AN ESSAY ON THE MATHEMATICAL PRINCIPLES OF PHYSICS. 341 



the highest scientific value. Of all such actions, that of gravitation is the most 

 universal and the most mysterious. Whatever theory of the constitution of 

 bodies holds out a prospect of the ultimate explanation of the process by which 

 gravitation is effected, men of science will be found ready to devote the whole 

 remainder of their lives to the development of that theory. 



The only theory hitherto put forth as a dynamical theory of gravitation 

 is that of Lesage, who adopts the Lucretian theory of atoms and void. 



Gravitation on this theory is accounted for by the impact of atoms of 

 incalculable minuteness, which are flying through the heavens with inconceivable 

 velocity and in every possible direction. These " ultramundane corpuscules " 

 falling on a solitary heavenly body would strike it on every side with equal 

 impetus, and would have no effect upon it in the way of resultant force. If, 

 however, another heavenly body were in existence, each would screen the other 

 from a portion of the corpuscular bombardment, and the two bodies would be 

 attracted to each other. The merits and the defects of this theory have been 

 recently pointed out by Sir W. Thomson. If the corpuscules are perfectly elastic 

 one body cannot protect the other from the storm, for it will reflect exactly 

 as many corpuscules as it intercepts. If they are inelastic, as Lesage supposes, 

 what becomes of them after collision ? Why are not bodies always growing by 

 the perpetual accumulation of them ? How do they get swept away ? and what 

 becomes of their energy ? Why do they not volatilise the earth in a few 

 minutes ? I shall not enter on Sir W. Thomson's improvement of this theory, 

 as it involves a different kind of hydrodynamics from that cultivated in the 

 Essay, but in whatever way we regard Lesage's theory, the cause of gravitation 

 in the universe can be represented only as depending on an ever fresh supply 

 of something from without. 



Though Professor Challis has not, as far as we can see, stated in what 

 manner his sethereal waves are originally produced, it would seem that on his 

 theory also the primary waves, by whose action the waves diverging from the 

 atoms are generated, must themselves be propagated from somewhere outside the 

 world of stars. 



On either theory, therefore, the universe is not even temporarily automatic, 

 but must be fed from moment to moment by an agency external to itself. 



If the corpuscules of the one theory, or the aethereal waves of the other, 

 were from any cause to be supplied at a different rate, the value of every 

 force in the universe would suffer change. 



