Читать книгу New lands within the Arctic circle - Julius Payer - Страница 4

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1. The ice-sheet spread over the Arctic region is the effect and sign of the low temperature which prevails within it. During nine or ten months of the year this congealing force continues to act, and if the frozen mass were not broken up by the effects of sun and wind, of rain, waves, and currents, and by the rents produced in it from the sudden increase of cold, the result would necessarily be an absolutely impenetrable covering of ice. The parts of this enormous envelope of ice sundered by these various causes now become capable of movement, and are widely dispersed in the form of ice-fields and floes.

2. The water-ways which separate these parts are called “leads,” or, when their extent is considerable, “ice-holes.” The meshes of this vast net, which is constantly in motion, open and close under the action of winds and currents in summer; and it is only in its southern parts that the action of waves, rain, and thaw produces any considerable detachments. Towards the end of autumn, the ice, forming anew, consolidates the interior portions, while its outer edge pushes forward, like the end of a glacier, into lower regions, until about the end of February the culminating point of congelation is attained. Motionless adhesion of the fields, which naturally reach their greatest size in winter, does not, however, exist even then; for during this period they are incessantly exposed to displacement and pressure from the currents of the sea and the air.

3. When the ice is more or less closed, so as to render navigation impossible, it is called “pack-ice,” and “drift-ice” when it appears in detached pieces amid predominating water. Since there are forces operating which promote the loosening process at its outer edge, and its consolidation within, it is self-evident, that the interior portions tend to the character of “pack-ice,” and its outer margin to that of “drift-ice.” This general rule, however, is so modified in many places, by local causes, currents, and winds, that we find not unfrequently at the outer margin of the ice thick barriers of pack-ice, and in the inner ice, ice-holes (polynia[1]) and drift-ice.

4. Ice navigation, during its course of three hundred years, has created a number of terms to designate the external forms of ice, the meaning of which must be clearly defined. Ice formed from salt-water is called “field-ice;” that from the waters of rivers and lakes “sweet-water ice.” The latter is as hard as iron, and so transparent that it is scarcely to be distinguished from water. Icebergs are masses detached from glaciers. The words “patch,” “floe,” “field,” express relative magnitude, descriptive of the smallest ice-table up to the ice-field of many miles in diameter. The term “floe,” however, is generally applied to every kind of field-ice, without reference to its size. The ice which lies along coasts, or which adheres to a group of islands within a sound, is called “land-ice.” Sledge expeditions depend on its existence and character. Along the coast-edge land-ice is broken by the waves and tide, and the forms of its upheaval and deposition on the shore constitute the so-called “ice-foot.” Broken ice, or “brash,” is an accumulation of the smaller fragments of ice which are found only on the extreme edge of the ice-belt. “Bay-ice” is ice of recent formation, and its vertical depth is inconsiderable.

5. Land-ice is less exposed to powerful disturbances, and its surface, therefore, is comparatively level, and is only here and there traversed by small hillocks called “hummocks” or “torrosy.” These are the results of former pressures, and they are gradually reduced to the common level by evaporation, by thawing, and by the snow drifting over them.

6. But ice-floes exposed to constant motion from winds and currents, and to reciprocal pressure, have a more or less undulating character. On these are found piles of ice heaped one upon another, rising to a height of twenty or even fifty feet, alternating with depressions, which collect the thawed water in clear ice-lakes during the few weeks of summer in which the temperature rises above the freezing point. The specific gravity of this water, where it does not communicate with the sea by cracks, is in all cases the same with the specific gravity of pure sweet water; and as the salt is gradually eliminated from the ice, the water produced is perfectly drinkable. In the East Greenland Sea ice-floes frequently measure more than twelve nautical miles across—these are ice-fields properly so called.[2] In the Spitzbergen and Novaya Zemlya Seas, they are much smaller, as Parry also found.

