458 HISTORY OF FORMAL ASTRONOMY. 



embarrassed by the deviations. His arguments 

 show a singularly clear and strong apprehension of 

 the features of the case, and their real import. He 

 says 3 , " these errours of the tables are alternately in 

 excess and defect; how could this constant com- 

 pensation happen if they were casual ? Moreover, 

 the alternation from excesg to defect is most rapid 

 in the moon, most slow in Jupiter and Saturn, in 

 which planets the errour continues sometimes for 

 years. If the errours were casual, why should they 

 not last as long in the moon as in Saturn ? But if 

 we suppose the tables to be right in the mean 

 motions, but wrong in the equations, these facts are 

 just what must happen ; since Saturn's inequalities 

 are of long period, while those of the moon are 

 numerous, and rapidly changing." It would be 

 impossible, at the present moment, to reason better 

 on this subject; and the doctrine, that all the appa- 

 rent irregularities of the celestial motions are really 

 regular, was one of great consequence to establish 

 at this period of the science. 



Sect. 3. Causes of the further Progress of 

 Astronomy. 



WE are now arrived at the time when theory and 

 observation sprang forwards with emulous energy. 

 The physical theories of Kepler, and the reasonings 

 of other defenders of the Copernican theory, led 



3 Astron. Kepler. Proleg. p. 17- 



