142 LIGHT SCIENCE FOR LEISURE HOURS. 



should find that it was always largest when the moon 

 looked largest, and vice versa. In other words, when 

 the moon is in perigee the lunar wave is largest. But 

 then there is another consideration. The lunar wave 

 would vary according to the moon's proximity to the 

 equinoctial ; and (other things being equal) would be 

 largest when the moon is exactly opposite the earth's 

 equator. If the two effects are combined, that is, if 

 the moon happens to be in perigee and on the equi- 

 noctial at the same time, then of course we get the 

 largest lunar tidal wave we can possibly have. 



Now this ' largest lunar wave ' occurs at somewhat 

 long intervals, because the relation on which it depends 

 is one which is, so to speak, exceptional. Still the 

 relation does recur, and with a certain degree of regu- 

 larity. When it happens, however, it by no means 

 follows that we have a very high tide ; because it may 

 occur when the tides are near ( neap ' ; in other words, 

 when the sun and moon exert opposing effects. The 

 largest lunar wave cannot stand the drain which the 

 solar wave exerts upon it at the time of neap tides. 

 Nor would the large lunar tidal wave produce an 

 exceptionally high tide, even though it were not the 

 time of 'neap,' or were tolerably near the time of 

 ' spring ' tides. Only when it happens that a large 

 lunar wave combines fully with the solar wave do we 

 get very high tides. And when, in addition to this 

 relation, we have the solar wave nearly at a maximum, 

 we get the highest of all possible tides. This is what 

 will happen, or all but happen, on October 5 next. 