7. The thickness which ice acquires in the course of a winter, when its formation is not disturbed, is about eight feet. In the Gulf of Boothia, Sir John Ross found the greatest thickness about the end of May; it was then ten feet on the sea and eleven feet on the lakes. In his winter harbour in Melville Island, Parry met with ice seven or seven-and-a-half feet thick; and Wrangel gives the thickness of a floe on the Siberian coast, which had been formed in the course of a winter, at nine-and-a-half feet. According to the observations of Hayes the ice measured nine feet two inches in thickness in Port Foulke. He estimates it, however, by implication, far higher in Smith’s Sound: “I have never seen,” he says, “an ice-table formed by direct freezing which exceeded the depth of eighteen feet.”

8. The rate at which ice is formed decreases as the thickness of the floe increases, and it ceases to be formed as soon as the floe becomes a non-conductor of the temperature of the air by the increase of its mass, or when the driving of the ice-tables one over the other, or the enormous and constantly accumulating covering of snow, places limits to the penetration of the cold.

9. While therefore the thickness which ice in free formation attains is comparatively small, fields of ice from thirty to forty feet high are met with in the Arctic Seas; but these are the result of the forcing of ice-tables one over the other by pressure, and are designated by the name of “old ice,” which differs from young ice by its greater density, and has a still greater affinity with the ice of the glacier when it exhibits coloured veins.

10. When the cold is excessive a sheet of ice several inches thick is formed on open water in a few hours; this, however, is not pure ice, but contains a considerable amount of sea-salt not yet eliminated; complete elimination of the saline matter takes place only after continuous additions of ice to its under surface. A newly-formed sheet of ice is flexible like leather, and as it becomes harder by the continued cold, its saline contents come to the surface in a white frosty efflorescence.

11. Hayes mentions that he met with fields of ice from twenty to a hundred feet thick in Smith’s Sound. But if it is difficult in many cases to distinguish glacier-ice, when found in small fragments, from detached portions of field-ice, it is often still more difficult to distinguish between old and new ice, and the attempt to do so is merely arbitrary, because their masses depend not on their age alone, but on other processes to which they are exposed. A floe of normal thickness is never more than two or three years old; and if it is to exist and preserve its size for a longer period, it must somewhere attach itself to land-ice, so as to escape destruction from mechanical causes, and dissolution from drifting southwards. Many floes run their course from freezing to melting within a year.

12. The perpetual unrest in the Arctic Sea, which continues undiminished even in the severest winter, and the incessant change in the “leads” and “ice-holes,” are the main causes of the increase of the ice, both in its area and in its vertical depth. Were this constant movement to cease, the result would be the formation of a sheet of ice of the uniform thickness of about eight feet over the whole Polar region.

13. A layer of snow, which, like the ice itself, is at a minimum in autumn, covers the whole surface of all the ice-fields. This snow, which in winter is sometimes as hard as a rock, sometimes as fine as dust, takes, towards the end of summer, more and more the character of the glacier snow of our lofty Alpine ranges. Its grains, in a humid state, exceed the size of beans, and when in motion they make a rustling noise like sand. This granular snow is the residuum of the incomplete evaporation of what fell in the winter, and of the surface of the ice which has become “rotten” and porous. Its crystals are frequently from a third to a sixth of an inch in length, and firm ice is found even in autumn only at the depth of one or two feet. In the North of Spitzbergen, Parry observed that the surface of the ice was frequently cut up into ice-needles of more than a foot long by the drops of rain, which in summer fall upon it, and in some places he found it overspread with red snow. We ourselves never saw the phenomenon observed by Parry, and the ice-crystals we met with seldom exceeded the length given above.

14. Field-ice is of a delicate azure-blue colour, and of great density, and there is, in these respects, no difference between that of the Arctic and Antarctic regions. Cook, indeed, calls the South Polar ice colourless, though Sir James Clark Ross speaks expressly of the blueness of its ice-masses. Sea-ice surpasses the ice of the Alps both in the beauty of its colour and in its density. The glorious blue of the fissures is due to the incidence of light, the blue rays of which only are reflected, while the other rays are absorbed. A spectrum observation made in 1869 on a Greenland ice-field gave brownish red, yellow, green and blue. The yellowish spots observed in ice are due to the presence of innumerable microscopic animalculæ.

