Page:Newton's Principia (1846).djvu/188

 be as $$\scriptstyle \frac{1}{AA}$$; and the foreign force subducted as cA, and therefore the remaining force as $$\scriptstyle \frac{A-cA^{4}}{A^{3}}$$; then (by the third Example) b will be equal to 1. m equal to 1, and n equal to 4; and therefore the angle of revolution be tween the apsides is equal to 180 $$\scriptstyle \sqrt{\frac{1-c}{1-4c}}$$ deg. Suppose that foreign force to be 357.45 parts less than the other force with which the body revolves in the ellipsis; that is, c to be $$\scriptstyle \frac{100}{35745}$$; A or T being equal to 1; and then 180 $$\scriptstyle \sqrt{\frac{1-c}{1-4c}}$$ will be 180$$\scriptstyle \sqrt{\frac{35645}{35345}}$$ or 180.7623, that is, 180 deg., 45 min., 44 sec. Therefore the body, parting from the upper apsis, will arrive at the lower apsis with an angular motion of 180 deg., 45 min., 44 sec, and this angular motion being repeated, will return to the upper apsis; and therefore the upper apsis in each revolution will go forward 1 deg., 31 min., 28 sec. The apsis of the moon is about twice as swift.

So much for the motion of bodies in orbits whose planes pass through the centre of force. It now remains to determine those motions in eccentrical planes. For those authors who treat of the motion of heavy bodies used to consider the ascent and descent of such bodies, not only in a perpendicular direction, but at all degrees of obliquity upon any given planes; and for the same reason we are to consider in this place the motions of bodies tending to centres by means of any forces whatsoever, when those bodies move in eccentrical planes. These planes are supposed to be perfectly smooth and polished, so as not to retard the motion of the bodies in the least. Moreover, in these demonstrations, instead of the planes upon which those bodies roll or slide, and which are therefore tangent planes to the bodies, I shall use planes parallel to them, in which the centres of the bodies move, and by that motion describe orbits. And by the same method I afterwards determine the motions of bodies performed in curve superficies. 




 * Any kind of centripetal force being supposed, and the centre of force, and any plane whatsoever in which the body revolves, being given, and the quadratures of curvilinear figures being allowed; it is required to determine the motion of a body going off from a given place, with a given velocity, in the direction of a given right line in that plane.

