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Sect, III.] 



HyDROGEAPHY 



81 



Imiar month. When you come away, mean the high and 



mean of them 



low water heights of each day^ and take a 



again for the mean -place of the water ^ and cut a mark in 



the rock corresponding with that mean level of the sea 

 before you remove the pole. As this is the true scientific 



level of reference in all matters relative to the tides, 

 ijfer this level again to some mark in a contiguous 



building, that a reference may at any time be made to it, 

 by persons who might not be able to find the rock. 



Let the w^atch be always at mean time at the place. 



Tlie hi^h and low water observations should be continued 





night and day with equal carefulness in order to deter- 

 mine the amount of diurnal tide ; and every observation 

 should be recorded, although it mav not seem to agree 



with the others. 



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If tides are taken 



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t coral islands, or a 



t stations within 



a belt of coral, it should alw^ays be noted in tlio journal 

 whether the sea or land breeze be blowing and with what 

 strength, and also whether the surf be h^gh upon the 



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reefs and sending its water into the lagoon, fillin 



it 



faster than it can escape. 



In the Appendix will be found two forms, one cf which 



(No, 8) is for registering the tides every half hour, th 





other (No. 7) is for the high and low water only. 



For further information upon the tides see that section. 



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Soundings. 



24. Before any soundings are inserted in the chart they 

 should be reduced to a standard obtained by meanin 

 the three or four successive lowest waters of c:ich spring- 

 tide, and meaning them again for a general mean. Tliis 



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