OUTLINES 



OF 



NATURAL PHILOSOPHY, 



ASTRONOMY. 

 PART II. 



PHYSICAL ASTRONOMY. 



SECTION L 



OF THE FORCES WHICH RETAIN THE PLANETS IN 

 THEIR ORBITS* 



36. IF a body gravitating to a fixed centre, 

 have a projectile motion impressed on it, in a 

 line not passing through that centre, it will 

 move in a curve ; and the straight line drawn 

 from the body to the centre, will describe areas 

 proportional to the times. 



Princip. Math. lib. i. prop. 1. 



. Conversely, 



