336 Tuttle — Changes in Elevation of Land and Sea. 



The above series of tidal observations from 1870-79 were care- 

 fully examined for use in determining the mean sea level — 

 M. S. L. — in New York harbor* and these values resulted : 



Mean sea level above Mean sea level above 



Year. 



zero of staff. 



Battery datum 



1870 



4-692 ft. 



2-033 ft. 



1871 



4-545 



1-886 



1872 



4-533 



1-874 



1873 



4-612 



1-953 



1874 



4*410 



1-751 



1875 



4-505 



1-846 



1876 



4-540 



1-881 



1877 



4-630 



1-971 



1878 



4-751 



2*092 



1879 



4-688 



2-029 



The zero of the tide staff at Governor's Island was found by 

 levelling to be 16*899 ft. below B. M. t in 1871-2 and 16*826 in 

 1875. Consequently the mean of these two values, 16*862 ft., 

 has been adopted as the correct value for the series of observa- 

 tions 1870-79. 



The elevations of half tide levelf— H. T. L.— from 1853 to 

 1869 are subject to considerable uncertainty owing to changes 

 in the elevation of the zero of the tide staff, of which no record 

 appears to have been made. Such changes must have taken 

 place in 4856, 1861, and 1870. In 1853 the zero of the tide 

 staff was recorded as 17*00 ft. below B. M. 1? but this location 

 does not harmonize with the succeeding observations and there 

 is much evidence to contradict it. 



We determine the probable elevations of H. T. L. at Gov- 

 ernor's Island for the years 1861-69, by making the average 

 H. T. L. for the four years 1866, '67, '68, and '69 have the same 

 elevation as the average for the four years 1870, '71, '72, arid 

 '73 : the elevations of H. T. L. for the years 1S56-60 are fixed by 

 giving the average of the observations for 1857, '58, '59, and '60, 

 the same elevation as those of 1861, '62, '63, and '64; and finally 

 we make the average H. T. L. for the three years 1853-55 the 

 same as the average of the three years 1856-58. Very nearly 

 the same values are arrived at if we start with the elevation 

 14*51 ft. of B. M.j above M. L. W. given in the Coast Survey 

 Report for 1853. We also observe that the series of 1861-1869 

 shows no tendency toward increased elevation with the time, 

 and for that reason they seem to confirm our location of the 

 1853-55 observations. 



These elevations are still further shown to be the most prob- 

 able ones, from the fact that in 1879 the elevation of B. M. x 



* Coast and Geodetic Survey Report for 1899, p. 404. 

 f Which is the average of high and low tides. 



