22 GREAT MEN OF SCIENCE 



gravitational forces would always exist, and the result would 

 be perpetual motion. Stevin, who had a practical know- 

 ledge of machines, takes for granted that perpetual motion, 

 an arrangement continuing to move without external aid, a 

 machine going by itself, is impossible; and hence likewise 

 the motion of the chain on his inclined plane. The chain 

 must therefore be in equilibrium, and we are thus immedi- 

 ately given the conditions for equilibrium on the inclined 

 plane, and the ratio of forces, if we imagine the freely hang- 

 ing part of the chain as abolished. Stevin thus connects 

 together the result of our experience that a perpetual mo- 

 tion is impossible, with the ratio of forces on the inclined 

 plane, and then further with the parallelogram of forces, 

 which first appears in his work and is there applied in very 

 many ways. This firm linkage of different laws, which can 

 be separately tested by experience, in such a way that they 

 all stand or fall together, is highly characteristic of exact 

 scientific research - the peculiar certainty of its results de- 

 pends upon such interconnection - and Stevin affords us the 

 fundamental example which we have just described. 



He connects this knowledge with the laws of the other 

 machines, such as sets of pulleys and levers, and thus arrives 

 at a connection with the work of Archimedes and Leonardo, 

 and in fact, at a complete understanding of all kinds of 

 machines, that is, of all arrangements, which allow the mag- 

 nitude, direction, and the point of application of given forces 

 to be changed as required. The study of pulleys leads him 

 for the first time to a real understanding of the 'principle of 

 virtual displacement,' which says, that in the case of small 

 displacements in the neighbourhood of the position of equili- 

 brium, force and distance are inversely proportional to one 

 another, or that what is gained in reduction of force is lost 

 by increase in distance. This principle is true of all 

 machines, even when they work with liquid, as in the sub- 

 sequently discovered hydraulic press. It was of the greatest 



