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most instructive. But the best and completest work that we have seen on the subject of the Northern fisheries, is a treatise in three volumes octavo, translated from the Dutch language into French by Bernard de Reste, and published at Paris in 1801, under the title Histoire des Pêches, des Découvertes, et des Etablissemens des Hollandois dans les Mers du Nord.
The Arctic Expedition, which has, for several months, attracted the attention of the public, proposes two distinct objects;-to advance towards the Pole-and to explore a northwest passage to China. These are, no doubt, splendid schemes; but, in order to form a right estimate of the plan, and some anticipation of its probable results, we must proceed with caution, and employ the lights of science to guide our steps. The facts alleged respecting the vast islands or continents of ice recently separated and dispersed from the Arctic regions, have given occasion to much loose reasoning, to wild and random conjectures, and visionary declamation. Glowing anticipations are confidently formed of the future amelioration of climate, which would scarcely be hazarded even in the dreams of romance, Every person possessing a slight tincture of physical science, conceives himself qualified to speculate concerning the phenomena of weather, in which he feels a deep interest; and hence, a very flimsy and spurious kind of philosophy, however trifling or despicable it may appear in the of the few who are accustomed to think more profoundly, has gained currency among certain classes of men, and engendered no small share of conceit. Meteorology is a complex science, depending on so many subordinate principles, that require the union of accurate theory, with a range of nice and various observations, as to have advanced very slowly towards perfection. Though little understood, or generally cultivated, it has yet made a decided progress, and at last attained to such degree of improvement, as will enable the judicious inquirer to draw his conclusions with safety and confidence. Nothing is required but the torch of geometry to illumine the results furnished by the application of delicate instruments.
With regard to the nature and real extent of the change which has now taken place in the condition of the Icy seas, we are persuaded that the reports are greatly exaggerated. * To
*So much has public credulity been abused by such tales, that a paragraph having appeared in a Scotch newspaper, stating that a vast mountain of ice had lately stranded on one of the Shetland Isles, the hoax was actually swallowed by sundry grave persons, especially in the South.
reduce them to their just amount, it would be necessary to estimate the annual effects produced in those regions, and likewise to compare the observations of a similar kind made by experienced navigators at former periods. From a critical examination of the various facts left on record, it will perhaps appear, that those Arctic seas have been, more than once, in the course of the last half century, as open as they are represented to be at present.
To discuss, with accuracy, the question of the periodical formation and destruction of the Polar Ice, it becomes necessary to explain the true principles which regulate the distribution of heat over the globe. This we shall attempt to perform, independent of every hypothesis, by the direct appeal to experiment and observation.
If, at any place we dig into the ground, we find, by the insertion of a thermometer, that, as we successively descend, we approach constantly to some limit of temperature, at a certain depth below which, it continues afterwards unchanged. This depth of equilibrium varies in different soils; but seldom exceeds thirty or fifty feet. If the excavation be made about the commencement of winter, the temperature will appear to increase in the lower strata; but, on the contrary, if the pit be formed in the beginning of summer, it will be found to grow colder as we descend. Hence, the mass of the earth merely transmits very slowly the impressions of heat or cold received at its surface. The external temperature of any given day, will perhaps take near a month to penetrate only one foot into the ground. By digging downwards in summer, we soon reach, therefore, the impressions of the preceding spring and winter; but the same progress into the ground brings us back to the temperatures of the autumn and of the summer. Still lower, all the various fluctuations of heat are intermingled and confounded in
one common mean.
Such observations are more easily and correctly made, by having thermometers, with long stems, sunk to different depths in the ground. From a register of four of those instruments, planted one, two, four, and eight feet deep, in a spacious garden on the northern shore of the Firth of Forth, we are enabled to quote the series of observations made during the years 1816 and 1817. Their mean indications in the month of January 1816 were respectively 33.0-36°.3—40°.7—and 43°.0; and in the same month of the following year, these were 35°.6-38°.7—45°.5— and 45°.1. But, in the month of June of the same years, the thermometers at the depths of one, two, four, and eight feet, marked at a medium 51°.6-50°.0-47°.1-and 45°.8; and again 51°.1-49.4-47°.6 and 47°.8. It is curious to remark,
that in this climate, the thermometers, at all those depths, nearly coincide at two different times of the year, or about the beginning and the end of summer. Thus, about the middle of May 1816, they all stood within a few tenths of a degree of 43°.7; and again, about the middle of September, they had reached nearly 510.2. In the following year, they indicated, in the same months, 45°.1, and 52°.1. It is obvious, therefore, that in the first of those means, the impression of spring was predominant, and that of summer in the second. The mean of the whole year, at the depths of one, two, four, and eight feet, was, in 1816, respectively 43°.8-44°.1-45°.1-and 46°.0; and in 1817, 44°.9-45°.9-46°.2 and 46°.8. Both those years, but especially 1817, had been unusually cold. The lowest thermometer, evidently affected by the impressions of preceding years, was partially returned again from the ground beneath it. On the 18th of April last, all those thermometers stood at 41°. That of one foot deep, rose at first slowly, and then with rapidity, to 61°, which it reached on the 13th of June, and had declined, on the 16th, to 58°, and at the end of the month sank to 55°. The thermometers, buried at the depth of two, four, and eight feet, had gradually mounted, on the 16th of June, to 55°, 50°, and 47°; and, by the end of the month, stood at 53°, 51°, and 49°. In preceding years, the thermometer, inserted one foot into the ground, did not reach its maximum till the middle or near the end of July; that range, in 1815, 1816 and 1817, being only respectively 58°, 54° and 56°; whereas it had, in this present summer, attained 61° early in the month of June. We may hence conclude, that the temperature of the ground is always the mean result of the impressions made at the surface during a series of years. The successive strata, therefore, at great depths, may be regarded as permanent records of the average state of the weather in distant ages. Perhaps the superficial influence will scarcely descend fifty feet in the lapse of a century. Copious springs which percolate the bowels of the earth, and rapidly convey the impressions of subterranean heat to the surface, will consequently furnish the most accurate reports of the natural register of climate. These, if rightly chosen, differ not sensibly in their temperature at all seasons; and, whether they have their seat at a depth of one hundred or of five hundred feet, they affect the thermometer alike. * We
* The celebrated fountain of Vaucluse, situate in the latitude of 43° 55', and 360 feet above the level of the Mediteranean sea, has been observed to acquire its highest temperature about the first day of September, and to reach the lowest at the beginning of April, the former being 56°.3, and the latter 54°.1 by Fahrenheit's scale; which gives 55°.2 for its mean heat. The waters are collected from the
are hence entitled to conclude, that however the weather may have varied from year to year, or changed its character at intervals of short periods of years, it has yet undergone no radical or sensible alteration during the efflux of many ages.
