124 J. FE. Hilgard—Tidal Waves and Currents. 
The proportion of the three first divisions in producing the 
depth of channel, may be approximately estimated by a com- 
parison of the areas and distances from the bar. In order to 
maintain the depth which we now have, it is important that the 
area of the tidal basin should not be encroached upon. 
proportion as that is diminished the depth of the channels will 
decrease. 
The flats, just bare at low water, but covered at high tide, 
form as important a part as any other portion, for it is obvious 
that it is only the volume of water contained between the 
planes of low and high water, the ‘tide prism,” that does the 
work in scouring the channels. The water on the flats is 
especially useful by retarding the outflow, thus allowing a 
greater difference of level to be reached between the basin and 
the ocean. 
When we yield to the demands of commerce any portion of 
the tidal territory, to be used for its wharves and docks, we 
must do so with full cognizance of the sacrifice we are about to 
make in the depth of water over the bar; and in order to form 
any well-founded judgment in regard to the effect of such en- 
croachments, it is necessary to be in possession of the fullest 
knowledge of all the physical facts involved in the problem, 
and no measure of encroachment should be determined upon 
except in pursuance of the advice of scientific experts. 
A proposition, frequently mooted by men of enterprise, and 
resisted by those interested in the welfare of the city of New 
York, is the occupation of the Jersey flats, from Paulus Hook 
to Robbins Reef, for docks and wharves. Without expressing 
any opinion as to the relative value of the gain of accommoda- 
tion for shipping and the loss of depth in the channel, I ven- 
ture to say that the withdrawal of that area from the domain 
of the tide would occasion a loss of not less than one foot in 
the depth of the bar off Sandy Hook, and certainly not more 
than two feet. 
e part which the fourth division in our classification of the 
basin of New York, that of the East river and Hell Gate pas- 
sage, plays in the outflow of the ebb-tide through the Sandy 
Hook channels depends less upon the area involved than upon 
the difference in point of time and height of tide in Hell Gate 
ready adverted to. The westerly current, usually called the 
ebb stream since it falls in with the ebb stream of New York 
harbor, taking place when the sound tide is highest, starts from 
a level of three and half feet higher than the easterly, and thus 
a much larger amount of water flows out through the cane 
Hook channels than through the narrows at Throg’s Neck. It 
is apparent, then, that this portion of the ebb stream re-entore- 
ing as it does the ebb stream of the harbor proper, at the most 
