546 ADDITIONS. 



in which this is described, did not appear till 1661, when it was pub- 

 lished by Hevelius of Dantzic. Some of his papers were destroyed by 

 the soldiers in the English civil wars ; and his remaining works were 

 finally published by Wallis, in 1673. The passage to which I here 

 specially wish to refer is contained in a letter to his astronomical ally, 

 William Crabtree, dated 1638. He appears to have been asked by 

 his friend to suggest some cause for the motion of the aphelion of a 

 planet ; and in reply, he uses an experimental illustration which was 

 afterwards employed by Hooke in 1666. A ball at the end of a string 

 is made to swino- so that it describes an oval. This contrivance Hooke 

 employed to show the way in w-hich an orbit results from the combi- 

 nation of a projectile motion with a central force. But the oval does 

 not keep its axis constantly in the same position. The apsides, as Hor- 

 rox remarked, move in the same direction as the pendulum, though 

 much slower. And it is true, that this experiment does illustrate, in a 

 general way, the cause of the motion of the aphelia of the Planetary 

 Orbits ; although the form of the orbit is different in the experiment 

 and in the solar system ; being an ellipse with the centre of force in 

 the centre of the ellipse, in the former case, and an ellipse with the 

 centre of force in the focus, in the latter case. These two forms of 

 orbits correspond to a central force varying directly as the distance, 

 and a central force varying inversely as the square of the distance ; as 

 Newton proved in the Principia. But the illustration appears to 

 show that Horrox pretty clearly saw how an orbit arose from a cen- 

 tral force. So far, and no farther, Newton's contemporaries could get ; 

 and then he had to help them onwards by showing what w r as the law 

 of the force, and what larger truths were now attainable. 



NewtorCs Discovery of Gravitation. 



[Page 402.] As I have already remarked, men have a willingness 

 to believe that great discoveries are governed by casual coincidences, 

 and accompanied by sudden revolutions of feeling. Newton had enter- 

 tained the thought of the moon being retained in her orbit by gravita- 

 tion as early as 1665 or 1666. He resumed the subject and worked 

 the thought out into a system in 1684 and 5. What induced him to 

 return to the question ? What led to his success on this last occasion ? 

 With what feelings was the success attended ? It is easy to make an 

 imaginary connection of facts. " His optical discoveries had recom- 

 mended him to the Pioyal Society, and he was now a member. He 



