A GREAT TIDAL WAVE. 141 



will pull with combined effect upon the waters of the 

 earth, and so cause what are called spring tides. This, 

 of course, happens at the time of every new moon. 

 But sometimes the moon exerts a more effective pull 

 than at other times ; and the same happens also in the 

 case of the sun ; and on October 5, it happens that 

 both the sun and the moon will give a particularly 

 vigorous haul upon the earth's waters. As regards 

 the sun, there is nothing unusual. Every October his 

 pull on the ocean is much the same as in preceding 

 Octobers. But October is a month of high solar tides 

 and for these reasons. In September, as everyone 

 knows, the sun crosses the equinoctial ; and other 

 things being equal it would be when on the equinoctial 

 that his power to raise a tidal wave would be greatest. 

 But other things are not equal ; for the sun is not 

 always at a fixed distance from the earth. He is 

 nearest in January; so that he would exert more 

 power in that month than in any other if his force 

 depended solely on distance. As matters actually 

 stand, it will be obvious that at some time between 

 September and January the sun's tidal power would 

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

 month of high solar tidal waves. 



But it is the lunar wave which will be most effectively 

 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 



