RESULTS AXD LIMITS OF SCIENTIFIC METHOD. 443 



It is far from impossible, again, that the planetary 

 system may be invaded by bodies of greater mass than 

 any comets. The sun seems to be placed in so extensive a 

 portion of empty space, that its own proper motion would 

 not bring it to the nearest known star (a Centauri) in less 

 than 139,200 years. But in order to be sure that this 

 long interval of undisturbed life is granted to our globe, 

 we must prove that there are no stars moving so as to 

 meet us, and no dark bodies of considerable size flying 

 through intervening space unknown to us. The intrusion 

 of comets into our system, and the fact that many of them 

 have hyperbolic paths, is sufficient to show that the sur- 

 rounding parts of space are occupied by multitudes of 

 dark bodies of some size. It is quite probable that small 

 suns might have cooled sufficiently to become non- 

 luminous ; for even if we discredit the theory that the 

 variation of brightness of periodic stars is due to the 

 revolution of dark companion stars, yet there is our own 

 globe as an unquestionable example of a smaller body 

 which has cooled below the luminous point. 



Altogether, then, it is a mere assumption that the 

 Uniformity of Nature involves the unaltered existence 

 of our own globe. There is no kind of catastrophe which 

 is too great or too sudden to be theoretically consistent 

 with the reign of law. For all that our science can tell, 

 human history may be closed in the next instant of time. 

 The world may be dashed, to pieces against some intruding 

 body ; it may be involved in a nebulous atmosphere of 

 hydrogen to be exploded a second afterwards ; it may be 

 scorched up or dissipated into vapour by some great ex- 

 plosion in the sun ; there might even be within the globe 

 itself some secret cause of disruption, which only needs 

 -time for its manifestation. 



There are even some indications, as alreadv noticed 



V 



(vol. ii. p. 327), that some violent disturbances have 



