T utile — Changes in Elevation of Land and Sea. 341 



1893-1902, give 2*304: ft. as the elevation of H. T. L. at Pier A, 

 while H. T. L. at Governor's Island 1870-79 was 1*858 ft., an 

 increase in the elevation of H. T. L. of *446 ft. in twenty-three 

 years, or 1*94 ft. per century. 



(4) The observations at Governor's Island 1870-79 give 



Elevation of M. S. L. 1870-74 1*899 ft. 



Elevation of M. IS. L. 1875-79 1*964 



An increased elevation in M. S. L. of 0*065 ft. in five years, 

 or 1*30 feet per century. 



(5) The observations at Sandy Hook for the seventeen years 

 1876-1892 give : 



Elevation of M. S. L. 1876-1881 6*883 ft. 



Elevation of M. S. L. 1887-1892 ..... 7*038 



An increased elevation in M. S. L. of 0*155 ft. in eleven years, 

 or 1*41 ft. per century. 



(6) The observations for the fourteen years 1889-1902 at 

 Pier A, when taken by themselves, give : 



Elevation of H. T. L. 1889-1895 . 2-127 ft. 



Elevation of H. T. L. 1896-1902 2-345 



An increased elevation in H. T. L. of 0*218 ft. in seven years, 

 or 3*1 ft. per century. 



(7) The average of the observations of H. T. L. for the ten 

 years 1870-1879 at Governor's Island, compared with all the 

 above mentioned observations at New York for the nine years 

 1894-1902, shows that H. T. L. has risen *326 ft. in twenty- 

 two and one-half years, or at the rate of 1*45 feet per century. 



(8) The Department of Docks and Ferries of New York City 

 found it necessary in 1898 to raise their plane of mean low 

 water (to which soundings are reduced) 0*24 ft. above that in 

 use since 1872 ; and the U. S. Engineers in 1900 raised their 

 plane of mean low water 0*32 ft. above that used in 1872. 



On account of the many disturbances to which tidal observa- 

 tions are subject, the frequent change of tide staffs in some 

 cases, and the settlement of bench marks, it is necessary to use 

 considerable caution in the interpretation of the observations, 

 and all possible checks should be used. Only when independ- 

 ent determinations show a reasonable agreement, can we feel 

 sure of the result arrived at. 



Besides the errors which may occur in the measurement of 

 the elevation of mean sea level, it is subject to changes from 

 many causes, among which may be mentioned changes in 

 atmospheric pressure, winds, temperature, various tidal com- 

 ponents having periodicities ranging from six hours to nine- 

 teen years, river outflow, evaporation, changes in ocean cur- 

 rents, melting of polar snow and ice, changes in salinity, 



