124 



TIDFS 



[Sect. IV, 





APP 



"*h 



V 



NOTE A. 



Note to Akticles 17, 19. and 37, 



To find the Hour of High Water on any day, at any place, tvhen the 



Establishment of the place is known. 



The rule is different (as to amount) according to the tidal force of the 

 sun : for though the tidal force of the sun in theory is the same at all 

 placcSj it is found by observation to be different at different places. 



This difference appears in the different ratio of the rise of spring- 

 tides to the rise of reap-tides : (this difference is the semimenstrual in- 

 equality of heights.) In general the rise of spring-tide is abou^ double 

 that of neap-tide, which gives the solar tide one'tldrd of the lunar tide. 

 But in some cases the spring-tide exceeds the nea])-tide only by one-third, 

 "which gives the solar tide only one-seventh of the lunar tide. 



Also the difference of the greatest and least lunitidal intervals (that 

 is, the semimenstrual inequality of times : see Art, 13 and 16) shows the 

 difference of the solar tidal force at different places. The difference of 

 the greatest and least intervals is 1 h. 28 m. at London and Liverpool, 

 but at Plvmouth it is 1 h. 36 m„ and at Portsmouth 1 h. 21 m. On the 

 coast of North America it is generally less than 1 h. 20 m., while at 

 some places on the coasts of France and Ireland it is above 2 h. 



We niav take 1 h. 28 m. as the mean value of this difference, which 

 agrees with the supposition that the solar tide is about one-third the 



lunar tide. 



In finding the hour of high water on any day when the vulgar esta- 

 blishment is known, the rule will also be different according to the age 

 of the tide= We shall give the rule M'hcn the tide is a day and a quarter 

 o!d, and also when the tide is two days and a half old. In general, the 

 tides will be between these limits. 



(L) Tide a day and a quarter old. Minutes to be added to or sub- 

 tracted from the establishments^ according to the hour of the moon's 



transit on the half-dav in question t 



Hour of the Moon's 



Transifc after Su 



Correction of the vul- 



^y^ r-* ablishment 

 to find the Lunitidal 

 Interval . , . 







m. j m. I m. 



161 



Oi 



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I 



J 



i 



47 --57 



-60i-47 -16 +15 



4-2?! + 25 



?*!■ 



For example— if the establishment be 2 h, 27 m., at what hour will tiie 

 high water come after a moon's transit which takes place at 4h. a.m.? 

 The minutes to he added to 2 h. 27 m. for 4 h. transit are. by the table, 

 - 57 m.: therefore the high water will be at 1 h. 30 m. after the moon's 

 transit^ that is, at 5h, 30 rn. 



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