Springs, Rivers, and the Sea. 135 



again (or the moon appears to move round 

 the earth from a given meridian to the 

 same again) in about 24 hours, hence in 

 that period there are two tides of flood 

 and two of ebb, and this alternate ebbing 

 and flowing continues without intermis- 

 sion. For instance, if the tide be now at 

 high -water-mark, in any port, or harbour, 

 ivhich lies open to the ocean, it will pres- 

 ently subside, and flow regularly back, for 

 about six hours, when it will be found at 

 low-water-mark. After this, it will again 

 gradually advance for six hours, and then 

 return back, in the same time, to its for- 

 mer situation; rising and falling alternate- 

 ly, twice a day, or in the space of about 

 twenty- four hours. 



The interval between its flux and reflux 

 is, however, not precisely six hours, but 

 about eleven minutes more ; so that the 

 time of high water does not always happen 

 at the same hour, but is about three quar- 

 ters of an hour later every day, for thirty 

 days ; when it again recurs as before. For 

 example, if it be high water, at any place, 

 to day at noon, it will be low water at ele- 

 ven minutes after six in the evening ; and 

 consequently, after two changes more, the 



