ARISTOTELIAN PHYSICS. 53 



characters of such motions in verse. The rule of 

 natural motion was 13 



Principium tepeat, medium cum fine calebit. 

 Cool at the first, it warm and warmer glows. 



And of violent motion, the law was 



Principium fervet, medium calet, ultima friget. 

 Hot at the first, then barely warm, then cold. 



It appears to have been considered by Aristotle 

 a difficult problem to explain why a stone thrown 

 from the hand continues to move for some time, and 

 then stops. If the hand was the cause of the mo- 

 tion, how could the stone move at all when left 

 to itself? if not, why does it ever stop? And he 

 answers this difficulty by saying 14 , "that there is a 

 motion communicated to the air, the successive parts 

 of which urge the stone onwards; and that each 

 part of this medium continues to act for some while 

 after it has been acted on, and the motion ceases 

 when it comes to a particle which cannot act after 

 it has ceased to be acted on." It will be readily 

 seen that the whole of this difficulty, concerning a 

 body which moves forwards and is retarded till it 

 stops, arises from ascribing the retardation, not to 

 the real cause, the surrounding resistances, but to 

 the body itself. 



One of the doctrines which was the subject of 

 the warmest discussion between the defenders and 

 opposers of Aristotle, at the revival of physical 



13 Alsted. Encyc. torn i. p. 687- u Phys. Ausc. viii. 10. 



