lo GREAT MEN OF SCIENCE 



activity in Milan, and then in Rome. Three years before 

 his death he migrated to France at the invitation of the King, 

 and died there at the age of 67. 



We see that to know him only as a painter, in which cap- 

 acity he is generally known, is not to do him justice. He was 

 in a quite special degree an investigator of nature, as appears 

 from the very extensive manuscripts which he left behind. 

 These are legible with difficulty, for he generally wrote with 

 his left hand. Illustrated with many drawings, they offer a 

 mass of natural knowledge derived from his own observation, 

 which points far into the future. He published nothing of 

 this in print himself; but he must have expounded most of 

 it in his lectures in the academy which he founded. We 

 find there for the first time ideas concerning the details of 

 the motion of falling bodies. As the result of experiments 

 with pieces of wood and lead falling from a tower, he re- 

 cognised that fall was an accelerated motion, although he 

 discovered nothing else of importance in this direction. The 

 dim disc of the moon which accompanies the bright sickle 

 shortly before and after new moon, was explained by him 

 quite correctly, one hundred years before Galileo, as due to 

 the reflected light of the sun-lit earth. He examined the 

 lever and other machines more closely than Archimedes, and 

 in considering forces acting at an angle upon a lever, he 

 already formed a correct conception of turning moment. He 

 also distinguished between sliding and rolling friction, re- 

 garded friction quite rightly as a special form of force, and 

 determined its independence on surface area. He was prob- 

 ably the first to clearly distinguish the concept of work from 

 that of force, inasmuch as he remarks that work implies a 

 distance moved in the direction of the force. He observed 

 waves in water, regarded sound correctly as wave motion in 

 the air, and concluded from observations of echoes that 

 it has a definite velocity of propagation. He noticed the 

 rise of liquid in narrow tubes, and regarded it as a special 



