SIR ISAAC NEWTON. 39 



nance mild, pleasant, and comely. I cannot say I 

 ever saw him laugh but once." 



In 1687. when Newton was forty-five, his Philo- 

 sophice Naturalis Principia Mathematica was pub- 

 lished. " The Principia consists of three books. 

 The First Book, besides the definition and axioms, 

 or laws of motion, with which it begins, consists of 

 fourteen sections, in the first of which the author 

 explains the method of prime and ultimate ratios 

 used in his investigations, and which is similar to 

 the method of fluxions. The other sections treat 

 of centripetal forces, and motions in fixed and 

 movable orbits. 



" The Second Book consists of nine sections, and 

 treats of bodies moving in resisting media, or 

 oscillating as pendulums. 



"The Third Book consists of five sections, on 

 the Causes of the System of the World, on the 

 Quantity of Lunar Errors, on the Quantity of 

 the Tides, on the Precession of the Equinoxes, 

 and on Comets." 



The great principle of the Principia is universal 

 gravitation, " That every particle of matter in the 

 universe is attracted by or gravitates to every 

 other particle of matter, with a force inversely pro- 

 portional to the squares of their distances." By 

 the laws of gravity, Newton was enabled to cal- 

 culate the quantity of matter in the sun, and in all 

 the planets, and even to determine their density, 

 results which Adam Smith said " were above the 

 reach of human reason and experience." He ascer- 



