TIDES 



165 



the day before, and that the time required for the comple- 

 tion of two high and two low tides is nearly 25 hours. Con- 

 tinued observation will show, as Julius Csesar stated many 

 centuries ago, that there is apparently a relation between 

 the phases of the moon and the height of the tides. The 

 greatest rise and fall of the water will be found to occur 

 about the time of full and new moon. 



It has been found that the position of the sun, as well as 

 that of the moon, 

 affects the height of 

 the tide. If the 

 earth, moon, and 

 sun lie in nearly 

 the same line, the 

 tidal range is great- 

 est. This is called 

 spring tide. When 

 the sun and moon 

 act at right angles 

 upon the earth, the 

 tidal range is least 

 and this is called 

 neap tide. The tidal 

 undulations have been proved by astronomers to be due to 

 the rotation of the earth and the gravitational attraction of 

 the sun and moon upon its water envelope. The moon is 

 much more effective because it is nearer. 



The tidal current as it sweeps between islands often 

 forms eddies and whirlpools which make navigation very 

 dangerous. An example of this is found at Hell Gate, 

 New York, and at the famous Maelstrom off the coast of 

 Norway. On the other hand, in flat countries where the 



Low TIDE AT THE SAME PLACE 



