12 HISTORY OF MECHANICS. 



scending force" of bodies, on inclined planes, was 

 compared, by a process, which, so far as it forms 

 a line of proof at all, is a somewhat curious ex- 

 ample of confused and vicious reasoning. When 

 two bodies are supported on two inclined planes, 

 and are connected by a string passing over the 

 junction of the planes, so that when one descends 

 the other ascends, they must move through equal 

 spaces on the planes ; but on the plane which is 

 more oblique (that is, more nearly horizontal,) the 

 vertical descent will be smaller in the same pro- 

 portion in which the plane is longer. Hence, by 

 the Aristotelian principle, the weight of the body 

 on the longer plane is less; and, to produce an 

 equality of effect, the body must be greater in the 

 same proportion. We may observe that the Aris- 

 totelian principle is not only false, but is here 

 misapplied ; for its genuine meaning is, that when 

 bodies fall freely by gravity, they move quicker in 

 proportion as they are heavier ; but the rule is 

 here applied to the motions which bodies would 

 have, if they were- moved by a force extraneous 

 to their gravity. The proposition was supposed by 

 the Aristotelians to be true of actual velocities ; 

 it is applied by Jordanus to virtual velocities, 

 without his being aware what he was doing. This 

 confusion being made, the result is got at by 

 taking for granted that bodies thus proved to be 

 equally heavy, have equal powers of descent on 

 the inclined planes; whereas, in the previous part 



