74 GALILEO AND HIS JUDGES. 



a lengthened pendulum, longer than at other times 

 by the semi-diameter of the lunar orbit; and therefore 

 (like any other pendulum) must vibrate more slowly. 

 I should say that he does not appear to have been 

 aware of the existence of two spring-tides in each 

 lunation, and therefore only tries to account for one ; 

 and it is obvious that this method of explaining 

 them is not only utterly inadequate, but even absurd. 

 The Moon truly enough exercises a certain disturbing 

 influence on the orbital motion of the Earth, but that 

 has nothing to do with the spring-tides. 



There remained the necessity of accounting for the 

 annual, or, more properly, semi-annual increase of the 

 ebb and flow of the sea. Galileo suggests that this 

 arises from the angle made by the plane of the 

 equator with the ecliptic at the equinoxes, owing to 

 which there would not be the same counteraction 

 exercised by the Earth's motion in its orbit on the 

 waters of the ocean at those periods as there would 

 at the solstices. But it seems that this would rather 

 tend to diminish the tides than to increase them, as, 

 indeed, would be the case as regards the last-men- 

 tioned explanation with respect to the ordinary 

 spring- tides. What really does happen at the equi- 

 noxes is, that the Sun and the full or new Moon 

 being at those times vertical to the equator (or nearly 

 so), they have a greater attractive force than at other 

 spring-tides over the vast expanse of the ocean, and 

 the tides are consequently greater. There is also 

 another increase which sometimes occurs when the 



