416 RISE AND PEOGEESS OF 



may be disregarded. There is equilibrium, therefore, between the 

 other two portions that which is equal to the length of the plane, 

 and that which is equal to its altitude ; and it follows that two 

 weights, which balance by means of the inclined plane, one hanging 

 freely, and the other resting on the plane, must be to one another 

 in the ratio of the height of the plane to its length. This singular 

 solution of the problem was given in the year 1585 ; and from 

 it the author has derived the general conditions of equilibrium 

 among any three forces meeting at a point. Thus, though he 

 does not seem to have been aware of the full value of the prin- 

 ciple, he touched the very corner-stone of statical science. 



The theory of the Equilibrium of Fluids is also largely indebted 

 to Stevinus ; and he was the first to determine the pressure which 

 the bottom of a vessel sustains from the contained liquid. 



The second grand division of mechanical philosophy the 

 Doctrine of Motion is of much later birth. We are here beyond 

 the circle of abstract truth, and experience alone can furnish the 

 principles of our reasonings. We are not to wonder, therefore, 

 that no progress was made in this department of science, when the 

 true principles of experimental inquiry were themselves unknown. 

 The school of Aristotle taught that motions are either natural or 

 unnatural The natural tendency of all terrestrial bodies was said 

 to be, either to fall directly to the ground, or to ascend from it 

 until they reached their place ; while bodies that were impelled 

 obliquely were believed to pursue a violent or unnatural course, 

 and the motions thus generated were supposed to tend continually 

 to decay. In the heavens all this was different : the natural 

 motions of the heavenly bodies were pronounced to be circular ; 

 and, as their matter was incorruptible, so their motions were 

 eternal and immutable. Amid these reveries, we can still trace 

 throughout the workings of genius ; and in some parts of the 

 mechanical speculations of Aristotle, we find very just views 

 respecting the nature of gravity, and of force in general. The 

 principle of the Composition of Motion that fertile principle which 

 served as the basis of the reasonings of Gralileo, and was more 

 fully developed by Newton is to be found distinctly stated, and 

 reasoned on in Aristotle's writings. 



For more than two thousand years the dogmas of Aristotle 

 continued to extort the assent of mankind, and though here and 

 there a bold inquirer might be found, who ventured to doubt the 



