mct. iv.] DISSERTATION SECOND. 121 



heavens becamein this way extremely complicated ; though, 

 when fairly examined, it will appear to be a work of great 

 ingenuity and research. The ancients, indeed, may be re. 

 garded as very fortunate in the contrivance of epicycles, 

 because, by means of them, every inequality which can 

 exist in the angular motion of a planet may be at least 

 nearly represented. This I call fortunate, because, at the 

 time when Apollonius introduced the epicycle, he had no 

 idea of the extent to which his contrivance would go, as he 

 could have none of the conclusions which the author of the 

 M&cunique Clleste was to deduce from the principle of 

 gravitation. 



The same contrivance had another great advantage ; it 

 subjected the motions of the sun, the moon, and the plan- 

 ets, very readily to a geometrical construction, or an arith- 

 metical calculation, neither of them difficult. By this means 

 the predictions of astronomical phenomena, the calculation 

 of tables, and the comparison of those tables with observa- 

 tion, became matters of great facility, on which facility, 

 in a great measure, the progress of the science depended. 

 It was on these circumstances, much more than on the sim- 

 plicity with which it amused or deceived the imagination, 

 that the popularity of this theory was founded ; the ascend- 

 ant which it gained over the minds of astronomers, and the 

 resistance which, in spite of facts and observations, it was 

 ao long able to make to the true system of the world. 



It does not appear that the ancient astronomers ever con- 

 sidered the epicycles and deferents which they employed 

 in their system as having a physical existence, or as serving 

 to explain the causes of the celestial motions. They seem 

 to have considered them merely as mathematical diagrams, 

 serving to express or to represent those motions as geo- 

 metrical expressions of certain general facts, which readily 

 furnished the rules of astronomical calculation. 



