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HISTORICAL VIEW OF THE

But how could this be connected with gravitation ?To this Newton replied, by not merely showing the way inwhich it was produced, but calculating its amount fromthe known laws of gravity, its agreement with whichthus furnished a fresh proof of their truth. If, arguedNewton, the earth were a sphere, it might he any howinclined, and no action of gravitation would alter its in-clination. If, however, it were spheroidal, and placed atrest with its equator inclined to the plane of the orbit,the attraction of the sun upon the protuberant equatorialpoints would bring it down into the plane of the orbit.But if it were in diurnal rotation, the effect of this at-traction would be insensibly small in actually dimin-ishing the obliquity, but would cause the point of theintersection to shift slowly in a direction opposite tothat of the rotation. # The mechanical reasoning is some-what of a refined character, and perhaps not easily ren-dered intelligible without going into greater detail thanwe have done. The attraction of the moon had to betaken into account, as well as that of the sun.

We have here, for the sake of illustration, spoken ofsimple elliptic orbits, and the observance of Kepler’s lawsas exactly true. But from the same principle of attrac-tion by which, in a general way, they would hold good,it follows, that, as all the different bodies of the systemwill exercise more or less of attraction one upon an-other, the exact fulfilment of these conditions must bein a certain degree disturbed. As, however, observationindicates that the concidence is very nearly exact, it fol-lows that the amount of these disturbances is very small.Newton recognised the existence of such disturbance;but it appears that, in regard to the planets, he conceivedit so small, that the consideration of it might be in generalsafely neglected, except, perhaps, in regard to Jupiterand Saturn, when near their conjunction.* With re-gard to the moon, he saw that the amount would bemuch greater. The observance, then, of Kepler’s lawswould give a certain general proof of the truth of the* Principia, iiu 5.