10-i HISTORY OF PHYSICAL ASTRONOMY. 



was led to investigate the subject, and was then again conducted, by 

 another road, to the same law of the inverse square of the distance. 

 This naturally turned his thoughts to his former speculations. Was 

 there really no way of explaining the discrepancy which this law gave, 

 when he attempted to reduce the moon's motion to the action of 

 gravity ? A scientific operation then recently completed, gave the ex- 

 planation at once. He had been mistaken in the magnitude of the 

 earth, and consequently in the distance of the moon, which is deter- 

 mined by measurements of which the earth's radius is the base. lie 

 had taken the common estimate, current among geographers and sea- 

 men, that sixty English miles are contained in one degree of latitude. 

 But Picard, in 1670, had measured the length of a certain portion of 

 the meridian in France, with far greater accuracy than had yet been 

 attained ; and this measure enabled Newton to repeat his calculations 

 with these amended data. We may imagine the strong curiosity 

 which he must have felt as to the result of these calculations. His 

 former conjecture was now found to agree with the phenomena to a 

 remarkable degree of precision. This conclusion, thus coming after 

 long doubts and delays, and falling in with the other results of me- 

 chanical calculation for the solar system, gave a stamp from that 

 moment to his opinions, aud through him to those of the whole philo- 

 sophical world. 



[2d Ed.] [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 1G66. 

 And M. Biot, in his Life of Newton, published in the Biographic Uni- 

 verselle, 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, 1838), that Picard'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 



