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become perceptible, did not nature, by the process of evaporation, raise the waters back to their sources; and thus, by again removing matter to a greater distance from the centre, destroy the velocity generated by its previous approach; so that the descent of rivers does not affect the earth's rotation. Enormous masses projected by volcanos from the equator to the poles, and the contrary, would indeed affect it, but there is no evidence of such convulsions. The disturbing action of the moon and planets, which has so powerful an effect on the revolution of the earth, in no way influences its rotation; the constant friction of the trade-winds on the mountains and continents between the tropics does not impede its velocity, which theory even proves to be the same as if the sea, together with the earth, formed one solid mass. But although these circumstances be inefficient, a variation in the mean temperature would certainly occasion a corresponding change in the velocity of rotation; for, in the science of dynamics, it is a principle in a system of bodies, or of particles revolving about a fixed centre, that the momentum, or sum of the products of the mass of each, into its angular velocity and distance from the centre, is a constant quantity, if the system be not deranged by a foreign cause. Now, since the number of particles in the system is the
same, whatever its temperature may be, when their distances from the centre are diminished, their angular velocity must be increased, in order that the preceding quantity may still remain constant. It follows then, that, as the primitive momentum of rotation with which the earth was projected into space must necessarily remain the same, the smallest decrease in heat, by contracting the terrestrial spheroid, would accelerate its rotation, and consequently diminish the length of the day. Notwithstanding the constant accession of heat from the sun's rays, geologists have been induced to believe, from the fossil remains, that the mean temperature of the globe is decreasing.
The high temperature of mines, hot springs, and, above all, the internal fires which have produced and do still occasion such devastation on our planet, indicate an augmentation of heat towards its centre; the increase of density, corresponding to the depth and the form of the spheroid, being what theory assigns to a fluid mass in rotation, concur to induce the idea that the temperature of the earth was originally so high as to reduce all the substances of which it is composed to a state of fusion, or of vapour, and that, in the course of ages, it has cooled down to its present state; that it is still becoming colder, and that it will continue to do so till the whole mass arrives at
the temperature of the medium in which it is placed, or rather at a state of equilibrium between this temperature, the cooling power of its own radiation, and the heating effect of the sun's rays.
Previous to the formation of ice at the poles, the ancient lands of our northern latitudes, long since obliterated, might, no doubt, have been capable of producing those tropical plants whose debris, swept into the deep at these remote periods, are preserved in the coal measures which must have been formed in the abysses of the ocean prior to the elevation of the modern continents and islands above its surface. But, even if the decreasing temperature of the earth be sufficient to produce the observed effects, it must be extremely slow in its operation; for, in consequence of the rotation of the earth being a measure of the periods of the celestial motions, it has been proved that, if the length of the day had decreased by the threethousandth part of a second since the observations of Hipparchus, two thousand years ago, it would have diminished the secular equation of the moon by 4" 4. It is therefore beyond a doubt that the mean temperature of the earth cannot have sensibly varied during that time; if, then, the appearances exhibited by the strata are really owing to a decrease of internal temperature, it either shows the immense periods requisite to produce geological
changes, to which two thousand years are as nothing, or that the mean temperature of the earth had arrived at a state of equilibrium before these observations.
However strong the indications of the primitive fluidity of the earth, as there is no direct proof of it, the hypothesis can only be regarded as very probable; but one of the most profound philosophers and elegant writers of modern times has found in the secular variation in the excentricity of the terrestrial orbit an evident cause of decreasing temperature. That accomplished author, in pointing out the mutual dependences of phenomena, says, 'It is evident that the mean temperature of the whole surface of the globe, in so far as it is maintained by the action of the sun at a higher degree than it would have were the sun extinguished, must depend on the mean quantity of the sun's rays which it receives, or—which comes to the same thing on the total quantity received in a given invariable time; and the length of the year being unchangeable in all the fluctuations of the planetary system, it follows that the total amount of solar radiation will determine, cæteris paribus, the general climate of the earth. Now, it is not difficult to show that this amount is inversely proportional to the minor axis of the ellipse described by the earth about the sun, re
garded as slowly variable; and that, therefore, the major axis remaining, as we know it to be, constant, and the orbit being actually in a state of approach to a circle, and consequently the minor axis being on the increase, the mean annual amount of solar radiation received by the whole earth must be actually on the decrease. We have therefore an evident real cause to account for the phenomenon.' The limits of the variation in the excentricity of the earth's orbit are unknown; but if its ellipticity has ever been as great as that of the orbit of Mercury or Pallas, the mean temperature of the earth must have been sensibly higher than it is at present; whether it was great enough to render our northern climates fit for the production of tropical plants, and for the residence of the elephant and other animals now inhabitants of the torrid zone, it is impossible to say.
The relative quantity of heat received by the earth at different moments during a single revolution varies with the position of the perigee, which accomplishes a tropical revolution in 21067 years. In the year 1245 of our era, and 19822 years before it, the perigee coincided with the winter solstice; at both these periods the earth was nearer the sun during the summer, and farther from him in the winter, than in any other position of the apsides; the extremes of temperature must