36 GREAT MEN OF SCIENCE 



played chiefly by Salviati, who in a manner pecuHarly 

 GaHleo's own, combines a respectful gentleness towards his 

 opponent with compelling power of argument. Simplicio is 

 the learned man of the Aristotelean school without however 

 being narrow-minded, as so many living opponents of Galileo 

 were; he suffers in the dialogue as many defeats as he puts 

 forward arguments. The third person, Sagredo, is a cheer- 

 ful and kindly character and witness, who is delighted to gain 

 new knowledge; he exhibits the feeling of a prisoner set free, 

 which was echoed by many contemporary readers of the 

 Dialogues in letters which are still preserved. Galileo here 

 gathers together every result of his life's work bearing upon 

 the question of the earth's movements. Much of this has 

 already been referred to above. But, as of fundamental 

 importance in these dialogues, we also have the knowledge 

 of the undisturbed super-position of motions of different 

 origin, and the law of inertia. Though all this is very simple, 

 no one had been clear about it before Galileo, not even 

 Kepler, and a mass of objections to the motion of the earth 

 had arisen from this want of clarity. 



But the abolition of these was not the only gain which 

 Galileo brought; we owe to him the actual foundation of the 

 whole science of motion, and hence of the physics of matter. 

 The Dialogues make use of experimental proof in the form of 

 observations which many people could have made before, 

 but which had not been grasped with sufficient clearness, 

 certainly for the most part through the hindrance produced 

 by learning Aristotle. Thus, for example, we have the fact 

 that the ball falling from the hand of a rider in full gallop, 

 does not remain behind him but retains the speed of the rider, 

 even when it falls at the same time. Or the experience which 

 we have upon ships sailing no matter how fast, that all pro- 

 cesses of motion take place on them in exactly the same way 

 as on stationary ships, so that if experiments are made in a 

 closed space, upon a ship, it is impossible to tell whether the 



