6S AMERICAN ASSOCIATION FOB THE ADVANCEMENT OE SCIENCE. 



inside of the Hook extending nearly one-third of the distance across Sandy Hook 

 Bay. These currents run to the north, during both the ebb and flood tide, with 

 varying rates, and result from those tides directly and indirectly. The inner cur- 

 rent is the one by which the flood and ebb tides draw, by the lateral communication 

 of motion, the water from Sandy Hook Bay, and the outer is similarly related to 

 those tides as they pass False Hook channel. The velocities and directions found, 

 favor this conclusively. 



An important observation for navigation results from this, for eleven hours out 

 of the twelve, there is a northwardly current running through False Hook channel, 

 which assists vessels entering New York harbor on the ebbtide, and is to be avoided 

 in passing out with the ebb. 



It is the conflict of these two northwardly currents outside and inside, and the 

 deposit of the materials which they carry to the point of the Hook, which causes its 

 growth. 



Within a century it has increased a mile and a quarter, and at about the rate of 

 one sixteenth of a mile a year, on the average, for the last twelve years. 



Flynn's Knoll, on the north side of the main ship channel, does not give way, as 

 the point of the Hook advances. The importance of watching this movement 

 cannot, therefore be over stated. 



The mode of controlling the growth is obviou3 from the result obtained. The 

 observations are still continued, to obtain the necessary numerical results. 



APPROXIMATE COTIDAL LINES OF DIURNAL AND SEMI-DIURNAL TIDES OF THE COAST OF THK 

 UNITED STATES ON THE GULF OF MEXICO-BY A. D. BACHE, SUPERINTENDENT UNITED 

 STATES COAST SURVEY. COMMUNICATED BY AUTHORITY OF THE TREASURY DEPART- 

 MENT. 



This paper is supplementary to those on cotidal lines of the Atlantic and Pacific 

 coasts heretofore communicated to the Association. Preparation was made at the 

 last meeting for these conclusions by presenting the type curves of the Gulf coast 

 The tides from Cape Florida to St. George's are of the usual type, with a large daily 

 inequality. From St. George's to the mouth of the Mississippi they are of the sin- 

 gle day type. Then the half-day tides reappear to extend beyond Galveston, the 

 day tides recurring at Aransas, in Texas, and southward. When the type curves 

 were presented, the mode of decomposing them with a diurnal and semi-diurnal 

 wave was described. The tide stations extend along our whole coast, but obser- 

 vations are much wanted beyond it to complete the investigation, on the south side 

 of the Straits of Florida, on the eastern coast of the Gulf of Mexico south of Texas, 

 and especially between Cuba and Yucatan, at the entrance of the Gulf from the 

 Caribbean sea. 



A table of the stations at which the observations were made, of the heights of 

 tide (rise and fall) observed, and of the half-day and day tides, was given ; and 

 another showing the period of observation and the name of the observer. The 

 first table is represented on a diagram by which a navigator may find the rise and 

 fall of tide approximately on any part of our Gulf coast. The least observed rise 

 and fall is at Brazos Santiago, Texas, and is nine tenths of a foot. The greatest is 

 at Cedar keys, Florida, and is two and a half feet. The difficulties of the problem 

 presented by these tides are explained, removable in part by the progress of the 

 survey of the Gulf, inherent in them in part. The labors of Mr. Pourtales and 

 other gentlemen concerned in the discussion of these tides are acknowledged. The 

 single-day tides have not been so elaborately discussed by former physicists or mathe- 



