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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 51°.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, 449.9—45°.9-46o.2—and 46°.8. Both those
years, cially 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 41o. 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 55o. 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 49o. 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°, 54o 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 farmer being 56o.3, and the latter 54o.1 by Fahrenheit's scale; which gres 55o.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 superficial 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 Fahrenheits 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 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 aërial currents leads to the same law of the gradation of temperature in different latitudes, as the celebrated Professor Mayer of Göttingen 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. Latitude Mean Temperature. Latitude. Mean Temperature. 0° 84° 50°
32° 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° 511'.
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 calledby 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 cons siderable change of temperature, the superficial stratum remains 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 104 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 equilibriuin 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, the accummulation 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 Encyclopædia Britannica, Vol. III. Part I. p. 177; or, Transactions of the Royal Society of Edinburgh, Vol. VIII. Part II. p. 465.