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Sect IV.] 



TIDES. 



121 



But the motion of tbis rwe-wave is not that motion of 

 the water which makes the stream of flood. Nor docs the 

 motion of the wave coincide with any motion of the parts 

 of the water. The tide-wave may be going one way 

 when the water is going another^ asliappens in some rivers 

 when the tide is travelling upwards in them. 



36. The establishment^ which is wanted in order to 



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determine one progress of the tide-wave (see Art. 34 



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and 35), may be known from observations made at any 





age of the moon, as well as at new or full moon, (See 

 Art. 17 and Note A.) 



37, Ii. tracing the progress of the tide-wave, instead of 

 using the vulgar estahlishmeM hitherto spoken of, it is 



better to use the mean estahlishment^ namely, the mean of 

 all the liinitidal intervals. 



For the vulgar establishment is affected by the age of 

 the tide (Art. 20), which the mean establishment is not. 



Th(^ mean establishment is (say) 10m., 20m.5 30m., or 

 40m, less than the vulgar establishment, according to the 

 age of the tide, (See Note A.) 



38. When the tides are regular, good observations, 



made for a few days or a week at each place, may give 



the establishment (either vulgar or mean) with sufficient 

 exactness to determine the progress of the tide-wave. 



39. But the progress of the tide-wave may be much 

 better determined by means of simultaneous ohservations • 

 namely, observations made at different places on the same 

 days for a few days or a week. 



For such a purpose persons must be posted at difE^rent 

 points of the shore or shores where tLo motion of the tide- 

 wave is to be traced ; say 10, or 20, or 40, or 80 miles 



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