448 THE WORLD MACHINE 



to the law of inverse squares. 1 It may be remarked that such 

 an assumption is required by the electro-dynamical theory of 

 gravitation ; but there is not the slightest evidence as yet that 

 such a diminution takes place. If it does not, it would follow 

 of course from an infinite universe that we should have stars 

 and other bodies moving at infinite speeds. 



We are undoubtedly obliged to make option between the two 

 possibilities, and to the writer it has always seemed that the idea 

 of a finite universe was preferable. Of course, either finite or 

 infinite is in reality unthinkable ; we can conceive the one as 

 little as the other. But since the human mind is so constituted 

 that it can frame no working conceptions of the universe which 

 are not mechanical conceptions, it is a part of the economy of 

 thought to believe that, for all our means of observation will 

 ever tell us, the mass and number of the stars is a finite quantity. 



If it be finite, it is possible that some other force, or forces, 

 exist to determine stellar motions than those of gravitation. 

 But the necessity for any such assumption may be dissipated 

 with the increase of our knowledge. There is distinctly such 

 a possibility in the probable number of dark bodies. Their 

 number and mass might be sufficient to produce any stellar 

 speeds with which we are at present acquainted. 



Even conceding this, if gravitation be the only effectual 

 inter-stellar force, we should probably have to give up the 

 notion that there exists any stable arrangement of stellar 

 systems. We should probably have to picture the starry 

 scheme as we have done, in something after the same fashion 

 as the kinetic theory of gases. The stars would simply be 

 flying in every direction without any conceivable order whatever. 



This is a part of the riddle. There still remains the larger 

 one of the nature of gravitation itself. 



When Newton revealed the presence of an attracting force 

 between all masses of matter, it was objected by his contem- 

 poraries that such a force was unthinkable. It remains so still. 

 Newton's theory (not Newton himself) conceived space as to all 

 intents and purposes empty. But so constituted is the human 

 mind that while we may picture that is to say, understand one 

 body imparting motion to another by impact, we cannot picture 

 an attraction or pull without an intervening medium. 



1 Sitzungsberichte d. Munchener Akad., 1896. 



