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corresponding perturbing forces, varying in the reverseway. At O 3 the force is wholly tangential, at M 4 it iswholly radial, and represented by the line M 4 E. At0 4 it is again wholly tangential; and, lastly, at M 4 , theforce is again wholly radial as at first, and representedby the line M, A.
It will be very obvious that, on the whole, the per-turbing action tends to diminish the earth’s influenceon the moon, since the forces acting outwards aregreater in amount, and act over larger arcs than thoseacting inwards. We see that M 4 A and M s A', themaximum outward forces, are twice as great as M 2 Eand M 4 E, the maximum inward forces; while the arcs0 4 0 4 and 0 2 0 3 each contain 109° 28', or in allnearly 219° out of 360°,—that is, more than three-fifths of the complete circumference. Hence we caninfer that there is a considerable balance of forceexerted outwards.
But it will be well to picture the radial forcesseparately.
Let M H, fig. 20, represent the disturbing forceon the moon at M, and draw E M B radially and MCtangentially. Then complete the rectangle B 0 bydrawing the perpendiculars H B and H C. By thewell-known rule for the resolution of forces, the forceM H is equivalent to the two forces represented byM B and M C, one radial, the other tangential. Sim 1-larly, if we had commenced by considering the forceM' H', fig. 21, exerted on the moon at M', we shouldhave found by a similar construction that the radial