174 ELEMENTS OF SCIENCE 



The grand result of this energy is that the planets of 

 our system, and doubtless those of other systems, revolve 

 round suns or central bodies, in ellipses, variously 

 attended by satellites, which in turn revolve around 

 their respective planets. In some distant systems there 

 may be more than one sun. Thus for our purpose we may 

 consider the universe as divided into two parts : (i) The 

 sun with its attendant bodies, i.e., the solar system ; and 

 (2) The rest of the universe which can by any means be 

 made visible to us nebulae, and all the bodies called 

 " fixed stars " because they have for us no obvious move- 

 ment. We class the latter, for convenient description, 

 in groups termed constellations, such as that of the 

 Great Bear (familiarly known as Charles's Wain) and 

 others. Such groups, however, have no natural con- 

 nection but are only associated together on account of 

 their conspicuousness and apparent proximity. 



Our own solar system is rushing at the rate of ten 

 thousand miles every half-hour in the direction of the 

 constellation, known as Lyra, and no doubt all the 

 other suns or fixed stars are similarly in motion, 

 although their great distances make such movements 

 inappreciable. The known universe, or cosmos, is made 

 up of bodies variously composed of solid, - liquid, or 

 gaseous matter, and these bodies differ greatly in density, 

 some, as before mentioned, being but masses of vapour, 

 the " nebulae." 



The cosmical bodies shine either by self-emitted light 

 (as does our sun and the variously distant stars) or by 

 reflected light, as do the planets and satellites of our 

 solar system and, probably, multitudes of planets of 

 other systems, though some planetary bodies themselves 

 may be very faintly self-luminous. 



All our heat is derived from the sun, and also almost 



