206 HAUPT—DEEPER NAVIGABLE CHANNELS. [April 2, 
comes the tool for the conversion of the effluent energy into useful 
work, with lateral transportation of the eroded material. 
REACTION VERSUS VELOCITY. 
The opinion is prevalent that the deep pockets frequently ob- 
served at the ends of spurs or obstacles or at contractions in rivers 
are due to velocity, and it has been stated that because a mean ebb 
velocity of two feet per second maintains a depth of over 100 feet 
at the Narrows of New York Harbor, therefore a similar contraction 
on the bar near Sandy Hook would produce some such depths. 
This was made the basis in 1886 for a proposition to build a jetty 
nearly five miles long, closing three of the channels across the 
New York bar. The great depth at the Narrows is not a velocity 
but a reaction depth, due to the resistance which the converging 
shores oppose to the passage of the flood, not the ebb tide, which 
increases the head before reaching the pass, depressing the resultant 
to the bottom, from which it reacts and scours out a depth to com- 
pensate for the lateral contraction. At other points in the harbor 
velocities of more than two feet per second do not scour to depths 
exceeding three feet, so that the results must be ascribed to some 
other cause than mere velocity. 
An extended investigation of these abnormal depths leads to the 
conclusion that they are caused by eddies operating in a vertical 
plane, these eddies being caused by obstacles placed in the path of 
a current in such manner as to retard the flow by the interference 
due to converging forces, thus creating a head with a downward 
resultant and scour until compensation is secured by enlarged aper- 
ture. Here the reaction produces a change of direction of the re- 
sultant, which is deflected upward with dispersion of energy, deposit 
of material and ultimately restored equilibrium. These facts are 
doubtless well known to many observers, but the particular point to 
which attention is directed in this connection is that the downward 
movement producing scour is supplemented necessarily by the up- 
ward resultant, accompanied by deposit in the same vertical plane, 
so that whichever way the eddy operates, whether with flood or ebb, 
the bar is a sequence of the pocket, unless other forces come to the 
rescue. Thus it appears that an eddy both scours and deposits. 
These effects are reciprocal results of the same eddy, and not of two 
separate ones. But eddies also operate in horizontal planes, and 
with like results. When the obstacle is limited in extent the effect 
