MECHANICS AND PHYSICS 189 



case. Here, it is the air which plays the part of 

 motive power. When displaced by the projectile 

 coming out of the catapult or sling, the air flows 

 back behind the projectile and pushes it forward. 

 Whilst the rectilinear natural movement is accelerated, 

 the violent movement is of necessity retarded (Phys., 

 230 b, 25). 



From the mechanical point of view, the interest of 

 Aristotle's teaching lies in the law of proportions which 

 he establishes, as we have seen, between the velocity 

 V, the force F and the resistance R. The same force 

 can move successively a heavy body and a light body ; 

 but it will move the heavy body slowly and the light 

 body quickly; thus the velocities of the movements 

 imparted to these bodies will be inversely proportional 

 to their weights. " The velocity of the Lighter body 

 will be to the velocity of the heavier body as the 

 weight of the heavier body is to the weight of the lighter 

 body " (De Coelo, 301 b). 



This law appears to be a faithful translation of com- 

 mon observation. At first sight, it even seems to 

 apply to the free fall of bodies in space. In this case 

 the motive force is the weight, the resistance is the air. 

 As a matter of fact, a light body like a feather falls 

 more slowly than a heavy body like a piece of lead. 

 If, however, we take two bodies of the same shape 

 but weighing respectively 1 lb. and 2 lbs., we ought 

 to have, since the resistance of the air is the same,, 



1 lb.= RV and 2 lbs.= R2V. 



The body weighing 2 lbs. should fall twice as quickly 

 as the one of 1 lb., which is contradicted by experience.. 

 Thus the law postulated by Aristotle, which persisted 

 until the Renaissance, is manifestly false. The resis- 

 tance of the air does not play the part attributed to 

 it by the Stagirite, and bodies fall with equal speed 

 in empty space as had been supposed by the Atomistic 



