« ПретходнаНастави »
more, form the first topic to which, as a matter of common concern, the conversation of casual acquaintance, or persons newly introduced, will be directed. Climates there no doubt are, so happy in their constitution, as to remove the anxiety which is in most others felt in regard to the state of the atmosphere, and in which remarks upon it might appear almost ridiculous; but in one so variable as ours, the fulness of the heart will overflow at the lips, and the hearty salutation of “a fine day,” however little new inforination it may convey to the hearer, will be the natural expression of the native and true urbanity, which desires to impart the feeling of pleasure itself enjoys.
Such being the constant attention which situation and circumstances lead a great portion of our race to pay to atmospheric changes, it might at first sight appear probable, that the strictly scientific part of the subject had received early notice, and reached a high state of perfection. This, however, is by no means the case; the consideration of the weather appears to have been thought too commonplace for the attention of philosophy, although the investigation of the causes of climate, requires the application of knowledge and skill of the very highest order.
« The complication of the processes carried on in the vast laboratory of nature, the wide circle of their agency, and the ever varying results of their compound influences, appear to have been too much for the mind to comprehend as a whole; and the powers of reason have been bewildered in the inextricable labyrinth of causes and effects.” Indeed, before the appearance of the treatise before us, we know of none that could be considered as a strict philosophical investigation of the general causes of atmospheric changes, in which the facts and theories derived from induction and experiment, were carefully and skilfully applied to the explanation of the phenomena.
The general and efficient causes of the vicissitudes of climate, are few, and simple in themselves ; at the present day, they require no tedious analytic process to diseover them ; and our author has consequently not found himself under the necessity of repeating the experiments or reasonings of Mariotte, Dalton, and Black. The mechanical nature both of gases and vapours is well understood, the cause of the presence of aqueous matter in our atmosphere is fully known, and the relations of the several elastic fluids, to latent, specific, and sensible heat, accurately established. When these are combined with the physical state of the nucleus of our earth, and the distribution of the masses of solid and liquid matter that occupy its surface, with the changes in temperature arising from its diurnal rotation and annual revolution, considered as a planet of the Solar System, they furnish the basis on which, by a synthetic process, properly conducted, we may proceed to the discovery of a correct and perfect theory.
One single set of facts was alone wanting; namely, the circumstances attending the evaporation of water in an atmosphere colder than itself, and this our author has himself illustrated. Such a theory will no doubt require, like all other physical propositions, to be brought to the test of observation and experiment. The discovery of new facts, may lead to the introduction of new circumstances in the course of reasoning; the theory itself, although its basis cannot be changed, may receive improvement, and derive confirmation from the development of more numerous facts; but we feel warranted in saying, that our author has proceeded as far as the present state of our knowledge would permit him, and has laid the sound foundation that must give support to all those who may hereafter pursue the study of Meteorology.
In ascribing this high degree of merit to the work before us, we would not be understood as undervaluing the labours of his predecessors, particularly the great names of Halley, Franklin, Dalembert, Laplace, Saussure, Deluc, and Howard, but we may confidently affirm, that, in none of their works, is to be found a theory so consistent with all known facts, and so complete in all
The body of our earth presents a surface principally covered with a mass of aqueous fluid, little more than one-fourth being encrusted with solid materials. But the phenomena of the tides show that this fluid, so extended upon the surface, is of comparatively small depth; and the density of the earth is such, that substances not only more dense than water, but even than the heavier substances that form this solid crust, and the bed of the ocean, occupy the interior. Observations made upon the temperature of mines, seem to indicate a greater degree of heat within the earth than upon its surface; and it has been surmised, that this temperature increases with the depth to such a degree, that the inner portions of the terrestrial sphere may still be in a liquid state under the influence of igneous fusion.* The figure of the earth is such as could only have been assumed spontaneously by a body in a liquid state, and of density increasing from the surface towards the centre. Its surface, therefore, during the process of creation, must have cooled, and thus consolidated. This cooling would necessarily take place from the physical property of the radiation of heat; and hence the earth must, at a time anterior to our histories, have diminished in temperature. It still continues to radiate heat to the empty space that surrounds it, and some have inferred, even at present, a regular and progressive cooling. This, however, is not the fact ; Laplace
* Laplace, Connaissance da Tems for 1824.
