NOTES TO BOOK VI. 125 



wagon on an inclined plane? is a statical question, de- 

 pending on simple conditions : but that the question, What 

 force will move the wagon ? requires additional considera- 

 tions to be introduced. 



In Chapter iv. of this Book, I have noticed Stevin's 

 share in the re-discovery of the Laws of the Equili- 

 brium of Fluids. He distinctly explains the hydrostatic 

 paradox, of which the discovery is generally ascribed to 

 Pascal. 



I must insert here the substance of a note which I 

 added to this Book in the first edition. Leonardo da 

 Vinci must have a place among the discoverers of the 

 Conditions of Equilibrium of Oblique Forces. He pub- 

 lished no work on this subject; but extracts from his 

 manuscripts have been published by Venturi, in his Essai 

 sur les Outrages Physico-Mathematiques de Leonard da 

 Vinci, avec des Fragmens tires de ses Manuscrits apportes 

 d'ltalie. Paris, 1797 : and by Libri, in his Hist, des Sc. 

 Math, en Italie, 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 center upon the directions of the forces. 

 This is quite sound and satisfactory. These views must in 

 all probability have been sufficiently promulgated in Italy 

 to influence the speculations of Galileo ; whose reasonings 

 respecting the lever much resemble those of Leonardo. 

 Da Vinci also anticipated Galileo in asserting that the time 

 of descent of a body down an inclined plane is to the time 



