  The second edition appeared in 1713, twenty six years after the first. It had five substantive changes of note. First, the structure of the argument for universal gravity at the beginning of Book 3 was made more evident, and the word ‘hypothesis’ was dropped from it. Second, because of disappointment with pendulum-decay experiments and an erroneous claim about the rate a liquid flows vertically through a hole in the bottom of a container, the second half of Section 7 of Book 2 was entirely replaced, ending with new vertical-fall experiments to measure resistance forces versus velocity and a forcefully stated rejection of all vortex  theories.[17] Third, the treatment of the variation of surface gravity with latitude (Book 3, Proposition 20) was significantly extended, partly in response to Huygens's alternative treatment of this variation, but also because of more recent data from near the Equator. Fourth, the treatment of the wobble of the Earth producing the precession of the equinoxes was revised in order to accommodate a much reduced gravitational force of the Moon on the Earth than in the first edition. Fifth, several further examples of comets were added at the end of Book 3, taking advantage of Halley's efforts on the topic during the intervening years. In addition to these, two changes were made that were more polemical than substantive: Newton added the General Scholium following Book 3 in the second edition, and his editor Roger Cotes provided a long anti-Cartesian (and anti-Leibnizian) Preface.