140 LIGHT SCIENCE FOR LEISURE HOURS. 



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 equinoctial 

 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 rela- 

 tion does recur, and with a certain degree of regularity. 

 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 excep- 

 tionally 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. 

 The combination of circumstances is almost the most 

 effective that can possibly exist. 



But, after all, high tides depend very importantly 

 on other considerations than astronomical ones. Most 