15. Sea-ice, which, when the cold is intense, is hard and brittle, loses this quality with the increase of temperature till it acquires an incredible toughness, far exceeding that of glaciers; and floes several feet thick bend under mutual pressure before they split. Hence the fruitlessness, especially in summer, of all attempts to loosen the connexion of its parts by blasting with gunpowder.

16. The specific gravity of sea-ice is 0.91, and accordingly about nine parts of a cubical block of ice are under water, while one part only rises above the surface. If, however, the ice of a floe be irregularly formed and full of bubbles, the specific gravity will be correspondingly reduced, and the volume submerged may diminish to two-thirds of the whole mass.

17. The irregularity of the forms of ice is so great, that no deduction can safely be drawn from them; cases may occur where a recently-formed ice-floe, which has been attached to old ice, is forced by its neighbour to sink under the normal level; hence the submergence of floes beneath the level of the sea is often overstated.

18. The temperature of the Arctic Sea at the surface is generally below the freezing point, and then increases slightly with the depth. Sir James Ross observed that the temperature in all oceans does not alter at great depths, and placed this constant temperature at 39° F. In summer the temperature of the atmosphere rises little above freezing point, and, according to Sir James Ross, it is still less at the South Pole, because he saw no thaw-water streaming down from the icebergs there as he did in the North. It was first observed in Forster’s days, that is about a century ago, that the salt was gradually eliminated from frozen sea-water. Of this fact Cook knew nothing; and even Sir James Ross endorses Davis’s remark that “the deep sea freezes not.” But the fact that ice is formed on the open sea, and far from the vicinity of land, was first asserted by Scoresby, and has been confirmed by all subsequent observers, though it was long disputed.

19. The crackling sound so commonly heard along the outer edge of the ice exposed to the action of the waves, is a consequence of the penetration of its pores by the sea-water, which is then immediately frozen, and disruption follows at once. But disruption on a far grander scale is due to a cause the very opposite of this, the sudden contraction and splitting of the ice, even in the great ice-fields, which is produced usually in winter by the sudden fall of the temperature.

20. When light falls on a field of pack-ice, it is reflected in the stratum of air above it, and this span of light, called the “ice-blink,” just above the horizon, warns the navigator of the impossibility of penetrating further. This phenomenon is often observed also over drift-ice, although not so intense nor so yellow in colour as over pack-ice.

21. Water spaces, on the other hand, show their presence by dark spots on the horizon, produced by the formation of clouds from ascending mists. These are the so-called “water-sky,” and faithfully indicate the “leads” beneath them. Above the larger “ice-holes,” they assume the dark colours of a thunder-sky, though they are never so strongly defined.

22. The annual evaporation from the surface of the ice, which even in winter is never entirely interrupted during the severest frost, and the destruction of ice by the action of rain and waves, are balanced, to speak generally, by its re-formation by frost. The maximum accumulation of ice takes place in spring, its minimum in the beginning of autumn. We observed in the autumn of 1873 not only the evaporation of the snow of the preceding winter, but also a vertical decrease of ice of about four feet. Evaporation is, therefore, the most potent regulator of the balance between waste and growth in the accumulation of ice; and next in importance is the drifting of its masses towards the south through all those openings by which the Polar waters mingle with the waters of lower latitudes.

23. However great the agitation of the sea may be in the open ocean, and though it may dash its waves with wild fury on the edge of the ice, within the icy girdle it is undisturbed, in consequence of the enormous weight of the superincumbent masses. It is only in the large “ice-holes,” and when the winds are very high, that the action of waves is discernible. An isolated accumulation of floes in a circular form, suffices to produce a calm interior sea, and its outer edge only encounters the beat of the ocean.

24. The ceaseless attack to which the ice is exposed on its outer edge is the cause of its excavation and undermining. Hence its centre of gravity is constantly displaced; and the overturning of its masses and its strange transformations are the consequences of this instability. The smaller the masses of the ice, the more fantastic are the shapes they assume.

