"«r- »v.) DISSERTATION SECOND. 151 



epicycle in the circumference of an eccentrick deferent, 

 both angular motions being uniform, lhat of Ihe planet in 

 the epicycle being retrograde, and double the other. The 

 figure thus described may be shown to be an ellipse, but 

 the line drawn from the planet to the focus does not cut oft* 

 areas proportional to the time. 



An hypothesis advanced by Ward, Bishop of Salisbury, 

 was simpler and more accurate than that of the French 

 astronomer. According to it, the line drawn from a planet 

 to the superiour focus of its elliptick orbit, turns with a 

 uniform angular velocity round that point. In orbits of 

 small eccentricity, this is nearly true, and almost coincides 

 in such cases with Kepler's principle of the uniform de- 

 scription of areas. Dr. Ward, however, did not consider 

 the matter in that light ; he assumed his hypothesis as true, 

 guided, it would seem, by nothing but the opinion, that a 

 centre of uniform motion must somewhere exist, and pleased 

 with the simplicity thus introduced into astronomical calcu- 

 lation. It is, indeed, remarkable, as Montucla has observ- 

 ed, how little the most enlightened astronomers of that time 

 seem to have studied or understood the laws discovered by 

 Kepler. Riccioli, of whom we are just about to speak, 

 enumerates all the suppositions that had been laid down con- 

 cerning the velocities of the planets, but makes no mention 

 of their describing equal areas in equal times round the sun. 

 Even Cassini, great as he was in astronomy, cannot be 

 entirely exempted from this censure. 



Riccioli, a good observer, and a learned and diligent 

 compiler, has collected all that was known in astronomy 

 about the middle of the seventeenth century, in a volumi- 

 nous work, the New Almagest. Without much originality, 

 he was a very useful author, having had, as the historian of 

 astronomy remarks, the courage and the industry to read, 

 to know, and to abridge every thing. He was, neverthe- 



