58 GALILEO AND HIS JUDGES. 



principle of the gravity of the atmosphere, I refer 

 him to the second day's dialogue. He was aware, no 

 doubt, that the air was carried round by the Earth 

 in its diurnal motion, but why it was so carried round 

 I do not think he quite understood ; indeed, as may 

 well be supposed, he did not clearly understand what 

 gravity was ; it was a mysterious force, drawing 

 heavy bodies towards the centre of the Earth, a force 

 to which we, indeed, give the name of gravity, but of 

 the essence of which we know nothing, as, in fact, we 

 know nothing of the nature of the force that moves 

 the heavenly bodies. This passage is remarkable 

 because it looks as if Galileo half suspected that the 

 force which acted on the Moon and the planets might 

 be akin to that which attracted terrestrial objects 

 towards the centre of the Earth. If he really had 

 arrived at such a conclusion, he would have antici- 

 pated the great discovery made thirty or forty years 

 later. I think, however, that he only wished to 

 illustrate the one by the other, and that the allusion 

 means no more. I give, however, the passage in 

 a note,* so that any reader may form his own 

 judgment ; and I may add that according to an 

 opinion commonly held by the Copernican school of 

 that age, the adherence of the atmosphere to the 

 Earth as it revolved was the effect of friction. 



* Simplicio having said that the cause why parts of the earth 

 are carried downwards was gravity, Salviati answers : " Voi errate, 

 Signor Simplicio, voi dovevate dire, che ciaschedun sa, ch' ella si 

 chiama gravita ; ma io non vi domando il nome, ma dell' essenza 

 della cosa : della quale essenza voi non sapete punto piu di quello, 



