70 GALILEO AND HIS JUDGES. 



few places, and notably at Venice, there is a sensible 

 tide, so it is said, causing a difference of a few feet 

 between high and low water. 



Now Galileo was under the impression that the ebb 

 and flow took each about six hours, following the 

 ordinary solar day ; whereas, if he had observed tlie 

 phenomenon on the shores of any sea, where the tidal 

 wave of the ocean made its full force to be felt, or 

 again, at the mouth of a great tidal river, he never 

 could have failed to perceive that the rise and fall of 

 the water follow approximately the lunar, and not 

 the solar day, the former being fifty minutes longer 

 than the latter. It must of course be understood that 

 the theory of the tides was first investigated fully 

 and scientifically by the same great genius to 

 whom we owe the theory of universal gravitation ; 

 and Galileo, who lived half a century earlier, may well 

 be excused for not having grasped it. But it had 

 long been known that the Sun and Moon had an 

 influence upon the tides, and as I have just stated, 

 any one who watched the movements of the sea from 

 day to day, and from week to week, at a place where 

 there is a great rise and fall as for instance, in the 

 Bristol Channel could not fail to perceive that the 

 Moon had the principal share in the work, however 

 unable he might be to comprehend the theory. 

 Besides which, the theory, however obvious to us (at 

 least in its main outlines), was not by any means so 

 intelligible to the men of Galileo's age. They might 

 just guess that the Sun exercised some attractive 



