CH. XVIII. LAW OF GRAVITATION EXPLAINED. 155 



6. He not only showed why the planets move 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, 

 thus showing that gravitation explains not only the general 

 laws but even apparent exceptions. 



7. Of all bodies comets are apparently the most irregu- 

 lar, yet Newton calculated that they probably move in a 

 peculiar curve called a parabola^ and since his time it has 

 been proved that the motions of all comets can be suffi- 

 ciently well explained by this theory, with the exception of 

 a few which move in ordinary ellipses like the planets, and 

 return periodically. These and many other problems of 

 the universe Newton showed could all be referred to the 

 action of gravitation ; and he concluded his work with a 

 grand description of the mechanism of the heavens, dwelling 

 with deep reverence upon the thought of that Infinite Mind 

 which gave rise to such a wonderful and complex machinery, 

 working in perfect order. 



Chief Works consulted, — Brewster's ' Life of Newton ; ' * Lives of 

 Eminent Persons ' — Lib. of Useful Knowledge ; Airy's ' Elementary 

 Astronomy;' Airy, • On Gravitation.' 



