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move with it, will describe equal areas in equaltimes.

In order to illustrate this by a figure, letA D B C be an ellipsis, whose transverse, orlongest diameter, is A B, and its conjugate, orshortest diameter C D ; then the two foci F f,are points so situated, that if right lines bedrawn from them to any point G in the curve,the sum of those lines will always be equal to thetransverse diameter A B. And if the sun beplaced in the focus F, and a planet be supposedto revolve round him in the curve B C A D,it will move in such a manner, that a linedrawn from it to the sun, will describe equalareas in equal times. That is, if the line F G,drawn from the fun F to the planet G, describesthe area or space B F G in a month, it will de-scribe an area GFg equally large the nextmonth; and soon through its whole revolution.

The distance between the centre of the ellipseO, and one of its foci F, is called its eccentri-city ; and the two extreme points A and B ofthe transverse diameter, are called the apsides.If the focus about which the equal areas are de-scribed, be at F, the point A, nearest that focus,is called the lower apsis, the point B the upperapsis, and the diameter A B the line of the ap-sides. When a planet, in revolving round thefun, is at its nearest distance from him, as in A,

it