too much evidence, since astronomers have observed that stars, which are in fact remote suns, have waxed and waned, and in some instances disappeared utterly.* Still, it is an interesting inquiry, to ascertain to what extent any slight changes of mean temperature which have been observed, or any difference between the mean or extreme temperatures of the northern and southern hemispheres, may be referred to an astronomical origin. We may consider, first, whether the position, and, secondly, whether the form, of the earth's orbit are exposed to changes which would influence the climate. The seasons are caused by the inclination of the earth's equator to the plane of the ecliptic; in other words, to the want of parallelism between its motions of rotation and revolution. The effect of this inclination is to make the sun run higher, and keep above the horizon longer, in the summer half of the year than in the winter half. If this inclination increased or diminished, the seasons would become to a corresponding degree more excessive or more moderate. This angle has one of those magnificent oscillations known in physical astronomy as secular variations, but the limits of it are very circumscribed, and its influence, within the historical period, must have been inappreciable. Poisson estimates the change in this angle too small, and too slow, to produce that increase in the descending strata of the earth's crust, the explanation of which he seeks in the changing temperature of the space traversed by the solar system. The earth, when in the perihelion of its orbit, is nearer to the sun, by of the mean distance, than when in the aphelion. Hence the heat received at the former point is greater than at the latter point. The perihelion is reached by the sun about the first of January, and the aphelion about the first of July. The effect would seem to be a diminution of the summer heat and the winter cold in the northern hemisphere, and an increase of both in the southern hemisphere, so that the seasons of the latter would be more excessive than those of the former. But Sir John Herschel remarks, in his Outlines of Astronomy, that "the elliptic form of the earth's orbit has but a very trifling share in producing the variation of temperature corresponding to the difference of seasons" on account of the greater velocity of the earth in the January half of the orbit, which shortens the time required for its passage by eight days, when compared with the period of describing the other half. Therefore, the northern hemisphere receives as much heat in its long summer as the southern hemisphere in its short one, though the latter is more closely exposed. And again, the southern hemisphere receives as much heat in its prolonged winter as the northern hemisphere in its short winter, though the momentary effect of the sun's rays is greatest for the northern winter. This reasoning would be satisfactory if all the heat received was retained. But radiation from the earth is always wasting the heat received from the sun; from the autumnal to the vernal equinox, the nights are longer than the days, and the earth loses more heat by night than it receives by day; and the southern hemisphere has eight more of these disadvantageous days than the northern hemisphere. Humboldt pointed out this defect in the argument of Mairan to prove the equal advantage belonging to the two hemispheres; an argument adopted, as we have seen, by Herschel, if we understand by his expression of " very trifling," that the distance of the earth from the sun is wholly balanced by the duration of the exposure. Humboldt says, in his Memoir on the Isothermal Lines, "The southern hemisphere receives the same quantity of light; but the accumulation of heat * Œuvres, VIII. p. 219. See also Buffon, IV. pp. 29, 30. Geol. Trans., III., Part 2, p 298. Memoires de l'Acad. de Paris, 1765, p. 166. Memoires de la Société d'Arceuil, III.; Edin. Phil Journ., IV. p. 262. in it is less, on account of the emission of the radiant heat, which takes place during a long winter." He might have added, that the summer heat would be greater, because, though no more heat was received, it was received in a shorter time, during which less would be lost. In 1835, Prof. Ladame published a memoir on the "Formation of the Actual Surface of the Globe," in which he discusses the sun's action in the distribution of temperature. Adopting the law of cooling of Dulong and Petit, he first proves, by mathematical analysis, that the mean temperature falls, if the daily increase of temperature becomes larger. Hence, the mean temperatures diminish with an increase of latitude, beyond what might be expected merely from the change in the inclination of the sun's rays; because the farther north the position on the earth, the greater the difference from winter to summer. "Another consequence to be derived from the preceding calculation refers to the temperature of the two hemispheres. In fact, in the present condition of the elements of the earth's motion, the northern summer