48 HISTORY OF MECHANICS. 



proportion of quickness and slowness, which are 

 palpably false. For a stone twice as great does 

 not move twice as fast." And Stevinus, in the 

 Appendix to his Statics, describes his having made 

 the experiment, and speaks with great correctness 

 of the apparent deviations from the rule, arising 

 from the resistance of the air. Indeed, the result 

 followed by very obvious reasoning ; for ten bricks, 

 in contact with each other, side by side, would 

 obviously fall in the same time as one ; and these 

 might be conceived to form a body ten times as 

 large as one of them. Accordingly, Benedetti, in 

 1585, reasons in this manner with regard to bodies 

 of different size, though he retains Aristotle's errour 

 as to the different velocity of bodies of different 

 density. 



The next step in this subject is more clearly 

 due to Galileo ; he discovered the true proportion 

 which the Accelerating Force of a body falling 

 down an inclined plane bears to the Accelerating 

 Force of the same body falling freely. This was 

 at first a happy conjecture ; it was then confirmed 

 by experiments, and, finally, after some hesitation, 

 it was referred to its true principle, the Third Law 

 of Motion, with proper elementary simplicity. The 

 Principle here spoken of is this : that for the same 

 body, the Dynamical effect of force is as the Sta- 

 tical effect ; that is, the Velocity which any force 

 generates in a given time when it puts the body 

 in motion, is proportional to the Pressure which 