25. Change of colour in the sea as we enter the ice-region is frequently, though not invariably, observed. Almost immediately on entering the ice, its normal dull green colour gives place to a deep ultramarine blue, especially in the East Greenland seas, and this colour is maintained under all changes of the weather, and is only modified by local currents. Two hundred and fifty years ago it appeared to Hudson, on the coast of Spitzbergen, that the sea, whenever it was free from ice, was green, and that its being covered with ice and its blueness of colour were intimately connected. Sir James Ross states that in both Polar oceans the colour of the sea changes in the neighbourhood of ice, and that the dull brownish colour sometimes seen near pack-ice in the Antarctic Ocean is owing to an infinite number of animalculæ. The rapid fall of the temperature of the water to the zero point is another indication that ice is near.

26. Of all the ice-formations in the Arctic Seas, icebergs are the most enormous. “It is well known that ice is not by any means so heavy as water, but readily floats upon its surface. Consequently whenever a glacier enters the sea, the dense salt water tends to buoy it up. But the great tenacity of the frozen mass enables it to resist the pressure for a time. By and by, however, as the glacier reaches deeper water, its cohesion is overcome, and large fragments are forced from its terminal front and floated up from the bed of the sea to sail away as icebergs.”[3] This process is sometimes called “the calving” of the glaciers; and the direction of the cleavage is a pre-indication of the forms of the masses when detached. The characteristic features of icebergs are their simple outline, differing widely from the fantastic shapes which the fragments of sea-ice tend to assume; their great height as compared with their breadth—their greenish-blue colour—their distinct stratification—their slight transparency—and the roughly-granulated character of their ice. Icebergs with long, sharp-pointed peaks, like those exhibited in numerous illustrations, have no real existence. It is only fragments of field-ice, raised up by pressure, exposed to the action of waves and the process of evaporation which are transformed into fantastic shapes. Icebergs are generally of a pyramidal or tabular shape, and in time they are usually rounded off into irregular cones. They vary in height from 20 to 300 feet. Sir John Ross (1818) mentions an iceberg of 51 feet; Baffin (1615) of 240 feet; Parry (1819) of 258 feet; Kane (1853) of 300 feet; and Hayes (1861) one 315 feet high, the depth of which below the water-line he estimated at half a mile. On the coast of East Greenland, Scoresby once counted 500 icebergs, some of which reached the height of 200 feet; and during the second German North-Pole expedition, we saw many at the mouth of the Kaiser Franz-Josef fiord which measured 220 feet in height. In Austria Sound, and on the east coast of Kron-Prinz Rudolph’s land, their altitude varied from 80 to 200 feet. From the covering of mist which envelops them, icebergs generally appear much higher than they really are, and their depth below the surface is not so considerable as is generally supposed. In an iceberg 200 feet above the water, a total height of 600 to 800 feet may, as a mean, be inferred. It is only glaciers of a very great size which shed icebergs; smaller glaciers, like those of Novaya Zemlya, only strew the sea with a multitude of fragments which resemble broken sea-ice. Hence the appearance of icebergs is connected with the proximity to glacier-covered lands, and with the currents which prevail along their coasts. Baffin’s Bay, Smith’s Sound, East Greenland, the South-East of Greenland, Austria Sound, are the principal places where they collect together and lie like fleets before the entrances of bays and gulfs. Under-currents of the sea take them not unfrequently in directions contrary to the drift of the field-ice, which depends only on upper-currents; and abnormal winds may sometimes carry them out to seas where they have been seldom or never seen.[4] This appears to be the case even with those met with on the north-west coast of Novaya Zemlya. On the other hand, they have never been seen on the coasts of Siberia, which have no glaciers.

27. The constant displacement of the centre of gravity of an iceberg, resulting from the unsymmetrical decrease of its form, causes its periodical oversetting; and the different temperature of the internal and external ice is the principal cause of its rending asunder with a noise like thunder; a process which occurs generally in the height of summer.