has shown most conclusively, that since the time of Hipparchus, the first observer whose records can be advantageously applied to the determination of the state of the motions of the heavenly bodies, the mean temperature of the earth has not diminished a quantity that can be estimated. This unexpected application of mathematical calculus, and astronomic observation, to a case that might at first sight appear entirely irrelevant, is one of the most beautiful investigations of that distinguished, and now lamented, geometer. Had the earth grown either warmer or cooler within the space to which authentic observations extend, its rotary motion on its axis, by which the length of the sidereal day is determined, must have varied, and thus the day must have increased or diminished. To ascertain whether either of these changes had occurred, might at first sight appear difficult, as this very rotation furnishes us with the unit in which time is measur
But it so happens, that, although the measure of time upon our planet does not furnish the means of inquiry, a variation in the length of the day would have affected the secular equation of the lunar motion. On comparing the rates of the moon's revolution at distant periods, they accord so fully with the hypothesis of an unvaried rate of diurnal motion, that it is absolutely certain, that the day has not varied one tenth part of a second since the days of Hipparchus. Consequently, its mean temperature has not changed more than a small fraction of a degree of Fahrenheit's thermometer. Some source must then exist, whence heat is derived by our earth to supply the waste by radiation ; and we find this in the sun ; whence, therefore, there flows to our earth a quantity of heat exactly equivalent to the quantity radiated. This is but one among innumerable instances of wisdom in the arrangement of forces and laws, that act in the universe, apparently unconnected, and originally unequal, but which have at length reached a state of the most exact and perfect equilibrium. But this Aow of heat from the sun, although its sum in any long period is thus exactly equal to the radiated heat, is, if estimated at any given place, unequal and variable. Radiation from the earth goes on continually, while the solar heat is received at intervals, and in unequal quantities, in consequence of the rotation of the earth, and its revolution in its orbit, causing the alternations of day and night, and the vicissitudes of the seasons. During the time, then, that the sun is above the horizon, twice as much heat, at a mean rate, is given to the earth, as radiates from the enlightened surface in the same number of hours. The axis of diurnal rotation is not perpendicular to the plane of the orbit of annual revolution; for this reason, the daily continuance of the sun above the horizon, (except at the equator,) and the inclination of his rays to any given point on the surface, is constantly varying, having a period corresponding to that of the annual repolution
earth. When the sun is longest above the horizon of any place, the surface of the earth, at that place, receives more heat in twenty-four hours, than it throws off, and vice versa. Hence arise the vicissitudes of the seasons; and as the accumulation of heat continues in summer after the solstice, and the diminution in winter goes on after the shortest day, the hottest and coldest weather, in temperate climates, falls in the months of August and February, instead of June and December. Nor are the two hemispheres into which the earth is divided by the equator, similarly situated in this respect; their seasons are opposite, but the length of them is not identical. The earth, instead of describing a circular orbit with equable motion, describes an ellipsis upon the principle of equal areas. The sun, for this reason, is seven days and twothirds longer in the northern, than in the southern part of the ecliptic; the summer of the northern hemisphere is therefore longer; under equal latitudes, the climate is warmer in the northern than in the southern hemisphere, and the mean temperature of the latter, less than that of the former.
Climate, however, is not affected merely by the position of a place in latitude. Numerous local circumstances influence it in a greater or less degree.
Elevation above the level of the ocean, is perhaps the most important of these. Our earth is surrounded by a mass of elastic fluid, called its atmosphere, which, from its distinguishing characteristic, is rarer as it rises from the mean surface of the earth. It is a property of elastic fluids, that their capacity for heat increases as they become more rare. Hence, in ascending from the level of the sea, the sensible heat of the atmosphere diminishes, and bodies situated in the higher regions, are cooled down to the diminished temperature. Even in the torrid zone, then, and immediately under the equator, high table lands enjoy a vernal temperature, and mountains of a great height are covered with perpetual snow; in higher latitudes, the region of perpetual congelation is more speedily reached. Not only is the elevated land itself reduced to a lower temperature, but the climate of the neighbouring districts may be sensibly affected by the vicinity of chains of mountains, particularly if covered with constant snow.
Seas and great lakes affect the climate of countries near them, not merely by the moisture they furnish, but by the peculiar manner in which a deep mass of water receives and parts with its heat. When surfaces of land and sea are similarly exposed to the rays of the sun, they are very differently affected. The soil being opaque, the heat of the sun penetrates to but a small depth; and hence its surface is much more heated than that of the water, into which the heat penetrates, before it is entirely absorbed, to the depth of many feet. On the other hand, the surface of the land is more rapidly cooled, by radiation, than that
of water. Water is among the very best radiators of heatfar better, indeed, than earth ; but when water cools from above, an intestine motion takes place among its particles; those first cooled, becoming denser, descend; their place is occupied by warmer portions from beneath; and the heat lost is derived from the entire mass. The open sea and very deep lakes, hence, hardly freeze in the coldest climates.
The climate of seas and great lakes will, for these reasons, be more equable than that of land, in equal latitudes, and will affect that of islands and their neighbouring coasts.
Shallow waters, such as small lakes, swamps, and morasses, act in a different manner. The rapidity of radiation from the surface, is not counteracted by the motion of the particles, which soon ceases, in consequence of the whole mass being speedily brought by it to one uniform temperature, or to the maximum density of water. Hence, while they may mitigate the heats of summer, such collections of water increase the cold of winter.
Cultivation affects climate, by draining marshes and small lakes; and still more, by clearing off the great forests that usulally cover the face of an unpeopled country. These last prevent the rays of the sun from penetrating to the soil; hence it is less heated in summer, and the snows of winter are more lasting. — Our own country is now feeling the influence of this last cause. If occasional returns of severe winters, seem to show that the minimum temperature of the year falls as low as it ever did, still there is no doubt that the duration of the inclement season is evidently lessening. The influence of cultivation is often very far extended, affecting even countries distant from that where the process is going forward. Thus, since the time of Virgil and Horace, the snows and frosts they attribute to Italy, have in a great measure disappeared; and the cause is to be sought, not in Italy itself, but in the cultivation of those more northern portions of Europe, which, in the days of Augustus, were covered with woods and marshes. Soracte no longer stands white with snow; frost no longer invades the plains of Lombardy. Other variations of climate are dependent upon the nature and constitution of the atmosphere, combined with the foregoing causes; these are most ably explained by our author, of whose investigations we shall proceed to present a synopsis.
The principal constituent part of the atmosphere that surrounds our earth, is a mixture of various permanently elastic fluids; or, to speak more in accordance with the present state of knowledge, of gaseous bodies, incondensible at any natural temperature. These, however heterogeneous in their chemical nature, are identical in their mechanical properties; and the combined mass may therefore, so far as the present object is concerned, be considered as homogeneous. Possessed of weight, and therefore gravitating, it