Page:Popular Science Monthly Volume 13.djvu/597

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THE ASTRONOMICAL HISTORY OF WORLDS.

vations and depressions, but also from a knowledge of the insignificant alterations in the outline of continents during the last 3,000 years.

In the imaginary systems of celestial architecture which Aristotle and Ptolemy gave to mankind, there was a very narrow limit assigned to the extent of the heavens; the entire stellar host was supposed to be confined to a very scanty domain, and the human mind was prevented by these erroneous dogmas from rising to a knowledge of the magnitude or the riches of the universe. If science, in former ages, had been crippled by being restricted to too narrow a region of space, it cannot avoid suffering, at the present day, from being subjected to similar restrictions with respect to the range of time which its researches should embrace. On grounds as uncertain as those which sustained many of the exploded doctrines of antiquity, it has been too hastily concluded that the past career of the earth and the duration of solar light must have been comprised within the course of a few millions of years. It is even supposed that, within a like circumscribed period of change and activity, the myriads of solar systems in the wide domains of space around us came into existence from chaotic fire-mist which filled the entire universe. The transitory character which modern speculation would thus assign to many important cosmical arrangements will appear more surprising if contrasted with the long endurance of others, as revealed by the older and the ripe fruits of physical astronomy. From the planetary theories of Lagrange and Laplace it would appear that the future life of the solar family, if not absolutely eternal, must be many thousand or even a few million times as long as the period into which certain modern scientific writers are endeavoring to squeeze geological history. According to Proctor, 20,000,000,000,000 years must elapse before even Mercury can meet a natural death by incorporating with the sun; and this estimate, which I believe to be somewhat too high, is introduced here to give an idea of the opinions prevailing on the subject in astronomical circles. It may, however, be safely asserted that the future age of the earth cannot be much less than a million times as long as the period during which the sun's contraction could supply heat and light, at the equable rate which the purposes of life require; and there is no reason to admire a course of creation which makes worlds outlive so long their term of utility, and condemns solar systems in coming time to endure an interminable reign of darkness.

In proceeding to trace the course of the great cosmical events throughout the universe, it seems necessary to begin with a careful study of the permanent alteration in the movements of the earth and many of the celestial bodies. During the prevalence of the ancient doctrine of the immutability of the heavens, the occurrence of such unperiodical changes was denied. Even in modern times they were long ignored, as they proceed so slowly that it is only in a few cases that they can be revealed by the most refined methods of theory and observation. During the last century the doctrine of the uniformity