144 THE THEORY OF GRAVITATION. 



edoou to the center of our globe is 60 times as great as our distance 

 from this same center, the spherical surface having the radius of the 

 moon's orbit is 3, GOO times as great as that of the earth. So that if 

 the outer surface were traversed by the same number of atoms as the 

 inner, their distribution would be 3,600 times rarer, and they would 

 in consequence cause a gravitation 3,600 times less. This would be 

 exactly that required by the theorems, 1 for this gravitating force would 

 suffice to sustain at a distance 60 times as great a moving body whose 

 absolute velocity was V60 times less than' that required by a body 

 revolving at the surface of the earth. 



V. 



The parallelism of path which Epicurus had introduced in the atomic 

 theory of Leucippus and Democritus was not exact, since had it been 

 so these atoms, all moving with equal velocity, could never have come in 

 collision. But Epicurus required that they should collide in order that 

 he might explain the formation of compound bodies without assuming 

 the intervention of a superior cause. Hence he supposed the paths of 

 the atoms to be slightly inclined to each other, and it is well known that 

 the introduction of the correction subjected him to many pleasantries 

 and objections from philosophers of other sects. 



VI. 



If, however, Epicurus had embraced the doctrine of the convergence 

 of the atoms toward a center, undoubtedly his opponents would have 

 attacked this hypothesis quite as vigorously. The Epicureans in reply- 

 ing would have been able 2 to explain this convergence by returning to 

 the system of Leucippus and Democritus as follows : Imagine the atoms 

 to move fortuitously in every direction, and let us trace the result in 



1 Combine the second and third theorems of Huygens published in 1673 following 

 his Horologium oscillatorium. 



2 It was natural enough to greatly diversify this motion which tended to deflect 

 the atoms. , 



Lucretius, even, despite his devotion to Epicurus, expressed himself several times 

 conformably to the system of Democritus. His first book with the lirst 216 lines of 

 the second ignored the imperfect parallelism that he lent to the paths of the atoms, 

 for instead of speaking of this parallelism he seems to say three times that they 

 come from all directions (undique, lines 986, 1041, and 1050), that they waver (voli- 

 tare, 951), trying several kinds of collisions (multimodis plagis, 1023 and 1024), essay- 

 ing all kinds of movements (orune genus rnotus, 1025), finding room to advance in 

 whatever direction they move (motus quacumque feruntur, 1075). He adds, in the 

 second book, that they wander in space (per Inane vagantur, line 82), that they are 

 agitated by various movements (varioque exercita motu, 96), and that all those 

 which have not been able to associate themselves together to form great masses are 

 always agitated in the great void (in inagno jactari semper Inani, 121) in the same 

 way as the dust that one sees in a dark chamber into which the sun's rays penetrate 

 is moved about in all directions (nunc hue nunc illuc, in cunctas denique parties, 

 130). Finally, several of his commentators convey the same idea. 



