Imágenes de página
PDF
ePub

the globe to the other, but, by their incessant play, they contribute, in the succession of ages, to spread them gradually over the intervening space.

The system of opposite aerial currents leads to the same law of the gradation of temperature in different latitudes, as the celebrated Professor Mayer of Gbttingen deduced from an empirical process. It would appear that the variation of the mean temperature at the level of the sea is always proportional to the sine of twice the latitude. Thus, for the parallels of every ten degrees, the arrangement is simple.

[table]

The arithmetical mean, or 58°, corresponds to the middle latitude of 45°. But the real mean of the temperature over the whole surface of the globe is 67°, which should occur on the parallel of 35° 51$'. . .;

The system of currents maintained in the atmosphere, likewise contributes essentially by its unceasing agency, in transferring and dispersing heat, to prevent the excessive inequality of seasons in the higher latitudes. But the motions produced in such a vast mass of fluid, must evidently follow, at long intervals, the accumulated causes which excite them. Hence probably the origin of those violent winds which, succeeding to the sultry warmth of summer and the sharp frosts of winter, prevail in the months of September and March, and are hence called , by seamen the Equinoxial Gales. In the Arctic seas, Nature has made a further provision for correcting the excessive irregularity of the action of the sun's rays. This luminary, for several months in winter, is totally withdrawn from that dreary waste; but, to compensate for his long absence, he continues, during-an equal period in summer, to shine without interruption. Now, by a beautiful arrangement, the surface of the ocean itself, by its alternate freezing and thawing, presents a vast substratum, on which the excesses of heat and of cold in succession, are mutually spent. In ordinary cases, the superficial water as it cools, and therefore contracts, sinks down into the abyss, by its superior gravity; but when it grows warmer, it expands, and consequently floats incumbent, communicating afterwards its surplus heat with extreme slowness to the mass below. But the seas within the Arctic circle being always near the verge of congelation, at which limit water scarcely undergoes any sensible alteration of volume from a con

siderable change of temperature, the superficial stratum remain* constantly stagnant, and exposed to receive all the variable impressions of the sweeping winds. The piercing cold of winter, therefore, spends its rage in freezing the salt water to a depth proportional to its intensity and continuance. The prolonged warmth of summer again is consumed in melting those fields of ice, every inch of which in thickness, requiring as much absorption of heat as would raise the temperature of a body of water 10£ feet thick, a whole degree. The summer months are hence nearly gone before the sun can dissolve the icy domes, and shoot with entire effect his slanting rays. It may be shown that under the Pole the action of the solar light is, at the time of the solstice, under the Pole, one-fourth part greater than at the Equator, and sufficient in the course of a day to melt a sheet of ice an inch and a half thick. If horizontal winds serve to balance the irregular action of the solar beams over the surface of the globe, the rising and descending currents excited in the body of the atmosphere still more effectually maintain the equilibrium of day and night. After the ground has become heated by the direct illumination of the sun, it warms the lowest portion of the incumbent air, which, being thus dilated, begins to ascend, and therefore occasions the descent of an equal portion of the fluid. But these vertical currents, being once created, will continue their motion long after the primary cause has ceased to impel them, and may protract, during the night, theaccummiilation of chilled air on the surface of the earth. This effect is further augmented, in general, by the frigorific impressions which are, it would seem, at all times darted downwards from a clear sky. * From the operation of this combined system, therefore, the diurnal vicissitudes of temperature are diminished in the temperate and torrid zones. Another consequence results from the rapid and continual interchange of the higher and lower strata, that the same absolute quantity of heat must obtain at every altitude in the atmosphere.

This equal distribution of heat at all elevations, is moulded, however, by another principle, which occasions the regular gradation upwards of a decreasing temperature. In fact, air is found to have its capacity for heat enlarged by rarefaction, so that any portion of the fluid carried to the higher regions, where it by consequence expands, will have its temperature proportionally diminished. The decrease of temperature in ascending the

* See Supplement to the Encyclopaedia Britannica, Vol. III. Part I. p. 177; or, Transactions of the Royal Society el' Edinburgh, 'Vol.. VIII. Part II. p. 465.

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 chv -cumstances 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 de-? pressed 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 seas 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 plac? 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 neat 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 die 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 uppei» 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 eontinent, 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 theyhappen 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 ia 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

« AnteriorContinuar »