SOLAR SYSTEM. 69 



system during that period, and when we reflect that 

 a comet within a given distance of our planet is 

 less attracted towards the sun than towards the 

 earth, is it not so, that the impact of comets upon 

 our world must have been inevitable ? 



102. If the impact of comets upon our world 

 during its past history must have been inevitable, 

 and a comet which traverses the solar system is an 

 ocean isolated in space, is it so that our world has 

 derived its ocean from the visitation of comets ? 



103. But what are those comets, the discovery 

 of which was made by Encke and Biela? Are 

 they portions of the atmosphere of that planet 

 which by a " cosmical convulsion," is supposed to 

 have burst into fragments ; and is it so, that other 

 portions of that atmosphere and other fragments of 

 that planet are still in reserve for future discovery ? 



104. If a comet which, in its course of revolution 

 about the sun, traverses the solar system, becomes a 

 solid, a liquid, and a vapour, will more of the sun's 

 rays be intercepted by it when it is a solid or a 

 liquid, than when it is a vapour, and more when it 

 is a vapour than when, from its proximity to the 

 sun, it is resolved into steam and becomes invisible ? 

 and is it because the mass of a comet in its course of 

 revolution about the sun never varies, but the impul- 



