28 



THE OPAL SEA 



The aflrnc- 

 tioji of the 

 vwon 



High and 

 low tides 



the nearest to the moon, draws them out of the 

 spherical on the near side of the earth, and 

 draws the earth itself away from the waters 

 on the far side. The result is a bulge or heap- 

 ing up of water along those portions of the 

 earth nearest and farthest removed from the 

 moon. At the intermediate points, between the 

 nearest and farthest remove, the pull of the 

 moon is a force directed inward toward the cen- 

 ter of the earth, and has a tendency to flatten 

 the waters in those regions. The result here is 

 a depression or hollowing instead of a bulge. 

 Thus with the water high in the places nearest 

 and farthest from the moon, and low at the 

 intermediate points, we have the sea raised 

 above its normal level in some places and de- 

 pressed below its normal level at other places. 

 These elevations and depressions are what we 

 call " high " and " low " tide. 



There are really four tides a day instead of 

 two — the moon and the sun causing two tides 

 each. But the solar tides are so much smaller 

 than the lunar, and so largely merged in the lat- 

 ter, that they are not iisually noticed. The in- 

 fluence of the sun is very noticeable, however, 

 when it joins with the moon (that is twice a 

 month) and there is a pull together. This re- 

 sults in the " spring " tides which are always 



