J. E. Hilgard—Tidal Waves and Currents. 123 
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 ebbtide from the 
tidal basin of New York harbor. 
Unlike Hell Gate passage, where permanence is the leading 
characteristic, the bar and channels of Sandy Hook have under- 
gone 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 increased a mile and a 
quarter. In the place where the beacon on the end of the hook 
now stands there were forty feet of water fifteen years before it 
was built. The cause of this growth is a remarkable north- 
wardly current along both shores of the Hook, running both 
during the flood and the ebb tides with varying rates, and result- 
ing from those tides directly and indirectly. 
The best water over the bar is about two miles east of Sandy 
Hook light, in a direct line with the Swash channel, which is 
the second opening, shown on the sketch, above the Hook ; the 
shoal lying between the main or Hook channel and the Swash 
channel being known as Flynn’s knoll. The greatest depth 
over the bar is twenty-two feet at mean low water; and very 
nearly the same depth can now be carried through the Swash 
channel, which formerly was three feet shallower, but has 
eepened since the cross section between the Hook and Flynn’s 
knoll has been diminished by one-third its area by the growth 
of the Hook. This relative change in the capacity of the chan- 
nels has not, however, affected the depth on the outer bar, 
which, according to the principles above laid down, is depend- 
ent mainly upon the area of the tidal basin within. 
The depth of twenty-two feet at mean low water, which is 
how maintained at the entrance, through the sands constantly 
thrown up by the waves of the sea, may be considered as de- 
oe Hila the following elements : 
_ Ast. The large basin between Sandy Hook and Staten Island, 
including Raritan bay, which furnishes more than one-half of 
the whole ebb scour. 
d. What is called the Upper bay, including the Jersey flats 
and Newark bay. 
3d. The North river, perhaps as far as Dobb’s Ferry, main- 
taming the head of the ah current, although not directly tak- 
ing part fs the outflow ; and, oe ene 
: j S i i ows in throu. 
Hell Bl a aus of the Sound tide, whic g 
