368 On the probable importance of Tide Obserrattoms 



This, tberefore, is oue main point to which the attentitm of 

 the tide observers at the Cape should be directed. On what 

 day of the moon's age is the highest high water ? But this 

 inquiry requires some additional cautions to malve the answer of 

 much value. One or two spring tides merely compared with 

 the preceding and setting tides, and with the moon's age, would 

 not give ns the accuracy whicli we want ; and which, with a 

 little additional attention, we can get. 



4. In the first place it is a very rough way of reckoning, to 

 talk of so many days after new or full moon ; for the full moon 

 may happen at any hour on the day in which it falls ; and thus 

 two full moons on two Mondaj's may differ by almost 24 hours 

 in their interval to the moon of Tuesday. This, however, is 

 easily mended. Instead of talking of the moon's age, we can 

 speak of the hour and minute when she passes ^Ae meridian; 

 this is given in most Almanacks. On the day of full moon she 

 passes, it may be, near 12 at night ; the next day she will pass 

 near 50 minutes later ; and so on. The hour and minute when 

 she passes (soZar time, observe not clock time*) are an exact 

 indication of her age ; and must be used instead, of the moon's 

 age, in all calculations about tides. 



5. Also we must have, not only two or three spring tides 

 observed, but several, in order to attain an exact result; for 

 any spring in particular, may be affected by casual irregu- 

 larities, (arising from wind and the like) which may show the 

 greatest tide on the wrong day, and this may be one, or two, 

 three days from the regular day of the greatest tide, so that the 

 observation would be worthless. 



But taking 77!rt»?/ such springs, a whole year for instance, 

 these casualties would not always occur, and would occur so 

 as to correct themselves. If one irregular tide was too high, 

 another would be too low ; if one made the maximum too late, 

 another would make it too early. We should have a compensa- 

 tion by properly taking the general effect of all spring tides. 



6. Suppose, then, we have observations of the height of 

 high water for several days at and near the spring tide : suppose 

 these observations have been long carried on and accumulated 

 in considerable number. The question will then occur, how 

 are we to take properly the general effect or mean result? I 

 will mention two ways, the first simpler, but giving only a 

 rough average ; the second more complex, but more exact. 



. First method of finding how long after full or new moon 

 the highest tide happens. 



• If the Almanack gives the moon's zenith in mean or clock time, we 

 apply the equation of time. 



