ON TIDES AND TIDAL ACTION IN HARBORS. 225 



waves of the sea, may be considered as dependiug upon the following 

 elements: 



1st. The large basin between Sandy Hook and Staten Island, includ- 

 ing Earitan Bay, which furnishes more than one-half of the whole ebb- 

 scour ; 



2d. What is called the Upper Bay, including the Jersey flats and 

 Newark Bay; 



3d. The ISTorth Eiver, perhaps as far as Dobbs' Ferry, maintaining 

 the head of the ebb-current, although not directly taking j)art in tbe 

 outflow ; and, 



4th.. A portion of the sound tide, which flows in through Hell Gate. 



The i)roportion of the three first divisions in producing the depth of 

 channel may be approximately estimated by a comparison 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. In 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 — tbe " 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 re- 

 gard to the eflect of such encroachments, it is necessary to be in pos- 

 session 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 occu- 

 pation of the Jersey flats from Paulus Hook to Bobbins Reef for docks 

 and wharves. Without expressing any opinion as to the relative value 

 of the gain of accommodation for shipping and the loss of depth in 

 the channel, I venture to ^ay 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 oft' Sandy Hook, and certainly not more than two 

 feet. 



The part which the fourth division in our classification of the basin 

 of New York, that of the East River and Hell Gate passage, phiys 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, already adverted to. The westerly 

 15 s 



