A HISTORY OF SCIENCE 



in one of his writings he declares, " II sole non si 

 motive" the sun does not move. 2 



Among his inventions is a dynamometer for de- 

 termining the traction power of machines and animals, 

 and his experiments with steam have led some of his 

 enthusiastic partisans to claim for him priority to 

 Watt in the invention of the steam-engine. In these 

 experiments, however, Leonardo seems to have ad- 

 vanced little beyond Hero of Alexandria and his 

 steam toy. Hero's steam-engine did nothing but ro- 

 tate itself by virtue of escaping jets of steam forced 

 from the bent tubes, while Leonardo's " steam-engine " 

 " drove a ball weighing one talent over a distance of 

 six stadia." In a manuscript now in the library of the 

 Institut de France, Da Vinci describes this engine 

 minutely. The action of this machine was due to the 

 sudden conversion of small quantities of water into 

 steam ("smoke," as he called it) by coming suddenly 

 in contact with a heated surface in a proper receptacle, 

 the rapidly formed steam acting as a propulsive force 

 after the manner of an explosive. It is really a steam- 

 gun, rather than a steam-engine, and it is not unlikely 

 that the study of the action of gunpowder may have 

 suggested it to Leonardo. 



It is believed that Leonardo is the true discoverer 

 of the camera - obscura, although the Neapolitan 

 philosopher, Giambattista Porta, who was not born 

 until some twenty years after the death of Leonardo, 

 is usually credited with first describing this device. 

 There is little doubt, however, that Da Vinci under- 

 stood the principle of this mechanism, for he describes 

 how such a camera can be made by cutting a small, 



