152 GALILEO AND HIS JUDGES. 



What I have just stated shows that Kepler's first 

 law corresponds with Newton's discovery ; but the 

 same is true of the two other laws. It would of 

 course be out of place here to go minutely into 

 all the evidence which can be gathered in support of 

 the doctrine of universal gravitation. I may briefly 

 state that all of Kepler's laws are simply explicable 

 by that hypothesis, and that the evidence derives 

 additional confirmation from the following curious 

 fact : observation shows that Kepler's laws, though 

 approximately true, are not strictly and accurately 

 so ; if the planets were mere particles revolving round 

 the Sun, they would then be quite rigidly true, but 

 the planets have a certain mass (though very small 

 compared to the Sun) and so do in some measure 

 attract the Sun as well as being attracted by him, 

 and they, moreover, exercise a disturbing in- 

 fluence on each other. These perturbations, how- 

 ever, have been calculated, and the result is that 

 they agree with what ought reasonably to be ex- 

 pected, supposing the theory of universal gravitation 

 to be true. This confirmatory proof has been ac- 

 quired, I need not add, since the time of Newton 

 by the labours of astronomers, Laplace and others, 

 who have succeeded him, and who have had the 

 advantage of that more manageable method of 

 mathematical calculation to which I have just 

 alluded. 



Supposing then the law of gravitation to be estab- 

 lished by sufficient proof, we may now ask what 



