Prof. A. D. Bache on Tidal Observations 



351 



mean -f- 0-007 in the curve computed from observation. The 

 prdinates o? the semi-diurnal curve are each zero. T^vo days 

 afterwards, viz. March 13, gives for the diurnal curve, 000 feet, 

 0'18, 0-34, 0-47, 61, 68, 074; corresponding to which is the 

 curve of sines 000 feet, 0-18, 0-37, Ool, 063, 0-72, 074, in 

 which the greatest difference is 004 feet, and the mean —002 

 feet, the curve of observation having the least ordinates. The 

 semi-diurnal curve is 000 feet, 00, 0-03, 0*02. 



The average of three months, taken by weeks, gives for the 

 mean curve and curve of sines the following table : 



Hour: 







1 



2 

 3 



4 



5 

 6 



Diurnal Curve. 



Semi-Diuruul Curve. 





} 



From ObsorviU'n. 

 Feet- 



000 

 0*17 



0-32 



0-52 

 0-56 

 0-58 



Of Sines. 

 Feet- 



000 



0-16 



0-80 

 0-42 

 0*52 

 0-68 



Diff. 



0-02 

 0-02 

 001 

 000 

 0-02 

 002 



From Oliservation. 



Of i^iQed 



Feet. 





O'OO 



0-00 



0-04 



0-04 



0-07 



0-07 



om 



0-08 



Sum 0-01 



These results are shown by a curve in the diagram herewith 

 presented {PL 7, or No. 5,) on the full scale,* the greatest differ- 

 ence between the curve from the observation and the curve of 

 sines being less than a quarter of an inch in the mean deduced 

 from three months observations. Whether this will disappear in 

 the mean of more observations or whether a modification of the 

 hypothesis of displacement of nine hours must be made to meet 

 It, further computations now in progress will show. 



S. VVhen this analysis has been made as complete as possible, 

 and applied to the year's observations, it will remain to take up 

 the two series into which we have divided the observations, and 

 to discuss them numerically in detail, as we have heretofore done, 

 generally, in regard to the known laws of the diurnal irregu- 

 'anty, and of the ordinary tides. 



Each determination gives a corresponding value of the maxi- 

 "^um, or of the ordinate of high water, and in the case of the 

 ^ean of the curves for January, February, and March, these 



maxima are 066 feet, 0*65, 0-60, 60, 0-58, O'SS. 

 feet, differin 



Of 



o 



Mean 0-61 

 ^ 03 of a foot,' from the maximum found directly 

 "om the observations, and if the discrepancies are accidental, 

 8'vj"g a mean probable error by the variations from the average 

 of 002 feet (one-quarter of an inch) of any one of the deter- 

 "unations, and for the mean. 0-0 1 feet nearly. 



. y- By the kindness of Col. Abert, of the Topographical En- 

 gineers, of Major Bache, of the same corps, and of Lieut. Maury, 

 tsuperuuendent of the National Observatory, I have been put in 



* Reduced in the plate. 



