8 U. S. COAST AND GEODETIC SURVEY 



Tide observations were commenced in 1847, about 16 years after 

 the completion of the dock. Ten years later, in 1857, the tide observer 

 reported opening of joints between the stones under the bench mark, 

 estimated to be as much as 2 inches in some cases. About two years 

 later, in 1859-60, repairs were made which involved the relaying of 

 the coping stones. Other repairs involving the relaying of the coping 

 stones were made 10 years later, in 1869-70. 



Leveling connections. — Until the year 1867 the tide staffs were 

 referred solely to bench mark 1, the top of the coping at the entrance 

 to the dry dock. In 1867 bench mark 2 was established on the coping 

 near the head of the dry dock. 



The elevation of the old bench mark No. 1 above the new bench 

 mark No. 2 has been reported from time to time as follows: 



Foot 



June, 1868, A. C. MitcheU, Coast Survey 0. 170 



Feb. 8, 1870, A. C. Mitchell, Coast Survev • . 155 



July 26, 1870, A. C. Mitchell, Coast Survey . 153 



1878, city survey office, Boston, Mass . 21 



1882, Gilbert Hodges . 23 



January, 1900, Hyde & Sherry, civil engineers, Boston . 35 



1903, F. M. Miner, citv surveyor's office, Boston . 34 



1903, W. E. Spear, Charles River Dam, Boston . 34 



1903, H. B. Wood, State harbor commission . 37 



Aug. 9, 1916, N. H. Heck, Coast and Geodetic Survev . 37 



Oct. 13-16, 1923, E. B. Roberts, Coast and Geodetic Survey . 36 



Nov. 15, 1927, Dwight P. Robinson & Co. (Inc.), New York . 36 



The change from 0.176 in 1868 to 0.155 in 1870 was probably due 

 to the repairs in 1869, while the gradual change from 1870 to 1900 may 

 be assumed to be due primarily to the action of frost on the seams 

 between the stones. Between 1900 and 1927 the changes are small 

 and irregular. 



In 1916 the third bench mark, designated as bench mark 3, which 

 consisted of a standard disk of the Coast and Geodetic Survey, was 

 established on Dry Dock No. 2 at the navy yard. This dock was 

 built in 1905. The relation of bench mark 3 to the two older benches 

 has been determined by levels as follows: 



Below 

 B. M. 2 



Aug. 9, 1916, N. H. Heck, Coast and Geodetic Survey 



October, 1923, E. B. Roberts, Coast and Geodetic Survey 



November, 1927, Dwight P. Robinson Co. (Inc.), New York. 



Foot 

 0.52 

 .50 

 .49 



Assuming that bench mark 3 has remained fixed, the above results 

 suggest that both the older bench marks may have settled from 0.03 to 

 0.04 foot iDctween 1916 and 1927, although the differences are too small 

 to be conclusive. 



Tide scale. — In 1903 measurements under the direction of John R. 

 Freeman, chief engineer of the committee on the Charles River Dam, 

 were made between the footmarks of the inlaid tide scales on the walls 

 of the dry dock. It was found that the spacing between marks on the 

 same stone was always exactly 1 foot, but between marks on different 

 stones the difference was a Uttle in excess of 1 foot. On the tide scale, 

 directly under bench mark 1, the excess varied from 0.04 to 0.07 foot, 

 with a total excess of 0.36 foot between the top of the coping and the 



