This second volume contains the first exchange of letters between Newton and Leibniz, which took place through the intermediacy of Oldenburg, as well as the beginning of Newton's correspondence of Flamsteed, which resulted from their common interest in the comet of 1680. Of prime interest is the correspondence with Halley, whose compelling zeal and energy played such a part in persuading Newton to write the Principia. This great work was published about midsummer 1687. As early as New Year 1684/5 it was known in some quarters that Newton was busying himself with applying his laws of motion to problems of celestial mechanics, for at that time Flamsteed wrote (Letter 275): 'if you will give me leave to guesse at your designe I beleive you are endeavoring to define ye curve yt ye comet in ye aether from your Theory of motion'.