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346 PTof. A. D. Bache on Tidal Observations. 



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tion, so as to give xVseful prediction tables. The foregoing results 

 point distinctly to a ruling dhuse depending upon the moon's de- 

 clination. 



3. The hoiyly observations for the year were thrown into the 

 form of curves, the abscissas representing the hours and the ordi- 

 nates the heights. Of these I present, as characteristic, the months 

 of January and March (PI. 4, 5, or Nos. 2, 3), In January the tides 



are single throughout the month, the rise and fall diminishing 



towards the zero of declination ; and in March two periods of 

 marked double tides occur. The times of new and full moon 

 coincide nearly with the zero of declination of March ; in January 

 the syzigies occur at times of greatest declination. A series of dia- 

 grams, prepared for periods of declination zero, shows irregulari- 

 ties or double tides near these times. Before disappearing, the tide 

 which is lost appears rather as an irregularity than as a real tide, 

 puzzling to the observer and a severe test of his faithfuUiess. A 

 similar set of diagrams for the periods of greatest declination 



show uniformly single tides and the greatest comparative rise 

 and fall at the same periods, whether coinciding with syzigies or 

 with first and last quarters. In computing the heights of spring 

 and neap tides by the common methods, four months gave zero 

 or negative differences. 



To discuss the epochs of the phenomenon, as compared with 

 greatest aiid least declinations, I prepared two sets of tables, 

 which require revision. 



They show sometimes an actual coincidence in the epoch of 

 least tides and zero of declination, sometimes a precedence and 

 sometimes a subsequence, which, when not caused by irregularity 

 of winds, I believe will find a satisfactory explanation ; at a 

 mean, there was little, advantage in' the discussion found from 

 displacing the epoch. The average rise and fall for the second 

 day before the greatest declination was I 68 feet, for the day 

 next preceding the greatest declination 1*78, for the day of great- 

 est declination ISl, for the next day ISO, and for the next 1-77. 

 Tracing a curve from these would give the epoch of greatest rise 



and fall about 0*75 days after the greatest declination. The av- 

 erage rise and fall on the corresponding days, in reference to de- 

 clination zero, were 0-96 feet, 0*75, GO, (dec. zero,) 0-G3, 73, 

 the curve giving the epoch about one-sixth of a day after the 

 zero of declination. The numbers, as stated, require revision; 

 and there are causes for an apparent displacement, which requn'e 

 further examination, 



4. This general examination tends to point to the diurnal 

 irregularity, as Mr. Whewell has stated, as the cause of the occur- 

 rence of these single day-tides; a view which is confirmed by 

 such examinations as I have been able to make of the hourly 

 tidal observations at Fort Morgan, at the entrance of Mobile Bay. 



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