198 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



The progress of high water in Chesapeake Bay is shown by the 

 location ofcotidal lines. Inside the entrance to the bay, high water 

 occurs 1 hour after the time of high water off the coast. High water 

 occurs on the lines shown at the second hour, third hour, and fourth 

 hour. At the fifth hour, high water occurs as the progressive wave 

 reaches Smith Island ; at the sixth hour it is high water in the lower 

 Potomac ; on the eighth hour, at Chesapeake Beach ; on the tenth, at 

 Love Point on Kent Island; and on the eleventh hour near the en- 

 trance to Baltimore Harbor. At the next line, high water occurs at 

 the end of the 12-hour cycle. At this time the stationary wave has 

 induced another high water along the cotidal line off the coast. For 

 the next 2 hours, high waters occur at the extreme upper and lower 

 sections of Chesapeake Bay at the same time. 



In lower Delaware Bay, between the first and second hour, high 

 water advanced about 25 miles ; during the next hour it advanced 15 

 miles farther. High water occurs at Chester about the fifth hour and 

 at Philadelphia about 1 hour later. 



The natural shapes of these two large bays have appreciable effect 

 on the range of the tide at various places. A tide-gage station on a 

 pier jutting several hundred feet into the ocean at Atlantic City, N. J., 

 has recorded a mean range of 4 feet. In Chesapeake Bay, owing to 

 the relatively shallow depth, which tends to absorb the tidal energy, 

 and the increased width above the entrance, the normal range of tide 

 is reduced ; near the mouth of the Potomac it is about 1 foot. On the 

 other hand, the funnel-like shape of lower Delaware Bay increases the 

 range from 4 feet offshore to 6 feet at Philadelphia. 



In both these bays, especially in the Chesapeake, high winds will 

 disturb the normal range and time of high and low waters. Strong 

 southeast winds cause water heights to rise and northerly winds to 

 reduce them several feet from the normal levels. Hurricane winds 

 have increased heights 5 to 6 feet. 



TIDAL PREDICTIONS 



In the early part of the last century there was need for predictions 

 of tides for many seaports of the world. It was necessary to know 

 when high and low water would occur, and what the amount of the 

 rise and fall of the sea would be. The depth of water in many ports 

 was a critical feature which determined the time of entry, loading, 

 and departure for sailing ships. It was widely recognized that the 

 predominant feature of the tidal movement was the relationship of 

 time of high and low waters to the time of the moon's passage over- 

 head; and that the higher or lower tides occurred during certain 

 phases of the moon. A series of observations of the tide at an iso- 

 lated port were usually sufficient to furnish this connection approxi- 



