SEQUEL TO THE EPOCH OF KEPLER. 457 



impossible not to think of extending the elliptical 

 theory to the moon. Horrox succeeded in doing 

 this ; and in 1638 sent this essay to his friend Crab- 

 tree. It was published in 1673, with the numerical 

 elements requisite for its application added by 

 Flamsteed. Flamsteed had also (in 1671 and 2) 

 compared this theory with observation, and found 

 that it agreed far more nearly than the Philolaic 

 Tables of Bullialdus, or the Carolinian Tables of 

 Street (Epilogus ad Tabulas}. Moreover Horrox, 

 by making the center of the ellipse revolve in an 

 epicycle, gave an explanation of the evection, as 

 well as of the equation of the center (T). 



Modern astronomers, by calculating the effects 

 of the perturbing forces of the solar system, and 

 comparing their calculations with observation, have 

 added many new corrections or equations to those 

 known at the time of Horrox; and since the mo- 

 tions of the heavenly bodies were even then affected 

 by these variations as yet undetected, it is clear 

 ' that the tables of that time must have shown some 

 errours when compared with observation. These 

 errours much perplexed astronomers, and naturally 

 gave rise to the question whether the motions of 

 the heavenly bodies really were exactly regular, or 

 whether they were not affected by accidents as little 

 reducible to rule as wind and weather. Kepler had 

 held the opinion of the casualty of such errours ; 

 but Horrox, far more philosophically, argues against 

 this opinion, though he allows that he is much 



