THE SOLAR SYSTEM. 81 



have yet investigated them, there does not prevail the same 

 planetary dependence in respect to relative motion and illu- 

 mination as that which characterizes our solar system : two 

 or more self-luminous heavenly bodies (whose planets and 

 their satellites, if they exist, escape the power of our tele- 

 scopes), do, indeed, revolve around a common centre of 

 gravity ; but this centre of gravity falls in a space occupied, 

 possibly, only by unagglomerated matter, *. e., cosmical 

 vapour ; whilst, in our system, the centre of gravity is in- 

 cluded within the surface of a visible central body. If 

 we choose to consider the sun and the earth, or the earth 

 ar?d the moon, as double stars, and our whole planetary 

 system as a multiple star, or group of stars, yet the 

 analogy suggested by such denominations fails altogether 

 when we regard illumination, and can only apply to motion 

 in conformity with the laws of gravitation. 



In such a generalisation of cosmical views as accords with 

 the plan of the present work, namely, with the sketch of a 

 picture of nature or of the universe, the solar system to 

 which our earth belongs may most properly be considered 

 under a two-fold aspect, first, in reference to the different 

 classes of the individual bodies which it contains, their mag- 

 nitudes, forms, densities, and disi-ances apart ; and, second, 

 in its relation to the other parts of the starry cluster of which 

 it forms a portion, and to its motion, or change of place, 

 within the same. 



The solar system (i. e., those various material bodies 

 which revolve round the sun) consists, according to our 

 present knowledge, of eleven principal planets, eighteen 

 moons or satellites, and myriads of comets, three of which 

 (called planetary comets) do not pass beyond the orbits of the 



