94 NOMOS. 



CHAP. II. 



SEARCH AFTER A CENTRAL LAW IN SOME OF 

 THE PHENOMENA OF NATURAL MOTION. 



THE philosophical interpretation of the movements 

 The laws of of the heavenly bodies began unquestion- 

 Kepier. a ^y w hen Kepler made the three great 



discoveries which are known as Kepler's laws. 



The first of these discoveries is that the radius 

 vector (or line which connects the planet or comet 

 with the sun, and the satellite with its primary,) is 

 carried over equal areas in equal times. It might be 

 expected that this radius would have been carried over 

 unequal areas in equal times, for the orbital velo- 

 city of the heavenly bodies is subject to considerable 

 variation ; but Kepler found that the radius increases 

 in length as the velocity diminishes, and vice versa, 

 and that this counterbalancing between distance and 

 velocity is such that the radius is always carried over 

 the same area in the same time. The second of 

 these discoveries is, that the planets and comets 

 move in ellipses with the sun in one of the foci, and 

 that the satellites describe a similar movement 

 around their primaries. The third discovery, which 

 is often spoken of as the harmonic law, is that the 

 squares of the periodic times of the revolution 



