TIDES AND CURRENTS IN BOSTON HARBOR / 



On February 9, 1870, Mr. Mitchell again wrote— 



I have altered the staff at the Charlestowia Navy Yard as you directed, so that 

 the 20-foot mark on the staff corresponds with the bench mark at entrance of 

 dry dock. The Vjench mark at foot of entrance of dry dock is 0.155 foot above 

 the bench mark at head of dry dock. These levels I ran over eight times on 

 three separate days (the only suitable weather I have had), and they all agree 

 with one another "within half a hundredth- — most of them nearer — so that I am 

 quite sure they are correct. The staff is attached to the same pile as before, liehind 

 the tide house, 8 or 10 feet from it. 



Change in location of gauge. — On July 19-21, 1870, the self-register- 

 ing tide gauge was moved to the next pier on the opposite side of the 

 same slip on which it had been established. The removal of the 

 tide staff was delayed to await the arrival of a leveling instrument, but 

 the staff was carried away by a colliding steamer on July 24 and 

 reinstalled July 26, 1870, with the 20-foot mark level with bench 

 mark No. 1. Apparently there was no further change in the position 

 of the tide station until it was discontinued on February 19, 1877. 



Later series at navy yard. — Since the primary station was dis- 

 continued in 1877 observations covering a few tides were made at 

 the navy yard in 1892, 1893, and 1894. These were taken in con- 

 nection with hydrographic work, but the series were too short to be 

 of any value except for comparison. 



In 1902 an automatic tide gauge was installed by the public works 

 department of the navy yard, and observations have been continued 

 to the present time. The records, however, have not been completely 

 tabulated, but such results as are available are included in this 

 publication. 



The datum to which these later observations are referred has been 

 reported as 14.54 feet below bench mark 2. If the elevation of bench 

 mark 1 above bench mark 2 is taken as 0.36 foot from the average of 

 all the levels between 1900 and 1923, this datum is found to be 14.90 

 feet below bench mark 1 . 



Reduction to a unijormdatum. — ^The reduction of all the observations 

 at the navy yard to a uniform datum must depend largely upon a 

 single bench mark, the top of the coping of the dry dock over the 

 inlaid tide scale used for the early observations. Unfortunately, 

 this bench mark has been subject to some changes, as already noted, 

 the principal changes being the expansion of the joints between the 

 stones of the dry-dock wall by the action of frost and the changes 

 due to relaying the stones at times of repairs. The exact amount of 

 these changes as they occurred at different times can not now be 

 determined, but we can draw some conclusion in regard to the prob- 

 ably maximum variations from such information as we have available. 



Dry DocTc No. 1, at Charlestown. — Together with one being built 

 at Norfolk, Va., at the same time, Dry Dock No. 1, at Charlestown, 

 is the oldest dry dock in the United States. It was authorized by 

 act of Congress passed March 3, 1827, and the coping was completed 

 in 1831. It was built under the direction of Loammi Baldwin, a 

 noted engineer of that period. In preparing for the foundation, exca- 

 vations were made through 18 inches of marsh mud, 5 feet of blue 

 clay, 13 feet of yellow sand and gravel, then hard clay interspersed 

 with small strata of sand, gravel, and bowlders. Piles were driven 

 3 feet from centers each way to hardpan or rock at depths of from 

 8 to 30 feet. The walls are of eastern granite, laid in cement mortar. 



