448 SPECIAL RESULTS IN THE UftANOLOGICAL 



relations to each other; of changes of form, as in tailed 

 comets ; and changes of light, amounting even to new appa- 

 rition and entire extinction of light in distant suns. The 

 quantity of existing matter in the Universe remains, it is 

 believed, always the same ; but, according to what has been 

 already investigated of the physical laws of nature in the tel- 

 luric sphere, we there see ever recurring, as if ever unsatisfied, 

 change ceaselessly prevailing in countless and indescribable 

 combinations, in the perpetual circle of the permutation of 

 substances. This manifestation of force or power in matter 

 is called forth by its, at least apparent, elementary hetero- 

 geneity. Exciting motion in portions of space 'immeasura- 

 bly small, the heterogeneity of substances complicates all 

 problems relating to terrestrial processes of nature. 



Astronomical problems are more simple in their character. 

 Celestial mechanics, as yet free from the complications 

 alluded to, and directed to considerations relative to the 

 quantity of ponderable matter, i. e. to mass, and to light- and 

 heat-exciting undulations, have, by reason of this simplicity, 

 in which everything can be reduced to motion, remained 

 amenable throughout to mathematical treatment. It is this 

 advantage which gives to treatises on theoretical astronomy 

 a great and peculiar charm. There is reflected in them 

 what the mental labour of the last few centuries has achieved 

 by analytical methods : we see in them how forms and 

 orbits have been determined; how, in the phenomena of 

 the motions of the planets, small fluctuations take place 

 round a mean state of equilibrium ; and how the preserva- 

 tion and permanence of the planetary system are provided 

 for by its internal structure, and by the equilibrium of mu- 

 tually compensating perturbations. 