corresponds to the aphelion, and the northern winter to the perihelion. Thus the burning heats of summer and the rigorous colds of winter are moderated, and this circumstance elevates, other things being equal, the mean temperature of the northern hemisphere. This is what experiments made in the two hemispheres to determine the mean temperature of different places, the limit of trade-winds on each side of the equator, and the relations of terrestrial magnetism to heat, (relations which M. Duperre has disclosed,) fully demonstrate. However, we are not to see in the eccentricity of the earth's orbit the only cause of this fact; the distribution of continents and seas is incontestably the most active; nevertheless, we should not set aside the former. For, before the appearance of the land above the water, that alone survived to produce differences in the superficial temperature, and the increase of the crust over different points of the carth's surface." Although there may be a residual effect depending on the shape of the earth's orbit, it will be certainly less than would result from the reasoning of Epinus upon the subject in 1761.t He gave full weight to the influence of the longer summer and the shorter winter in the northern hemisphere, as compared with the longer winter and the shorter summer in the southern hemisphere, without allowing any force to the partial compensation which the southern hemisphere would receive from the shorter distance of the sun during its summer. Buffon makes a similar mistake.‡ If the position of the perihelion of the earth's orbit is of any importance, then the shifting of this point, by which it is gradually carried forward from one month into another, becomes of some moment in the explanation of the changes of climate. If this consideration is of any value in the study of the earth's climate during the vast periods contemplated by geology, it has none in relation to the alleged changes which have been assumed in the tempera ture of certain spots of the earth, within historical times, because the perihelion has moved within these times only by about 36°, and the time of the earth's arriving at it has only shifted from about the 24th of November to the 1st of January. Is the form of the earth's orbit subject to any variation which would influence the climate? Astronomers § have demonstrated that the mean distance of the sun is unchangeable, but that the shape of the orbit is exposed to fluctuations. Sir John Herschel || has proved that this invariability of the mean distance will not secure the permanence of the amount of heat received from the sun; that this amount will increase as the shorter axis of * Memoires de la Société des Sciences Naturelles de Neuchatel, I. pp. 159, 160. † De Distributione Caloris, pp. 16, 17. Hist. Nat., III. p. 308; Epoq. de la Nature. Laplace, Bk. II. 57. the earth's orbit becomes less, and decrease as the shorter axis of the orbit becomes greater, and the orbit approaches more nearly to the form of a circle. Now astronomers have demonstrated that the planetary disturbances are bringing the earth's orbit nearer and nearer to the circular form, and hence diminishing the amount of heat which the earth receives from the sun. This influence, however, Arago declares to be a pure abstraction. For in 10,000 years it would barely become perceptible, and in historical times it is wholly insignificant. Herschel contemplates the possibility of the earth's orbit, after becoming a circle, relapsing again into an eccentricity as great as that of Pallas, or of the semi-transverse axis of the ellipse, or of having been of this shape formerly. But such a change in the shape of the orbit would alter the whole amount of heat received from the sun in a year, by only three per cent. Though the mean temperature would not sensibly alter, the summer and winter temperatures might. For the heat at the perihelion would (other things being equal) exceed that at the aphelion, in the ratio of 3 to 1. It would then be of vast importance whether the perihelion corresponded with the summer solstice or the winter solstice. In the former case, the midsummer heat and midwinter cold would be as intensified to one hemisphere as if there were three suns in summer and only one in winter. However interesting such discussions may be in the study of the meteorology of geological eras, it does not concern historical meteorology. For such changes as are here supposed in the earth's orbit could not take place in less than six hundred thousand years, and carry us therefore so far back into the past or forward into the future. If the earth depended on the sun exclusively for heat, calculation shows that at a certain depth (about 100 feet) the temperature would be invariable throughout the year, though its value would vary with the latitude. All this accords with observation. But it appears that this constant temperature of the earth's interior mass increases with the depth of the place examined (as is found by experiments in artesian wells), at the rate of