206. REPORT— 1871. 



Series fi. 



89. Here again we have an abrupt diminution in the height of mean level 

 for the first two years, which the following extract from a letter received 

 'from I. E. Hilgard, Esq., fully explains: — 



" The change in the mean-level reading ^t Fort Point is a matter of much 

 " annoj'ance to ns. The tide-gauge was put up in a small building near the 

 " end of a wharf, and the tide-stafi' used for comparison was close to it. I^ow 

 " it Vas observed after the observations had continued some time-that the 

 " wharf was settling,— ^t least the part where the gauge stood. Then the 

 " gauge was moved to a point a little nearer to the shore believed to be firm, 

 " but we think the whole wharf settled and continued to do so for years. 

 " There seems to be a bog formation uuderlpng the surface deposit at that 

 "place. There is probably- no way of ascertaining the amount of settling 

 " except from the observations themselves. We are now having frequent 

 " levellings made, referring the tide-staff to a rocky ledge further inland." 



It is contemplated including the new tide-components now evaluated 

 in the calculation of the tide-heights shown in § (J9, doubtless to their 

 improvement. 



90. It having come to the knowledge of the Tide.Committee that the United 

 States' Coast Survey Office was in possession of a series of hourly tide observa- 

 tions, taken at Cat Island in the Gulf of Mexico, and which were of a very 

 remarkable and interesting character, it was thought a favourable opportunity 

 of testing the value of the harmonic analysis for the evaluation of the com- 

 ponents of the tides of this place, which appeared very complicated ajid pe- 



'euliar. Application having been made, a series of about thirteen months 

 were received through the kindness of J. E. Hilgard, Esq. These are now 

 in course of reduction. 



The following results represent the tide-components as far as they have at 

 present been evaluated. Datum 10 feet beJoiu datum of United States' Coast 

 Survey : — 



Year 1848. 10=4-8574 ft. I = i8°-45 



Series S. Series M. Series L. Series N. 



Rj 0-0442 o-oioi 



61 io°-o4 95°'ii 



~" R., 0-0677 0-1195 0-0118 0-0269 



e', 23^-80 io°-75 222°-40 33°'57 



Series K. Series O. Series P. Series J. Series Q. 

 Rj 0-4627 o"3855 0-1559 0-0292 0-0733 



55°-20 224°-29 230°-65 28°-22 2I5°-32 



e 



R., 0-0205 

 el 288°-73 



Retardation of phase of Spring-tides o<> izi- 5 1'-M ,, ^. j^qo^'s svivgies 



Coincidence of phase of Declinational tides o" 6'' 1 5™ J •' •' ° 



