342 Tuttle — Changes in Elevation of Land and Sea. 



detritus carried into the sea by rivers, and the wearing away of 

 shores, as well as changes in the position of the earth's axis 

 and speed of rotation. 



High and low waters only were measured in most of the 

 observations, and consequently EL T. L., which is the mean of 

 the above quantities, has been given in the tables, except in a 

 few instances. 



In general M. S. L. obtained from hourly ordinates, which 

 determines a level surface, differs from H. T. L. to a small 

 extent, depending in amount on the shape of the tide wave. 

 In deep water this difference is usually quite small, but in 

 shallow water, or where the tide wave is obstructed, it requires 

 to be taken into account. 



It has been computed from the tidal constants that M. S. L. 

 is 0-01 ft. above H. T. L. at Sandy Hook, and 0'067 ft. above 

 at Governor's Island. The differences are about the same at 

 Pier A, West 57th st., and East 24th st., as they are at Gov- 

 ernor's Island, so far as present information goes, and at Fort 

 Hamilton the difference is estimated to be about 0*03 ft. The 

 Sandy Hook observations, only, have been corrected to H. T. L. 

 at Governor's Island in the above tables. 



The settlement of bench marks would make the sea appear 

 to rise in relation to the land by the entire amount of the set- 

 tlement. Consequently all bench marks should be carefully 

 checked to guard against any change, and this has been done 

 in the case of B. M., and B. M. L. mentioned above. 



The accuracy of the tidal averages of H. T. L. obtained 

 from an automatic tide gauge varies somewhat with the type 

 of the instrument employed, and the attention given to it. 

 The results are most accurate w T here the tidal range is small, 

 and the working scale of the gauge large. An examination of 

 the yearly means of the various tidal observations goes to show 

 that the yearly averages of half tide level should not have a 

 probable error greater than 0*05 ft., although the maximum 

 error may be nearly three times as great. The greater part of 

 this probable error is due, however, not to the inaccuracy of 

 the tide gauge, but to fluctuations of the mean sea level from 

 meteorological causes. 



When we examine the daily changes of mean sea level in 

 JSTew York Harbor, we find that they are mainly determined by 

 the velocity and direction of the wind. N.W. and W. winds are 

 the effective agents in depressing the sea level and JST.E. and E. 

 winds in raising it.* The mean level has been at times raised 



*DesBarres in his charts of New York Harbor published in 1780, says, 

 *■ Tides rise perpendicularly about seven ft., but are sometimes checked to 

 such a degree by the Westerly or North Westerly winds, as to lower the 

 water on the bar to three fathoms and one-quarter, and Easterly or North 

 Easterly winds have frequently risen it to five fathoms." 





