GALILEO AND HIS JUDGES. 73 



left behind by the diurnal rotation, and in the neigh- 

 bourhood of the tropics, where the effect is chiefly 

 felt, a constant wind blows accordingly from east to 

 west. Our philosopher had evidently heard of the 

 trade winds, though he had not acquired an accurate 

 knowledge of their course or of their origin. It 

 is undoubtedly true that they do help strongly 

 to prove the revolution of the Earth, because they 

 arise from cold currents of air flowing in from 

 the north and from the south respectively towards 

 the tropics, to supply the place of the atmo- 

 sphere rarefied by the sun's heat, and consequently 

 ascending, as is the case in those regions. Then 

 these cold currents, coming from latitudes where 

 there is a less velocity of rotation, tend to preserve 

 that velocity and lag behind the Earth as it revolves, 

 so that they have the effect of north-easterly winds 

 in the northern hemisphere, and south-easterly in the 

 southern hemisphere. Galileo's imperfect information 

 prevented him from using this important argument. 



However, to return to the tides. He had to 

 account for other phenomena, besides the daily rise 

 and fall, namely, for the much greater rise and fall 

 which take place soon after new and full moon, and 

 which are known as the spring-tides. Unable to 

 deny that these were in some way due to lunar 

 influence, he took refuge in the supposition that the 

 Moon, when at the full, retarded the motion of the 

 Earth in its orbit, since as the two travel together round 

 the Sun at those particular times, they form, as it were, 



