234 THE SEAS 



it is so small as to have no effect in drawing it vertically 

 away from the earth's surface. But the further we go 

 from this point on the hemisphere directed towards the 

 moon the more horizontal does the moon's pull become. It 

 does not take nearly so great a force to make water slide 

 horizontally over the earth as to draw it vertically upward, 

 and the moon's attraction is sufficient to do the former. The 

 result is that water is drawn over the surface of the earth 

 from all around and towards the point beneath the moon 

 so that the water becomes piled up there. Where this 

 bulge of water occurs the tide is high. 



On the hemisphere of the earth pointing away from the 

 moon exactly similar forces are acting to cause high tide 

 at the point furthest from the moon. How these forces 

 ;ome about can be proved mathematically, but it is some- 

 what beyond the scope of this book to enter into an explana- 

 tion. 



In this way there are two high tides simultaneously on 

 the earth, one at a point beneath the moon, and the other 

 on the other face of the earth opposite to the first. These 

 two high tides cause the water lying between them to be 

 drawn away, so that at two points lying on either side 

 of the earth midway between the two high tide regions 

 there will be low tides. 



But the earth is revolving, so that approximately once 

 each day the moon exerts its influence on every meridian 

 on the earth's surface in turn. Under these conditions 

 the points at which high tide occurs change with the 

 changing position of the moon, and a " tidal wave" sweeps 

 round the surface of the globe. (This must not be confused 

 with the popular and wrongly named " tidal wave " of 

 great dimensions and destructive force, which is usually 

 a wave caused by a submarine earthquake and quite 

 unconnected with the moon's attraction.) If the earth 



