318 HISTORY OF MECHANICS. 



statue in honor of him in one of the public places of his lative city 

 He was born in 1548, as I learn from M. Quetelet's notice of him, and 

 died in 1620. Montucla says that he died in 1633 ; misled apparently 

 by the preface to Albert Girard's edition of Stevin's worts, which was 

 published in 1634, and which speaks of a death which took place in 

 the preceding year ; but on examination it will be seen that this refers 

 to Girard, not to Stevin. 



I ought to have mentioned, in consideration of the importance of 

 the proposition, that Stevin distinctly states the triangle of forces ; 

 namely, that three forces which act upon a point are in equilibrium 

 when they are parallel and proportional to the three sides of any plane 

 triangle. This includes the principle of the Composition of Statical 

 Forces. Stevin also applies his principle of equilibrium to cordage, 

 pulleys, funicular polygons, and especially to the bits of bridles ; a 

 branch of mechanics which he calls Chalinothlipsis. 



He has also the merit of having seen very clearly, the distinction of 

 statical and dynamical problems. He remarks that the question, What 

 force will support a loaded wagon on an inclined plane ? is a statical 

 question, depending on simple conditions ; but that the question, What 

 force will move the wagon ? requires additional considerations to be 

 introduced. 



In Chapter iv. of this Book, I have noticed Stevin's share in the re- 

 discovery of the Laws of the Equilibrium of Fluids. He distinctly 

 explains the hydrostatic paradox, of which the discovery is generally 

 ascribed to Pascal. 



Earlier than Stevinus, Leonardo da Vinci must have a place among 

 the discoverers of the Conditions of Equilibrium of Oblique Forces. 

 He published no work on this subject ; but extracts from his manu- 

 scripts have been published by Venturi, in his Essai sur les Ouvrayes 

 Physico-Mathematiques de Leonard da Vinci, avec des Fragmens tires 

 de ses Manuscrits apportes d' Italic. Paris, 1797: and by Libri, in 

 his Hist, des Sc. Math, en Italic, 1839. I have also myself examined 

 these manuscripts in the Royal Library at Paris. 



It appears that, as early as 1499, Leonardo gave a perfectly correct 

 statement of the proportion of the forces exerted by a cord which acts 

 obliquely and supports a weight on a lever. He distinguishes between 

 the real lever, and the potential levers, that is, the perpendiculars drawn 

 from the centre upon the directions of the forces. This is quite sound 

 and satisfactory. These views must in all probability have been suffi- 

 ciently promulgated in Italy to influence the speculations of Galileo 



