صور الصفحة
PDF
النشر الإلكتروني

atmosphere, is not far from being uniform, at the rate of about one degree for every hundred yards of elevation. Hence the limit of perpetual congelation forms a curve, which is nearly the same as the Companion of the Cycloid, bending gradually from the Equator, reverting its inflexure at the latitude of 45°, and grazing the surface at the Pole. The mean heights of eternal frost, under the Equator, and at the latitudes of 30° and 60°, are respectively 15207, 11484, and 3818 feet.

It is important to remark, that the heat of large collections of water will seldom agree precisely with the mean temperature corresponding to the latitude. The variable impressions received at the surface from the atmosphere, will not, as on land, penetrate slowly into the mass, and become mingled and equalized at a moderate depth. Heat is conducted through liquids chiefly by the internal play resulting from their partial expansion.— In the more temperate regions of the globe, the superficial waters of lakes or seas, as they grow warmer, and, therefore, specifically lighter, still remain suspended by their acquired buoyancy. But whenever they come to be chilled, they suffer contraction, and are precipitated by their superior density. Hence the deep water, both of lakes and seas, is always considerably colder than what floats at the surface. The gradation of cold is distinctly traced to the depth of twenty fathoms, below which, the diminished temperature continues nearly uniform as far as the sounding line can reach. In shallow seas, however, the cold substratum of liquid is brought nearer to the surface. The increasing coldness of water, drawn up from the depth of only a few fathoms, may, therefore, indicate to the navigator who traverses the wide ocean his approach to banks or land.

These principles, however, will not apply to the peculiar circumstances of the Arctic seas. Water differs essentially, in its expansion by heat, from mercury, oil or alcohol: Far from dilating uniformly, a property which fits the latter substances for the construction of thermometers, it swells from the point of congelation, or rather a very few degrees above it, with a rapid progression, to that of boiling. Near the limit of its greatest contraction, the volume of water is scarcely affected at all, by any alteration of heat. When the surface of the ocean is depressed to a temperature between 28 and 44 degrees of Fahrenheit's scale, it will remain almost stagnant, and therefore exposed to the full impression of external cold. Hence the Polar scas are always ready, under the action of any frosty wind, to suffer congelation. The annual variations of the weather are in those seas expended on the superficial waters, without disturbing the vast abyss below. Contrary to what takes place

under milder skies, the water drawn up from a considerable depth is warmer within the Arctic circle than what lies on the surface. The floating ice accordingly begins to melt generally on the under side, from the slow communication of the heat sent upwards.

The patience of our readers, we fear, will be exhausted by this laborious discussion, rendered necessary, however, by the loose and inconclusive manner in which the subject of climate is usually treated. We shall next endeavour to sketch the features of the revolving year as observed within the Arctic Circle.

After the continued action of the sun has at last melted away the great body of ice, a short and dubious interval of warmth occurs. In the space of a few weeks, only visited by slanting and enfeebled rays, Frost again resumes his tremendous sway. It begins to snow as early as August, and the whole ground is covered, to the depth of two or three feet, before the month of October. Along the shores and the bays, the fresh water, poured from rivulets, or drained from the thawing of former collections of snow, becomes quickly converted into solid ice. As the cold augments, the air deposits its moisture, in the form of a fog, which freezes into a fine gossamer netting, or spicular icicles, dispersed through the atmosphere, and extremely minute, that might seem to pierce and excoriate the skin. The hoar frost settles profusely, in fantastic clusters, on every prominence. The whole surface of the sea steams like a limekiln; an appearance, called the frost-smoke, caused, as in other instances of the production of vapour, by the water's being still relatively warmer than the incumbent air. At length the dispersion of the mist, and consequent clearness of the atmosphere, announce, that the upper stratum of the sea itself has become cooled to the same standard; a sheet of ice spreads quickly over the smooth expanse, and often gains the thickness of an inch in a single night. The darkness of a prolonged winter now broods impenetrably over the frozen continent, unless the moon chance at times to obtrude her faint rays, which only discover the horrors and wide desolation of the scene. The wretched settlers, covered with a load of bear-skins, remain crowded and immured in their hut, every chink of which they carefully stop against the piercing external cold; and, cowering about the stove or the lamp, they seek to doze away the tedious night. Their slender stock of provisions, though kept in the same apartment, is often frozen so hard, as to require to be cut with a hatchet. The whole of the inside of their hut becomes lined with a thick crust of ice; and, if they happen for an

instant to open a window, the moisture of the confined air is immediately precipitated in the form of a shower of snow. As the frost continues to penetrate deeper, the rocks are heard at a distance to split with loud explosions. The sleep of death seems to wrap up the scene in utter and oblivious ruin.

At length the sun reappears above the horizon; but his languid beams rather betray the wide waste, than brighten the prospect. By degrees, however, the further progress of the frost is checked. In the month of May, the famished inmates venture to leave their hut, in quest of fish on the margin of the sea. As the sun acquires elevation, his power is greatly increased. The snow gradually wastes away-the ice dissolves apace-and vast fragments of it, detached from the cliffs, and undermined beneath, precipitate themselves on the shores with the noise and crash of thunder. The ocean is now unbound, and its icy dome broken up with tremendous rupture. The enormous fields of ice, thus set afloat, are, by the violence of winds and currents, again dissevered and dispersed. Sometimes impelled in opposite directions, they approach, and strike with a mutual shock, like the crush of worlds,-sufficient, if opposed, to reduce to atoms, in a moment, the proudest monuments of human power. It is impossible to picture a situation more awful than that of the poor crew of a whaler, who see their frail bark thus fatally enclosed, expecting immediate and inevitable destruction.