Some philosophers attempt to explain such facts as are now stated, from the supposed internal heat of the globe, caused by the action of central fires; and pretend, in support of their favourite hypothesis, that the temperature always increases near the bottom of very deep mines. But this observation holds only in particular situations, where the warm exhalations from the burning of lamps and the breathing of the workmen are collected and confined under the roofs of the galleries. The water which trickles from the crevices of the strata, and runs along the floor of the mine, has still the medium temperature belonging to the place. It should likewise be remarked, such is the very slow conducting quality of earthy and stony matters, that the volcanic fires have no sensible influence on the climate of those countries where they still exist in full activity.
The permanent heat of the ground is, therefore, produced by the mere accumulation of incessant external impressions. These impressions are received, either directly from the sun's rays, or circuitously, through the medium of atmospheric influence. But air is better fitted for diffusing than for storing up heat. The whole mass of the atmosphere, it may be easily shown, does not contain more heat than a stratum of water only 10 feet thick, or one of earth measuring 15 feet. According to their relative temperature, the winds, in sweeping along the ground, either abstract or communicate warmth. But the sun is the great and original fountain of heat, which the internal motion excited in the atmosphere only serves to distribute more equally over the earth's surface. The heat imparted to the air, or to the ground, is always proportional to the absorption of the solar beams; and the consequences are, therefore, still the same, whether we embrace the simple theory, that heat is only the subtle fluid of light, in a state of combination with its substratum; or prefer the opinion, that light has always conjoined with it an admixture of the invisible matter of heat.
Owing to the spherical form of the earth, and the obliquity of its axis, very different quantities of light or heat are received
fissures of an extensive limestone rock, and seem to receive the su• perficial impressions in the space of three months. They burst forth with such a volume as to form, only a little below their source, the translucid Sorgue, a river scarcely inferior in its discharge to the Tay above Perth.
in the several latitudes. The same portion of heat which would raise the temperature of 135 pounds of water a degree on Fahrenheit's scale, is only capable of melting one pound of ice. The measure of ice dissolved is hence the simplest and most correct standard, for estimating the quantity of heat expended in that process. If we apply calculation, therefore, to actual experiment, we shall find that the entire and unimpaired light of the sun would, at the Equator, at the mean latitude of 45°, and at the Pole, be sufficient to melt a thickness of ice expressed by 38.7, 25.9, and 13.4 feet. Of this enormous action, the greatest portion is no doubt wasted in the vast abyss of the ocean; and, of the remainder, a still larger share is perhaps detained and dissipated in the grosser atmosphere. Yet the light which, after those defalcations, finally reaches the surface of the earth, if left to accumulate there, would create such inequality of temperature, as must prove quite insupportable. It is indeed remarkable how very small a part of the extended scale of heat is wanted for the support of animal life, or compatible with its existence. The absolute zero probably descends at least 1400 degrees below the commencement of Fahrenheit's divisions, and the intense power of our furnaces perhaps rises ten times higher. Yet few plants will bear a change of 50 degrees of temperature; and man, the hardiest of all animals, would find the transition of 100 degrees quite insupportable.
The slow conducting quality of the ground, if not balanced by extraneous influence, would fix the heat where it was received, and thus perpetuate the effect of the unequal action of the sun's beams. The mobility of the atmosphere hence performs an important office in the economy of Nature, as a great regulator of the system, dispensing moderate warmth, and attempering the extremes of climate over the face of the globe. As the heat accumulates within the tropics, it will occasion currents of cold air from the higher latitudes. But the activity of the winds thus raised being proportional to their exciting cause, must prevent it from ever surpassing certain limits. A perpetual commerce of heat between the Poles and the Equator is thus maintained, by the agency of opposite currents in the atmosphere. These currents will often have their direction modified; and they may still produce the same effects, by pursuing an oblique or devious course. The actual phenomena of climate only require the various winds throughout the year, to advance southwards or northwards at the mean rate of almost two miles an hour, or to perform in effect three journeys of transfer annually from the Equator to either Pole. Not that these carry the impressions of heat or cold directly from one extremity of