TIDES 229 



The conformation of the coast line and the relation of the tidal 

 wave to it modify this interrelation of current and tide. Thus at 

 the mouth of the Elbe, at Cuxhaven, the reversal of the current 

 occurs I hour and 30 minutes after low water, and i hour and 25 

 minutes after high water. Thus during the first hour and a half 

 of falling tide a current still runs up the Elbe, and during the 

 same interval of rising tide the current still runs down. On Lon- 

 don Bridge one may observe that in the center of the Thames the 

 current still runs up stream, even after the water has fallen 2 feet, 

 while at the mouth of the Thames, at the Mouse lightship, the re- 

 versal of the current occurs 2 hours after high and low water. 



At the heads of small bays, and on shores where the tide comes 

 on broadside, slack water agrees with high and low tides ; all the 

 rising tide having a flood current, all the falling tide an ebb current. 

 This is, however, not the case when the tide progresses obliquely 

 along the shore. 



Interference of Tides. In bays or channels open in two direc- 

 tions, remarkable interferences may occur by the meeting of high 

 and low tides or by cross tides. "At New York high tide entering 

 from the harbor reaches the rocky narrows of Hell Gate when a 

 low tide arrives through Long Island Sound, and six hours later a 

 low tide from the harbor meets a high tide from the Sound." 

 (Davis-iSic?/. ) This produces a rapid back and forth flowing 

 current or tidal race, which made this passage a dangerous one to 

 vessels until the channel was widened by blasting away the rocks. 



Even more complicated interferences occur in the English Chan- 

 nel, especially near the Dover Straits, where the tides from the 

 Atlantic and the North Sea meet, with the production of the famil- 

 iar strong series of currents and waves. In the Irish Sea the meet- 

 ing of two tides of equal height from opposite directions, and with 

 a difference of phase of 12 hours, produces excessive tides (5 to 7 

 meters), but, owing to the direct opposition of movements of the 

 two tidal streams, complete slack water results, as is the case at 

 the Isle of Man. When, on the other hand, the phase difference is 

 6 hours, no rise or fall of the tide will occur, since a crest ap- 

 proaching, say, from the right, balances a trough approaching from 

 the left. In both the movement is to the left, that of the crest 

 being forward and that of the trough being backward, and so a 

 current of double strength will alternately flow in the one and the 

 other direction, slack water being halfway between high and low 

 water time. 



Where the waters are crowded in narrow channels, as between 

 islands or headlands, the tidal stream likewise becomes greatly 



