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Sect. IV.] 



TIDES- 



119 



^me 



that there is 07ihj one tide in 24 hours 



have single-day tides) . 



But this doe 



(and then we 

 not generally 



if possible, oftener, say 



every 



happen through a whole lunation ; it happens only fo. a 

 few days in each semi-lunation ; ?aid at other times there 

 are two tides as usual. Ca.^es of one tide in 24 hours 

 should be particularly observed, making the observations 

 every half-hour, or, 



5 minutes. 



30. In some places the tide rises and falls /oi/r times in 



the 24 hours. The cases where this occurs arc to be par- 

 ticularly observed. 



They may be observed, as in Art. 29, by making 

 observations every half-hour, 10 minutes, or 5 minutes. 



•J 



half- 



• f 



31. A^Hiere double half-day tide 



s e 



s. 



'I 



xist, they do not 

 commonly extend over any considerable lengtli of coast. 

 If there be time and opportunity, it will be well to 

 examine, by observation, liov/ far tbey do extend. Bnt if 

 the object be to determine the laws of the tides in a larger 

 area, it is better to make the observations out of the 

 resion of these anomalies. 



32. It is well to observe the direction of the stream of 

 flood and of ebb, and the time at which the stream 



turns. 



We must take care not to confound the time of the 

 turn of the tide-stream with the time of high water. 

 Mistakes and errors have often been produced in t'de 

 observations by supposhig that the turn of the tide-stream 

 is the time of high water. But this is not so. The turn 

 of the stream generallv takes place at a different time 



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