ARISTOTELIAN PHYSICS. 71 



ground diminishes and finally ceases ; the motion of a body which 

 falls from a height goes on becoming quicker and quicker ; this was 

 accounted for on the usual principle of opposition, by saying that the 

 former is a violent, the latter a natural motion. And the later writers 

 of this school expressed the characters of such motions in verse. The 

 rule.of natural motion was' 4 



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 calct, 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 motion, 

 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, 15 " 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 forward and is retarded till it stops, 

 arises from ascribing the retardation, not to the real cause, the sur- 



O 



rounding resistances, but to the body itself. 



One of the doctrines which was the subject of the wannest discus- 

 sion between the defenders and opposers of Aristotle, at the revival ol 

 physical knowledge, was that in which he asserts, 16 " That body is 

 heavier than another which in an equal bulk moves downward 

 quicker." The opinion maintained by the Arisotelians at the time ot 

 Galileo was, that bodies fall quicker exactly in proportion to their 

 weight. The master himself asserts this in express terms, and reasons 

 upon it. 17 Yet in another passage he appears to distinguish between 

 weight and actual motion downwards: 18 " In physics, we call bodies 

 heavy and light from their power of motion ; but these names are not 

 applied to their actual operations (eve'pysiai^) except any one thinks 



4 Alsted. Encyc. torn. i. p. 637. I5 Phys. Ausc. viii. 10. 10 De Coelo, iv. 1, p. 308 

 17 Ib. iii. 2. " Ib. iv. 1, p. 807. 



