and condemned them to sterility. When the snow of the mountains began to thaw under the warm breath of spring, it was now no longer retained by the spongy soil of the forests, and gradually dissolved under their cover; but, rapidly melting, filled with its impetuous torrents the bed of the rivers, and overflowing their banks, spread ruin and devastation far around.

Unfortunately, forests when once destroyed are not so easily restored, and it requires many centuries ere the bared mountain side reassumes its pristine vesture of shady woods. First lichens, mosses, and other thrifty herbs, content to feed upon nothing, have to prepare a scanty humus for the reception of more pretentious guests. In course of time some small stunted shrub makes its appearance here and there in some peculiarly favoured spot, and after all requires vast powers of endurance to maintain itself on the niggard soil, exposed to the full enmity of wind and weather. This paves the way for a more vigorous and fortunate offspring; and as every year adds something to the vegetation on the mountain's side, and opposes increasing obstacles to the winds, the falling leaves and decaying herbage accumulate more and more, until dwarfish trees first find a sufficiency of soil to root upon, and finally, the proud monarch of the forest spreads out his powerful arms and raises his majestic summit to the skies.

While Greece and Asia Minor have seen their fertility decrease or vanish with the trees that once covered their hills, other countries have improved as their vast woods have been thinned by the axe of the husbandman. In the time of the Romans all Germany formed one vast and continuous forest, and its climate was consequently much more rigorous than it is at present. All the low grounds were covered with impervious morasses, and the winter is described by historians in terms like those we should employ to paint the cold of Siberia.

But the scene gradually changed as tillage usurped the sylvan domain. The excessive humidity of the soil diminished, the swamps disappeared, and the heat of the sea, penetrating into the bosom of the earth, developed its productive powers. Thus the chestnut and the vine now thrive and ripen their fruits on the banks of the Rhine and the Danube, where 2000 years ago they could not possibly have existed. But Germany would also see her fertility decline, if the destruction of the forests which still crown the brow of many of her hills should continue in a considerable degree. Numerous rivulets would then be dried up during the warm season, in consequence of the more rapid descent and thaw of vernal rains and wintry snows, and most likely, refreshing summer showers would be far less frequent. Even now the inundations which almost annually desolate the banks of the Elbe, the Oder, and the Rhine, are ascribed by competent judges to the excessive clearing of the forests in the mountainous countries where those rivers originate. These few examples suffice to prove to us the power of man in modifying the climates of the earth, and the vast importance of the study of terrestrial physics. By planting or destroying woods, he is able to compel nature to a more equitable distribution of her gifts. In marshy and low countries, he may remove the superfluous waters by drainage, and increase the productiveness of arid plains by judicious irrigation. Thus man is the lord and master of the earth; but hitherto he has done but little to reap all the advantages he might have obtained from his dominion, or even used it to his own detriment. Drainage, irrigation, and a judicious management of forest-lands, are only beginning to be understood even among the most enlightened nations. A great part of our damp island still remains undrained, and we allow the rivers of India to pour their waters into the sea, instead of diverting them upon her thirsty plains. But there can be no doubt that as knowledge increases, man will gradually learn to provide every soil with the exact measure of humidity that is requisite to make it bring forth its fruits in the greatest abundance. Views such as these teach us, that, far from having attained the summit of civilisation, we are still on the threshold of her temple, and that most likely our descendants will look down upon our present condition as we do upon that of our barbarous ancestors.

Rocky Mountains at the bend of Bear Lake River.