162 ANNUAL OF SCIENTIFIC DISCOVERY. 



TIDES OF THE GULF OF MEXICO. 



AN interesting paper was presented to the American Association at 

 New Haven by Prof. Bache, on the tides of the Gulf of Mexico, as ob- 

 served by the officers of the Coast Survey, at Cat Island. The tides of 

 the Gulf of Mexico, said Prof. Bache, present interesting peculiarities. 

 They are generally, not universally, single day tides. Those at Cat 

 Inland are the type of this class. The discussions applicable to them, 

 reduce;! to rule, would answer for all others of the class. The prog- 

 ress of such a discussion he proposed to present. The tides at the en- 

 trance to Mobile Bay are in part reduced, and new observations are be- 

 ing made there and at other points in the Gulf. The observations at Cat 

 Island were made hourly, day and night, for a year. The average rise 

 and fall is but one foot. There is one high and one low water, as a 

 rule, in the twenty-four hours. The wind is supposed by navigators 

 generally to cause these tides, but the hypothesis, when carefully ex- 

 amined, falls to the ground. The time of high water advances as the 

 lunar day gains on the solar, until suddenly it shifts nine, ten, and 

 twelve hours. The low waters follow the same law. The times of 

 change are at or near the period when the moon crosses the equator. 

 This points to the diurnal inequality, as shown by Mr. Whewell, as 

 the source of the phenomenon. 



An establishment useful to the navigator may be obtained by con- 

 sidering the luni-tidal intervals for the superior and inferior transits 

 according to the moon's place north or south of the equator. Ordi- 

 nary modes of discussion fail entirely. The curves of hourly observa- 

 tions for the year leave no doubt that the declination changes of the 

 moon are those first to be looked to. Ordinary double or six-hour tides 

 occurred always at and near the period of no declination, when they 

 were near the quadratures. The tables for the whole year showed 

 this, and the comparison of tides at the zero and maximum of declina- 

 tion showed that the time of occurrence (epoch) corresponded very 

 nearly, at a mean, with the moon's position. The explanation of the 

 tides was to be found in the interference of the semi-diurnal and of the 

 diurnal tide waves. 



Prof. Bache went on to develop the theory, and show that it accords 

 nearly with the observations, and remarked that, should farther exam- 

 ination confirm his conclusions, the two tides could be separately ob- 

 tained from the observations, and discussed according to the known 

 laws of lunar and solar action. 



CURRENT CHARTS. 



PROF. BACHE described to the American Association at New Haven 

 the method used in the Coast Survey for showing the results of cur- 

 rent observations. The paper was illustrated by diagrams of Boston 

 Harbour. 



" Observations being multiplied at different periods of the current, 

 from slackwater to slackwater, they are projected upon diagrams show- 

 ing at a glance the direction and velocity at any particular station. 





