116 



TIDES. 



[Sect. IV. 



any day, when the establishment of the place is known a^ 

 m Art. 17, is given in Note A. 



The establishment of the place may be known by 

 observation, made at any age of the moon, as well as at 

 new and full moon, by the same kind of calculation. 

 18. It is also advisable to set off the height of hioi. 



ordinates, and to draw a curve through the 

 extremities. This curve abo will ascend and descend 



(ascending at spring tides and flpsr.,.n.1,-^„ 



Sf aJ-tC>i* clS 



at neap tides). 



nm 



■A 



Hie heights may be set off as ordinates, taking for the 



abscissas equal intervals to represent successive half-days, 

 as in Art. 16. 



But tiie curve will be better determined if we take for 

 the abscissas the hour of the moon's transit, as in Art. 17. 

 ;. The maxirmmi or greatest ordinate of this curve of 

 heights (that is, the spring-tide height) follows the day of 

 new and the day of full moon, by one, two, or three 

 days ; and as the new or full moon is supposed to produce 

 the spring tide, this interval of one, two, or three days is 



M 



called the age of the tide. 



th 



20. If the heights 



be set off from an abscissa which is 

 hour of the moon's transit (see Art. 18), the distance 

 of the maximum ordinate from the hour of transit, Oh. or 

 12h. (which are the same thing \ wdll give the age of the 

 tide more exactly than Art. 19. 



21. The lunitidal intervals and heights of low tvater 

 may be laid down in curves in the same manner as those 

 of high water, 



of the semi-mensual inequality of 

 times and heights should be determined, when opportunity 



2. The 





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