126 DISSERTATION SECOND. [rknx i 



It appears extraordinary, that so natural a thought should 

 have occurred, at so late a period, for the first, or nearly 

 for the first time. We are assured, by Copernicus him- 

 self, that one of the first considerations which offered. itself 

 to his mind, was the effect produced by the motion of a 

 spectator, in transferring that motion to the objects ob- 

 served, but ascribing to it an opposite direction. ' From 

 this principle it immediately followed, that the rotation of 

 the eaifh on an axis, from west to east, would produce the 

 apparent motion of the heavens in the direction from east 

 to west. 



In considering some of the objections which might be 

 made to the system of the earth's motion, Copernicus rea- 

 sons with great soundness, though he is not aware of the 

 full force of his own argument. Plolemy had alleged, that, 

 if the earth were to revolve on its axis, the violence of the 

 motion would be sufficient to tear it in pieces, and to dis- 

 sipate the parts. This argument, it is evident, proceeds 

 on a confused notion of a centrifugal force, the effect of 

 which the Egyptian astronomer overrated, as much as he 

 undervalued the firmness and solidity of the earth. Why, 

 says Copernicus, was he not more alarmed for the safety 

 of the heavens, if the diurnal revolution be ascribed to 

 them, as their motion must be more rapid, in proportion as 

 their magnitude is greater ? The argument here suggested, 

 now that we know how to measure centrifugal force, and to 

 compare it with others, carries demonstrative evidence 

 with it, because that force, if the diurnal revolution were 

 really performed by the heavens, would be such, as the 

 forces which hold together the frame of the material world 

 would be wholly unable to resist. 



1 Astronomia Inataurata, Lib. 1. cap. 5. 



