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dynamical theory then current. One of the chief 

 difficulties was that if a stone be thrown straight up 

 from the earth, it falls again to sensibly the same 

 place from which it started, whereas, according to 

 the dynamics of the time, the earth, if it had a 

 motion of revolution, should have moved onwards 

 during the time of flight, leaving the stone to fall 

 behind its original position. 



Copernicus insisted again on the astronomical 

 simplification to be gained by supposing the earth 

 to rotate, and on the farther simplification gained 

 by supposing it and the planets to revolve round 

 the sun. With fortunate audacity he disregarded 

 the dynamical difficulty, but it was not till the 

 publication in 1632 of Galileo's " Dialogues on the 

 System of the World " that this difficulty was finally 

 overcome by the introduction of a better dynamical 

 theory. Then, and not till then, did the Copernican 

 theory afford a mental picture of relationship among 

 a whole class of reminiscences, clearer, more direct, 

 and better adapted for complete comprehension 

 than any previous syntax, and accordingly it was 

 promptly accepted by all astronomers. The theory 

 of astronomy had been stationary since the time of 

 Hipparchus : the practice had progressed but little, 

 and very slowly, since the time of Ptolemy ; yet in 

 little more than half a century from the publication 

 of the simpler system of Copernicus, this had led 

 to the discoveries of Kepler, and these again in 



