GALILEO. 261 



The theories of the lever, of the inclined plane and of the screw were 

 familiar to Leonardo.* 



The ideas of Aristotle as to motion and rest were not physical, hut 

 metaphysical. An example will illustrate his mode of reasoning which 

 satisfied the scientific world for something like two thousand years. 

 When a stone is thrown from the hand why does it continue to move 

 for a time, and why does it eventually come to rest? Where is the 

 cause of motion — in the hand ? — or in the stone ? If in the hand, how 

 can the stone continue to move after it has left the hand ? If in the 

 stone, why does it ever come to rest ? Aristotle's answer is that ' a 

 motion is communicated to the air, the successive parts of which urge 

 the stone onward; each portion of the air continues to act for some 

 little time after it has heen acted upon, and the motion ceases when it 

 comes to a particle which can not act after it has been acted upon/ 

 The confusion of this explanation is complete. 



The mechanical ideas of Aristotle and of his successors, as to fall- 

 ing bodies, are expressed in these words : ' That body, is heavier than 

 another which, in an equal bulk, moves downward quicker.' Trans- 

 forming the phrase, we may say, that if two bodies, A and B, are of 

 equal bulk but of different weights, then the heavier body will fall the 

 quicker; or, again, if A weighs ten pounds and B one pound, A will 

 fall faster than B. The Aristotelians of Galileo's time further main- 

 tained that A would fall exactly ten times faster than B. Galileo's 

 experiments proved that they fell in precisely the same time. Sixteen 

 hundred years earlier Lucretius had come near to the same truth: 

 " For whenever bodies fall through water and thin air they must 

 quicken their descents in proportion to their weights, because the body 

 of water and subtle nature of air can not retard everything to an equal 

 degree; on the other hand, empty void can not offer resistance to any- 

 thing in any direction at any time, but must continually give way ; and 

 for this reason all things must be moved and borne along with equal 

 velocity, though of unequal weights, through the unresisting void." 



While Kepler was determining the empirical laws according to 

 which the planets move in their orbits, Galileo was laying the founda- 

 tions of the science of mechanics by which, eventually, Newton was to 

 explain why they so move. The foundations of mechanics rest on 

 experiments made by Galileo, at Pisa, on the laws of falling bodies. 

 It was the opinion of the time that heavy bodies fell faster than light 

 ones, and it was a matter of common observation that a square foot of 

 wood reached the ground before a square foot of paper released at the 

 same time. The fact was explained by Galileo as due to the resistance 

 of the air. In a vacuum they would fall at the same time. By crump- 



* Leonardo's investigations in mechanics were not published during his 

 lifetime, but a correct theory of the lever was known to him as early as 1499. 



