CH. xviii. LAW -OF GRAVITATION EXPLAINED. 151 



received, and his name became one of the most renowned 

 and honoured in the world, yet it was more than fifty years 

 before his work was thoroughly appreciated. 



It may therefore easily be imagined that it is not possible 

 to give a simple sketch of what is contained in the 'Principia;' 

 but some idea may perhaps be formed of the grandeur of 

 the law of gravitation from an enumeration of some of the 

 problems which Newton explained by its action. 



1. He explained those laws of motion which Galileo had 

 proved by experiment, and showed that it is the force of 

 gravity which causes the weight of bodies ; and determines, 

 when combined with other laws, the rate at which they fall, 

 and the path they describe. 



2. He worked out the specific gravity of the planets, 

 showing, for example, that the matter of which Saturn is 

 composed is about nine times lighter than the matter of our 

 earth. 



3. He showed how the attractions of the sun and of the 

 moon cause the tides of the sea, and worked out accurately 

 the reason of the spring and neap tides. 



4. He proved that the earth could not be a perfect globe, 

 and measured almost exactly how great the bulge at the 

 equator and the flattening at the poles must be. And this 

 he did entirely by calculation, for no measurements had then 

 been made, to lead any one to doubt that the earth was a 

 perfect globe. 



5. He gave a complete explanation of the cause of the 

 1 precession of the equinoxes,' the occurrence of which, as 

 you will remember, Hipparchus had discovered (see p. 30). 



6. He not only showed why the planets moved in ellipses 

 while a line joining the sun and a planet cuts off equal areas 

 in equal times ; but he also accounted for many irregularities 

 in these movements, arising from their mutual attractions, 



