A GREAT TIDAL WAVE. 157 



have a maximum value. Thus it is that October is a 

 month of high solar tidal waves. 



But is it the lunar wave which will be most effective- 

 ly strengthened at the next spring tide. If we could 

 watch the lunar tidal wave alone (instead of always 

 finding it combined with the solar wave) we should 

 find it gradually increasing, and then gradually dimin- 

 ishing, in a period of about a lunar month. And we 

 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 some- 

 what 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 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 



