28 THE TIDES. 



the earth is between the sun and the moon, the 

 attractions of these two bodies acting in concert 

 upon the earth, cause the increase of the ebb and 

 flow or the spring-tides. Again, at the time of the 

 first and last quarter of the moon, that is when the 

 places of the sun and moon are a quarter of a 

 circle apart, the moon-flood must happen together 

 with the ebb due to the sun, and conversely; so 

 that by reason of this contest between the two 

 forces, resulting from the contrast of their po- 

 sition, the tides are brought down to their lowest 

 amount. 



It is true that when the moon is new, that planet 

 stands between the sun and the earth, but the three 

 bodies do not then always lie in a straight line ; 

 neither are they in the same line when the moon is 

 full. But you will see that the sun and moon will 

 aid each other the more powerfully in raising the 

 tides, the more nearly the three bodies approach 

 the condition of lying in a true right line. This 

 happens most perfectly in autumn and in spring 

 at the times of the equinoxes, when the sun and 

 moon are over the equator at the same time. This 

 shows why the strongest spring-tides occur at 

 these seasons. 



The formation and progress of the tide-wave, 

 although in general it is connected with the con- 

 ditions which I have thus explained, is yet sub- 

 jected to very considerable changes by the peculiar 



