297 



NOTES TO BOOK VII. 



(F.) p. 169. DR.ROBISON (Mechanical Philosophy, p. 288,) 

 says that Newton having become a member of the Royal 

 Society, there learned the accurate measurement of the 

 earth by Picard, differing very much from the estimation 

 by which he had made his calculations in 1666. And M. 

 Biot, in his life of Newton, published in the Biographie 

 Universelle, says, "According to conjecture, about the month 

 of June, 1682, Newton being in London, at a meeting of 

 the Royal Society, mention was made of the new measure of 

 a degree of the earth's surface, recently executed in France 

 by Picard ; and great praise was given to the care which 

 had been employed in making this measure exact." 



I had adopted this conjecture as a fact in my first 

 edition ; but it has been pointed out by Prof. Rigaud 

 (Historical Essay on the First Publication of the Principia, 

 1 838,) thatPicard's measurement was probably well known 

 to the Fellows of the Royal Society as early as 1675, 

 there being an account of the results of it given in the 

 Philosophical Transactions for that year. Newton appears 

 to have discovered the method of determining that a body 

 might describe an ellipse when acted upon by a force 

 residing in the focus, and varying inversely as the square 

 of the distance, in 1679, upon occasion of his correspond- 

 ence with Hooke. In 1684, at Halley's request, he re- 

 turned to the subject, and in February, 1685, there was 

 inserted in the Register of the Royal Society a paper 



