THE ASTRONOMICAL HISTORY OF WORLDS. 579 



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 magni- 

 tude 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 incor- 

 porating 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 



