Page:The Mathematical Principles of Natural Philosophy - 1729 - Volume 2.djvu/532

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1 [ § 3'] fugal, it will contribute to lengthen the f ace and time of the lib ration; by lengthening the ſpace, it increaſes ' the eccentricity; and by lengthening the time of the lib ration, it prmlracts the time of the revolution to the apſis, and cauſes what is improperly called a motion of the apſis forward. But when the line of apſides is moving to the quadratures, the external force in the apſides, is at that time centripetal 5 which will contribute to ſhorten the ſpace and time of lib ration; and by ſhortening the ſpace will thereby leffen the eccentricity, and by ſhortening the time of lib ration, will thereby contract the time of the revolution to the apſis; and cauſe what is im# properly called a retrograde motion of the apſis. I ſhall only add a few remarks, which ought to have been made in their proper places. A As to the motion of the Moon in the elliptic epicycle (page 9.) it ſhould have been mentioned, that there is no need of any accurate and perfect deſcription of the curve called an ellipſis, it being only rg ſhew the elongation of the Moon; from the center of the epicycle; which doth not require any ſuch accurate def@fiPIiQ11=

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