about 1° for every fifty-five feet. Hence the supposition that the earth was once much hotter than at present, an incandescent, molten mass, acquiring its present form under the action of the centrifugal force and gravity, as the clay of the potter under its rotation becomes a figure of revolution; that the earth is, in fact, an encrusted sun, the central heat of which may be invoked in any exigency of science. This result of positive science harmonizes with the speculations of the Plutonic school, and with the imaginations of Descartes and Leibnitz. If it were known how many thousand years ago the earth began to cool, then the rate of cooling could be calculated. If, on the contrary, the rate of cooling could be found by direct observation, then it would be possible to recover the date of the commencement of the process, and hence the minimum age of the planet. It has been concluded that the mean temperature of the earth's mass has not sensibly altered for two thousand years. If the same average law of contraction is assumed for the earth as for glass, a change of temperature of 1° (Centigrade) would have shortened the day by 176 seconds, in conformity with the ordinary laws of the moments of rotation. But the comparison of ancient and modern astronomical observations do not warrant the admission that the length of the day has been changed, since the Alexandrian school of astronomy, by the To of a second. Therefore, the supposition even of a change of of a degree (Centigrade) is inadmissible. And even if the average law of contraction for the earth's materials were very different from that for glass, the conclusion would be substantially the same. The importance of this interior source of heat has been very little during historical times, whatever it may have been at carlier and geological epochs. Mairan,* Buffon,t and Bailly made an exaggerated estimate of it, which Mém. de l'Acad. des Sciences, 1755, p. 143. † Epoques de la Nature, V11. pp. 92 and 264. Lettres à Voltaire. for France amounted to almost five hundred times the heat which that country received directly from the sun in winter, and even twenty-nine times all it received in midsummer. Buffon's speculations in regard to the internal heat of the earth were built upon his cosmogony of the solar system; viz. that the earth and the other planets were fragments of the sun, struck from that glowing orb by the blow of a comet. Originally, therefore, the earth was a melted mass of matter, and has been gradually cooling from this excessively high temperature. Guided by experiments which he had made on the cooling of iron balls, Buffon calculates that the earth would cool down to such a point as to be solidified to the centre in 2,936 years; that its temperature would sink to such a point that animals might touch it with impunity in 34,270 years; that it would reach its existing temperature in 74,832 years, and that it would descend to one twenty-fifth of its present temperature in 168,123 years. A similar computation was also made for all the other planets and satellites then known, and for Saturn's rings. In these computations allowance was given for the influence of the sun's rays, and those of the planets and satellites upon each other, as also for the different density of the various bodies. The smaller bodies cooling with the greatest rapidity, and vice versa, Jupiter, the largest of all, would become solidified throughout in 9,433 years; cooled so as to admit of safe contact in 110,118 years; it would arrive at the existing temperature of the earth in 240,451 years (or 165,619 years from the present epoch); and descend to one twenty-fifth of the existing temperature of the earth not until 483,121 years after the time when it was chipped from the sun. Buffon drew the following important conclusions from the results of his calculations: That the fifth satellite of Saturn was the first body in the solar system which cooled down to a temperature which made it inhabitable. This began 4,916 years after the origin of the planetary system and continued till 47,588. But as the system is now 74,832 years old, that satellite became too cold, thousands of years ago, for the existence of organized beings, such as we are familiar with at the present time. Our moon came next, and had a career of organized existences for 60,000 years; but it was all frozen out 2,318 years ago. On Mars, also, life has become extinct. On the other hand, Jupiter still remains, and will continue for 35,000 years longer, too hot to be touched with safety by plant or animal, though a career of nearly 400,000 years awaits it in the future. In all the other bodies of the solar system then discovered organized life is in the ascendant, though it began at very different periods, and is destined to enjoy a longer or a shorter future, according to the individual peculiarities of each planet. Buffon estimates the mean effect of the sun's rays upon the earth's superficial temperature at the present time as equal to only one-fiftieth of the effect which comes