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HISTORICAL VIEW OF THE

gravitation of the moon and earth, the moon must beacted on by the sun; and the same force which wassufficient to bend the earth’s course into an ellipse, couldnot but have a sensible effect on the orbit of the moon.Newton then proceeded to estimate the difference of theforces of the earth and sun, by which the moon’s mo-tion is affected. This investigation, and those connectedwith it (in a collective point of view, termed “ thelunar theory ”); the deviations from the regular law ofvelocity which belongs to an elliptic orbit ; the retro-gradation of the nodes of the moon’s orbit (or inter-section of its plane with that of the earth’s) ; thedecrease of the angular motion, and the moon’s gravity ;were the points which Newton successfully investigated.With respect to some others, many of the data were byno means perfectly well determined, on the one hand,and the methods of analysis had not yet the requisiteextension to fit them for such delicate enquiries, on theother. Newton’s labours, therefore, on these points,were unavoidably such as to leave much for his suc-cessors to do in bringing them into a more completeand exact form. Nevertheless, even with these neces-sary imperfections, his results must be regarded asastonishing proofs of that intuitive skill which, notwith-standing these adverse circumstances, enabled him toseize upon and preserve the clue which, through alldifficulties, brought him to some conclusion never veryfar from the truth.

After all that related to the motions of the earth, themoon, and the other planets bad been satisfactorily in-vestigated, there still remained one important and verystriking class of phenomena, the appearance and mo-tions of comets. We have before observed that theseapparently singular and anomalous bodies had beenalready elevated from the rank of meteors, engenderedin our atmosphere, in which the ancients had placedthem, to that of celestial hodies moving through theimmensity of space, by the observations of Galileo andCassini. Several conjectures had been broached as to