p. 49 – in Iones –

In 1673 Dr Hook wrote to him to send him something new for the transactions whereupon he sent him a little dissertation to confute that common objection that if it were true that the earth moved from West to East all falling bodies would be left to the West, & maintained that on the contrary they would fall a little Eastward & having described a curve with his hand to represent <2> the motion of a falling body he drew a negligent stroke with his pen from whence Dr Hook took occasion to imagine that he meant the curve would be a spiral, whereupon the Doctor wrote to him that the curve would be an Ellipsis & that the body would move according to Kepler's notion which gave Sir I. N. an occasion to examine the thing thoroughly & for the foundation of the calculus he intended he <3> laid down this proposition that the areas described in equal times were equal which though assumed by Kepler was not by him demonstrated of which demonstration the first glory is due to Sir I.

Memorandum Pemberton in his preface which vide mentions this in another manner – He says particularly – Upon this principle he found the line described by a falling body to be an Elipsis the center of the earth being one focus <4> & the primary planets moving in such orbits round the sun he had the satisfaction to see that this enquiry which he had undertaken merely out of curiosity could be applied to the greatest purposes – Hereupon he composed near a dozen propositions relating to the motion of the primary planets about the sun, & from no other materials but those few propositions only he composed his principia in the space of one year & half —– Sir Isaac Newton says in one of his papers relating to Leibnitz that he wrote the Principia in 17 or 18 months beginning the end of December 1684 – & sending it to the Society in May – 1686 except about 10 or 12 of the propositions which were composed before Viz. two in 1679 – & the rest in Iune & Iuly – 1684 —

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Professor Rob Iliffe
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