ON TIDES AND TIDAL ACTION IN HARBOES. 223 



ing the sound at a rate indicated by the Eoman figures, with increasing 

 heights indicated by the Arabic numerals, it reaches Sand's Point 

 eleven and a half hours after the transit of the moon with a height of 

 seven and seven-tenths feet. The observed time of transmission from 

 the Eace to Sand's Point is two hours one minute, and the time com- 

 puted from the depths, according to the law developed by Airy, is two 

 hours fourteen minutes — a very good approximation, when we consider 

 the irregularities in the configuration of the sound, which could not be 

 taken into account. Advancing still farther, the height somewhat de- 

 clines in consequence of the changes of direction in the channel and 

 its shallowness. At Hell Gate this tide- wave is met by that which had 

 entered at Sandy Hook, and advanced more slowly, owing to the nar- 

 rowness and intricacies of the channel, especially in the East River. 



These two tides, which meet and overlap each other at Hell Gate, dif- 

 fering from each other in times and heights, cause contrasts of water- 

 elevations between the sound and harbor which call into existence the 

 violent currents that traverse the East Eiver. The conditions of the 

 tidal circulation through Hell Gate are such, that if there were a parti- 

 tion across it, the water would sometimes stand nearly five feet higher, 

 and at other times five feet lower, on one side than on the other. In the 

 actual case of the superposition or compounding of the two tides the 

 difference of level existing at any time is of course much less; but 

 the difference of one foot is often observed within the space of 100 

 feet in the most contracted portion of Hell Gate, off Hallett's Point 

 Eeferring now more particularly to the diagram representing iSTew 

 York Bay and Harbor, it is important to observe that the entrance from 

 Long Island Sound is a natural depression or arm of the sea, which is 

 not changed by the forces now in operation. The tidal currents which 

 flow through it do not change the channel, but are obliged to follow it 

 in its tortuous course. The Sandy Hook entrance, on the contrary, is 

 characterized by a cordon of sands, extending from Sandy Hook to 

 Coney Island, intersected by channels, which are maintained against 

 the action of the sea, that tends to fill them up, by the scour of the ebb- 

 tide from the tidal basin of New York Harbor. 



Unlike Hell Gate passage, where permanence is the leading charac- 

 teristic, the bar and channels of Sandy Hook have undergone continual 

 changes within the brief period of our history. The advance of Sandy 

 Hook upon the main ship-channel is among the notable and important 

 instances of the effect of tidal currents. Within a century it has in- 

 creased a mile and a quarter. In the place where the beacon on the 

 end of the Hook now stands were forty feet of water fifteen years before 

 it was built. The cause of this growth is a remarkably northwardly 

 current along both shores of the Hook, running both during the flood 

 and the ebb tides with varying rates, and resulting from those tides 

 directly and indirectly. 



The best water over the bar is about two miles east of Sandy Hook 



