ON TIDES AxND TIDAL ACTION IN HARBORS.^ 



By Professor J. E. Hilgard, of the U. S. Coast Survey, Washington, D. C. 



Ladies AND Gentlemen: I propose to eugage your attention this 

 evening with the subject of the tides of the ocean and the influence ex- 

 erted by tidal currents on our harbors. I shall first briefly describe the 

 phenomena of the tides as they present themselves to an observer, then 

 consider the physical causes to which these phenomena are due, next 

 examine more in detail the phases of the tide on our own coasts, and 

 finalh^ describe the tidal hydraulics of the magnificent harbor of Kew 

 YoTk. 



The most obvious change in the surface of the ocean to be noticed 

 upon our shores is the alternate rising and falling regularly twice in 

 every day. Closer attention will show that the tides of each day occur 

 somewhat later than those of the preceding day, the average time of re- 

 tardation being fifty-two minutes, and that this retardation corresponds 

 to that of the moon. It will pass as a fair approximation to say, that it 

 is high water at New York with a southeast moon, or similarly for New 

 Castle, on the Delaware, that high water occurs when the moon is south 

 In fact, so closely is the time of tide connected with the position of the 

 moon, that in order to give the time of high water upon any day ap- 

 proximately it is customary to state the time of high water on the days 

 of the new and full moon, when the moon passes the meridian at twelve 

 o'clock, nearly. This time is called the " establishment of the port." 

 Then, to find the time of high water on any other day, it is only neces- 

 sary to add the " establishment" to the time of the moon's meridian 

 passage on that day. On closer examination, it will be found that the 

 interval between the time of the moon's passage over the meridian and 

 the time of high water, called the luni-tidal interval, varies with the 

 moon's age very sensibly. Moreover, the elevation at high water and 

 depression at low water will not always be the same, but will be great- 

 est about the times of new moon and full moon, and least about the first 

 and third quarters. The details of these variations will be best traced 

 out in connection with the explanation of their causes, to which we will 

 now proceed. 



The popular explanation of the tides, as depending on the law of gravi- 

 tation, is sufficiently simple, although the complete mathematical inves- 

 tigation of the subject, by which we should be enabled to predict their 

 * Delivered before the American Institute, January 27, 1871, with revision. 



