42 CLASSICS OF MODERN SCIENCE 



ess would occur in the same way, if, without disturbing so vast a part 



of the universe, the earth alone should be revolved. 



The difficulty is however doubled since a second very important one 

 is added. That is, if one attributes to the firmament this mighty mo- 

 tion, one must regard it as necessarily opposed to the particular move- 

 ments of all the planets, all of which indisputably have their own 

 movements from west to east, and in comparison very moderate move- 

 ments at that. One is then forced to the conclusion that they depart 

 from that rapid daily motion, namely from east to west, to go in the 

 opposite direction. But, if we suppose that the earth moves, the op- 

 position of motions disappears and the single movement from west 

 to east fits in with all the facts and explains them most satisfactorily. 



SiMPL. : As far as this opposition of motions is concerned that has 

 little importance, since Aristotle proves that the circular motions are 

 not opposed to one another and that the apparent opposition cannot 

 actually be called so. 



Salv. : Does Aristotle prove that or merely suppose it, because 

 it aids him for a certain purpose? If, according to his own declara- 

 tion, those things are opposed which mutually destroy one another I 

 do not see how two moving bodies which meet one another in a 

 circular motion should do one another less harm than if they meet on 

 a straight line. 



Sagr. : Wait a moment, I pray. Tell me, Signore Simplicio, if 

 two knights run into one another with leveled lances on the open 

 field, if two squadrons or two streams on their way to the sea break 

 through and unite with one another, would you call such collisions 

 opposed movements? 



SiMPL. : Of course we would call them opposed. 



Sagr. : How then is there no opposition in circular motions ? For 

 the movements mentioned take place upon the surface of the earth or 

 water, both of which are recognized to be circular in form and so the 

 motions must be circular. Do you understand, Signore Simplicio, 

 what circular motions are not opposed to one another? Two circles 

 which touch each other on the outside and of which the revolution 

 of one is in a reverse direction from that of the other. If, however, 

 one circle is within the other, then motions in different directions must 

 be opposed to one another. 



Salv.: Whether opposed or not opposed is merely a strife of 



