The present paper contains a full transcription, with commentary and introduction, of two hitherto unknown manuscripts by Leibniz on Newton's Principia mathematica. Both manuscripts were probably written in Rome in 1689. Leibniz's interest focused in particular on Newton's concept of vanishing quantities and last ratios, on the notion of force and on the cause of gravity. An edition of further unknown manuscripts by Leibniz on the Principia and on planetary motion is in progress and will appear in the sequel.