PHYSICAL ASTRONOMY. O-17 



there learned the accurate measurement of the Earth by I'icard, diil'er- 

 iiiLC very much from the estimation by which he had made his calcula- 

 tion in 1606 ; and he thought his conjecture now more likely to be 

 just," 2 M. Biot gives his assent to this guess. 3 The English transla- 

 tion of M. Biot's biography 4 converts the guess into an assertion. But, 

 says Professor Rigaud, 5 Picard's measurement of the Earth wa- 

 .known to the Fellows of the Royal Society as early as 1G75, there be- 

 ing an account of the results of it given in the Philosophical Ti-<ut*- 

 actions for that year. Moreover, Norwood, in his Seaman s Practice, 

 dated 1636, had given a much more exact measure than Newton em- 

 ployed in 1666. .But Norwood, says Voltaire, had been buried in ob- 

 livion by the civil wars. No, again says the exact and truth-] ovino; 

 Professor Rigaud, Norwood was in communication with the Royal 

 Society in 1667 and 1668. So these guesses at the accident which 

 made the apple of 1665 germinate in 1684, are to be carefully distin- 

 guished from history. 



But with what feelings did Newton attain to his success ? Hoiv 

 again we have, I fear, nothing better than conjecture. "He went 

 home, took out his old papers, and resumed his calculations. As they 

 drew near to a close, he was so much agitated that he was obliged to 

 desire a friend to finish them. His former conjecture was now found 

 to agree with the phenomena with the utmost precision." 5 This con- 

 jectural story has been called " a tradition ;" but he who relates it 

 does not call it so. Every one must decide, says Professor Rigaud, 

 from his view of Newton's character, how far he thinks it consistent 

 with this statement, Is it likely that Newton, so calm and so indiffer- 

 ent to fame as he generally showed himself, should be thus agitated 

 on such an occasion ? " No," says Sir David Brewster ;' ' ; it is not sup- 

 ported by what we know of Newton's character." 7 . To this we may 

 assent ; and this conjectural incident we must therefore, I conceive, 

 separate from history. I had incautiously admitted it into the text of 

 the first Edition. 



Newton appears to have discovered the method of demonstrating 

 that a body might describe an ellipse when acted upon by a force re- 

 siding in the focus, and varying inversely as the square of the distance, 

 in 1669, upon occasion of his correspondence with Hooke. In 1681, 



2 Eobison's Mechanical Philosophy, vol. iii. p. 94. (Art. 195.) 



3 Biograptye UnicerseUe. * Library of Useful Knowlcljf. 

 s Historical Essay on the First Publication of the Principia (183S). 



Robison, ibid. 7 Life tf foicton, vol. i. p. 292. 



