CHAPTEK XX. 



TIDES. 



Anyone who has spent a summer at the sea- 

 shore has observed that the water level of the 

 ocean changes twice in about twenty-four 

 hours, or perhaps it would be a better state- 

 ment to say that it is continually changing 

 and that twice in twenty-four hours there is a 

 point when it reaches its highest level and an- 

 other when it reaches its lowest. It swings 

 back and forth like a pendulum, making a 

 complete oscillation once in twelve hours. 

 When we come to study this phenomenon 

 closely we find that it varies each day, and 

 that for a certain period of time the water 

 will reach a higher level each succeeding day 

 until it culminates in a maximum height, 

 when it begins to gradually diminish from day 

 to day until it has reached a minimum. Here 

 it turns and goes over the same round again. 

 It will be further observed that the time occu- 

 pied between one high tide and the next one is 

 a trifle over twelve hours. That is to say, the 

 two ebbs and flows that occur each day require 



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