748 THE PRINCIPLES OF SCIENCE. [CHAP. 



It is far from impossible, again, that the planetary 

 system may be invaded by bodies of greater mass than 

 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 

 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 surround- 

 ing parts of space are occupied by multitudes of dark 

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

 may 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 in our own globe 

 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 a wandering 

 star; 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 

 explosion 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 some indications, as already noticed (p. 660), 

 that violent disturbances have actually occurred in the 

 history of the solar system. Olbers sought for the minor 

 planets on the supposition that they were fragments of an 

 exploded planet, and he was rewarded with the discovery 

 of some of them. The retrograde motion of the satellites 

 of the more distant planets, the abnormal position of the 

 poles of Uranus and the excessive distance of Neptune, are 

 other indications of some violent event, of which we have 



