ent the great inland basins on the southern portion of Long
Island communicate with the sea only by narrow passes obstructed by bars
and shoals; yet, in spite of the dangers which are always presented,
large fleets of market vessels pass out daily through the inlets, laden
with farm produce and shell fish. It requires no thought to perceive
that if these inlets were made safe and permanent by suitable marine
constructions, and were furnished with the proper buoys and beacons,
there would spring up in their neighborhood great commercial
enterprises.

While, in the case of the lower harbor and its approaches, it was the
design of the observations to detect in the movement of the waters the
causes of alterations in the physical geography, the same kind of
studies, undertaken afterward in Hell Gate, had for their object the
reverse inquiry, viz., to ascertain to what degree and in what manner
the form of the rocky channel influenced the tides and currents, in
order that some prediction might be made of the consequences likely to
follow the removal of obstructions from the waterways. The propagations
of the tide wave meet at Hell Gate, so that here the observations, when
plotted, exhibit compound curves, in which the portion due to the wave
from Sandy Hook is easily distinguishable from that due to the wave from
Long Island Sound. The Sandy Hook tide wave differs so widely in height
and time from that of Long Island Sound, that there is over three feet
difference of level between the harbor and the Sound at certain stages
of the tides; and at these times the currents rush through the Gate,
vainly endeavoring to restore the inequalities.

The problem of referring a current to a _tidal head_ is a very difficult
one. The current, for instance, which renders Hell Gate so dangerous, is
not at any time so great as a _permanent head_, equal to the difference
of the tides observed, would engender. The currents are so very slow in
their movements, compared with the undulations of the tide wave, that it
cannot be ascertained as yet, what are the magnitudes of such elements
as _inertia_ and _friction_, and how they are to be corrected for, so as
to predict the time and velocity of the current from observations of the
vertical rise and fall.

It is due to the officers of the Coast Survey to state that their
services to the Harbor Commissioners were rendered gratuitously; the
work offered to them only an opportunity for research.

Th