to the surface from the fires below. Formerly, the sun's influence was comparatively less; hereafter, it will be comparatively greater, as the internal heat diminishes. But its accumulated power is only able to increase the period of the earth's inhabitability (which without it would be 148,000 years) by about 20,000 years. But as soon as Fourier subjected the problem to rigorous calculation, he found a relation between the excess of the total temperature of the earth's surface above what it receives from the sun, and the increase of temperature at different depths; and from this relation he proved that the earth's surface was indebted to the interior for only of a Centigrade degree. However great the temperature and its changes which still prevail in the central parts of the earth, and however important such changes may have once been at the surface, it is of little moment to those who live now upon the surface, even if the internal fires were wholly extinguished; and, as Arago remarks, the * Nat. Hist., Vol. VII.; Introduction à la Hist. des Mineraux, p. 78, &c. frightful picture drawn by Buffon of a vast planetary congelation, when the interior source of heat is exhausted, is only a reverie of his own brain. The centre itself would lose its heat only at the rate of 3000 of a second in a century. In 1824, Fourier introduced a new element into the theory of climate, viz. the temperature of the interplanetary spaces, as it is produced by the cross radiation of countless stars. Looking at the rate of diminution in the heat of the atmosphere as the strata become higher and rârer, -a diminution so rapid, that even under the equator the mountains are covered with everlasting snow at the height of three or four miles, meteorologists had concluded that outside of the atmosphere the temperature would be excessively low, hundreds and thousands of degrees below the ordinary zero of thermometers.* Fourier subjected this opinion to a rigorous examination. He found that upon this hypothesis the polar regions would be subject to an enormous degree of cold, much more excessive than observation indicates, and the decrease of temperature between the equator and the poles would be vastly greater than is observed. The change from day to night would produce consequences more marked than any which are noticed. "The surface of bodies would be exposed suddenly, at the approach of night, to an infinitely intense cold. Animals and vegetables could not resist so powerful and prompt an influence, which would act again in a contrary direction at the dawn of day." Fourier comes to the conclusion, that the temperature of space through which the earth pursues its track is only about 60° or 70° below the zero of Fahrenheit. It has been thought that a remarkable confirmation of Fourier's calculation was furnished by Svanberg, who found the temperature of space to be about 58° or 59°, from the capacity of the air for caloric and on the assumption that the absorption of heat by the different strata of air was proportional to the absorption of light, as given by Lambert's experiments.† The immense number of bodies which contribute by their united rays to the general temperature of space, will compensate for any individual irregularities, such as the observations of astronomers, upon the temporary, periodical, and fluctuating light of certain stars, would lead us to anticipate. The temperature of space may not be the same in the different regions of the universe. But the dimensions of the earth's orbit are too small, compared with the distance of the stars, to give any influence upon the seasons to this variable temperature of space. Yet the grand march of the solar system through space may be sufficient to make the mean temperature of the earth experience a sensible change from this cause in the course of ages. In 1838, Pouillet published a memoir ‡ in which he discusses these large questions: the quantity of solar heat which falls perpendicularly in a given time on a given surface; the proportion of this heat which is absorbed by the atmosphere in the vertical passage; the law of absorption for different obliquities; the total quantity of heat which the earth receives from the sun in the course of a year; the total quantity of heat which is emitted at each instant by the whole surface of the sun; the elements which must be known in order to ascertain whether the mass of the sun cools gradually from century to century, or whether there is a cause destined to reproduce the quantities of heat which escape incessantly from it; the elements which would allow its temperature to be determined; the absolute quantity of heat emitted by a body whose surface, temperature, and radiating powers are known; the laws of cooling of a body which loses its heat without receiving * Ann. de Chim. et Phys., XIII. et XXVII. p. 136; Mém. de l'Acad. Roy., Paris, IV., V., et VII. p. 598. † Bib. Univ., XLIII. p. 362; Edin. Journ. Sci., N. S., III. p. 13. Compt. Rend., 1838; Sci. Mem., IV. p. 44; Elem. de Physique, II. p. 642. |