280 REPORT— 1876. 



Eemark an abrupt diminution in the height of mean level after the first 

 two years, which the following extract from a letter received from Prof. 

 J. E. Hilgard fully explains : — 



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

 " annoyance to us. The tide-gauge Avas put iip in a small building near the 

 " end of a wharf, and the tide-staff used for comparison was close to it. 

 *' Now it was observed after the observations had continued some time that 

 " the wharf was settling, at 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 underlying 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 

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



Cat Island, Gulf of Mexico (Lat. 30° 23' N., Long. 5" 56'° W. of Greenwich). 



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

 present been evaluated. Datum 10 feet helow datum of United-States Coast 

 Survey : — 



Year 1848. Ao=4-8574ft. I = i8°-45. 



It is extremely interesting to find that, although the lunar and solar 

 semidiurnal tides are very small in value, the series of means from which 

 they were obtained being extremely regular and good, the consequent de- 

 termination of the phase of spring-tides from their respective epochs is 

 probably correct within a few minutes. The proportion between the ampli- 

 tudes of the lunar and solar semidiurnal tides is the nearest approach to 

 equality yet obtained, being in the ratio of 11 to 6. The comparatively large 

 value of 11 of Series S is undoubtedly a genuine tide ; but the smallness of the 

 corresponding value of Series M must forbid the conclusion of its being purely 

 astronomical. It is perhaps produced by temperature or wind, its time of 

 maximum being about 40 minutes after noon. There are also indications of 

 a similar and large annual tide of 0-274 foot amplitude, and maximum about 

 Aug. 16, which is also probably meteorological in its origin. The proportion 

 between the lunar and solar diurnal (Declinational) tides (E^ of Series and 

 P) will be, on the assumption of the variation of E^ of Series being as the 

 square of the sine of the declination, about 4 to 1. 



The following are the values of the long-period tides : — 