Before the end of June, the shoals of ice in the Arctic seas are commonly divided, scattered, and dissipated. But the atmosphere is then almost continually damp, and loaded with vapour. At this season of the year, a dense fog generally covers the surface of the sea, of a milder temperature indeed than the frost smoke, yet produced by the inversion of the same cause. The lower stratum of air, as it successively touches the colder body of water, becomes chilled, and thence disposed to deposit its moisture. Such thick fogs, with mere gleams of clear weather, infesting the northern seas during the greater part of the summer, render their navigation extremely dangerous. In the course of the month of July, the superficial water is at last brought to an equilibrium of temperature with the air, and the sun now shines out with a bright and dazzling radiance. For some days before the close of the summer, such excessive heat is accumulated in the bays and sheltered spots, that the tar and pitch are sometimes melted, and run down the ship's sides.

The ice which obstructs the navigation of the Arctic seas, consists of two very different kinds; the one produced by the congelation of fresh, and the other by that of salt water. In those inhospitable tracts, the snow which annually falls on the

islands or continents, being again dissolved by the progress of the summer's heat, pours forth numerous rills and limpid streams, which collect along the indented shores, and in the deep bays enclosed by precipitous rocks. There, this clear and gelid water soon freezes, and every successive year supplies an additional investing crust, till, after the lapse perhaps of several centuries, the icy mass rises at last to the size and aspect of a mountain, commensurate with the elevation of the adjoining cliffs. The melting of the snow, which is afterwards deposited on such enormous blocks, likewise contributes to their growth; and, by filling up the accidental holes or crevices, it renders the whole structure compact and uniform. Meanwhile, the principle of destruction has already begun its operations. The ceaseless agitation of the sea gradually wears and undermines the base of the icy mountain, till, at length, by the action of its own accumulated weight, when it has perhaps attained an altitude of a thousand, or even two thousand feet, it is torn from its frozen chains, and precipitated, with tremendous plunge, into the abyss below. This mighty launch now floats like a lofty island on the ocean; till, driven southwards by winds and currents, it insensibly wastes and dissolves away in the wide Atlantic.

Such, we conceive, to be the real origin of the icy-mountains or icebergs, entirely similar in their formation to the glaciers which occur on the flanks of the Alps and the Pyrennees. They consist of a clear, compact and solid ice, which has the fine green tint verging to blue, which ice or water, when very pure and of a sufficient depth, always assumes. From the cavities of these icebergs, the crews of the northern whalers are accustomed, by means of a hose, or flexible tube of canvas, to fill their casks easily with the finest and softest water. Of the same species of ice, the fragments which are picked up as they float on the surface of the ocean, yield the adventurous navigator the most refreshing beverage.

It was long disputed among the learned, whether the waters of the ocean are capable of being congealed; and many frivolous and absurd arguments, of course, were advanced to prove the impossibility of the fact. But the question is now completely resolved; and the freezing of sea water is established both by observation and experiment. The product, however, is an imperfect sort of ice, easily distinguishable from the result of a regular crystallization: It is porous, incompact, and imperfectly diaphanous. It consists of spicular shoots, or thin flakes, which detain within their interstices the stronger brine; and its granular spongy texture has, in fact, the appearance of congealed >vrup, or what the confectioners call water-ice. This saline ice

can, therefore, never yield pure water; yet, if the strong brine, imprisoned in it, be first suffered to drain off slowly, the loose mass that remains will melt into a brackish liquid, which in some cases may be deemed potable.

To congeal sea-water of the ordinary saltness, or containing nearly the thirtieth part of its weight of saline matter, it requires not an extreme cold, this process taking effect about the 27th degree on Fahrenheit's scale, or only 5 degrees below the freezing point of fresh water. Within the Arctic circle, therefore, the surface of the ocean being never much warmer, is, in the decline of the summer, soon cooled down to the limit at which congelation commences. About the end of July, or the beginning of August, a sheet of ice in the space of a single night is formed, perhaps an inch thick. The frost now maintains ascendency, and shoots its increasing energy in all directions, till it has covered the whole extent of those seas with a solid vault to the depth of several feet. But, on the return of spring, the penetrating rays of the sun gradually melt or soften that icy floor, and render its substance friable and easily disrupted. The first strong wind, creating a swell in the ocean, then breaks up the vast continent into large fields, which are afterwards shivered into fragments by their mutual collision. This generally happens early in the month of June; and a few weeks are commonly sufficient to disperse and dissolve the floating ice. The sea is at last open, for a short and dubious interval, to the pursuits of the adventurous mariner.

While icebergs are thus the slow growth of ages, the fields or shoals of saline ice are annually formed and destroyed. The ice generated from melted snow, is hard, pellucid, and often swells to enormous height and dimensions. But the concretion of salt-water wants solidity, clearness and strength, and never rises to any very considerable thickness. It seldom floats during more than part of the year; though, in some cold seasons, the scattered fragments may be surprised by the early frost, and preserved till the following summer.

The whale-fishers enumerate several varieties of the salt-water ice. A very wide expanse of it, they call a field, and one of smaller dimensions, a floe. When a field is dissevered by a subaqueous on grown swell, it breaks into numerous pieces, seldom exceeding forty or fifty yards in diameter, which, taken collectively, are termed a pack. This pack again, when of a broad shape, is called a patch; and, when much elongated, a stream. The packs of ice are crowded and heaped together by violent winds; but they again separate and spread asunder in calm weather. If a ship can sail freely through the floating pieces of

« السابقةمتابعة »