82 ROUGH WAYS MADE SMOOTH. 



orb, assigns, so to speak, the duration of that orb's existence, 

 or rather of the various stages of that existence. The 

 smaller body must cool more rapidly than the larger, and 

 hence the various periods during which the former is fit for 

 this or that purpose of planetary life (I speak with purposed 

 vagueness here) are shorter than the corresponding periods 

 in the life of the latter. Thus the sun, viewed in this way, 

 is the youngest member of the solar system, while the tiniest 

 members of the asteroid family, if not the oldest in reality, 

 are the oldest to which the telescope has introduced us. 

 Jupiter and Saturn come next to the sun in youth ; they are 

 still passing through the earliest stages of planetary existence, 

 even if we ought not rather to adopt that theory of their 

 condition which regards them as subordinate suns, helping 

 the central sun to support life on the satellites which circle 

 around them. Uranus and Neptune are in a later stage, 

 and perchance when telescopes have been constructed large 

 enough to study these planets with advantage, we may learn 

 something of that stage, interesting as being intermediate to 

 the stages through which our earth and Venus on the one 

 hand, and the giant brothers Jupiter and Saturn on the 

 other, are at present passing. After our earth and Venus, 

 which are probably at about the same stage of planetary 

 development (though owing to the difference in their 

 position they may not be equally adapted for the support of 

 life), we come to Mars and Mercury, both of which must be 

 regarded as in all probability much more advanced and in a 

 sense more aged than the earth on which we live. In a 

 similar sense, even as an epherheron is more aged after a 

 few hours of existence than a man after as many years, the 

 small planet which we call * our moon ' may be described as 

 in the very decrepitude of planetary existence, nay (some 

 prefer to think), as even absolutely dead, though its lifeless 

 body still continues to advance upon its accustomed orbit, 

 and to obey the law of universal attraction. 



Considerations such as these give singular interest to the 

 discussion of the past history of our moon, though they add 



