TIDES AND CURRENTS IN BOSTON HARBOR 



43 



Page 

 Boston Light, Little Brewster 



Island 43 



Lo veils Island 43 



Georges Island 44 



Chelsea Creek Entrance 44 



Boston (Charlestown) 44 



Boston (central portion) 45 



South Boston 46 



Page 



Cam-bridge 47 



Brookline 47 



Neponset River Railroad Bridge. _ 47 



Nut Island 48 



Weymouth Fore River Bridge 48 



Crow Point, Hingham Harbor 



Entrance 49 



The elevations of the bench mark are referred to the principal 

 tidal datums. Many of them have been connected directly by 

 spirit levels with the basic bench marks used in defining the datums, 

 and others are connected with the datums through simultaneous 

 water levels. The method used in referring the bench marks to the 

 datums is indicated at the beginning of each group of marks. 



BOSTON LIGHT, LITTLE BREWSTER ISLAND 



The following bench marks have been referred to the Boston 

 standard datums, through the primary tide station at Common- 

 wealth Pier No. 5, by simultaneous water levels covering the period 

 June 9 to July 21, 1921; the references to the local mean low water 

 are based upon the relations established through the several series 

 of observations at Boston Light: 



Bench mark 1 (established in 1860) is the center of a circle cut on top of a 

 ledge of stone and marked thus "C O S." It is about 340 feet westerly from 

 the lighthouse and 15 feet southeast of the southeast corner of a boathouse 

 (new in 1899). Elevation, 20.77 feet above sea-level datum; 25.64 feet above 

 Boston low-water datum; 25.30 feet above local mean low water. 



Bench mark 3 (established by United States Engineers) is the center of a 

 cross cut on a granite post on the western mound of the island. It bears the 



U. S. E. 

 inscription . Elevation, 20.60 feet above sea-level datum; 25. 47 feet above 



1885 

 Boston low-water datum; 25.13 feet above local mean low water. 



Bench mark 4 (established in 1899) is the top of an eyebolt, without a ring, 

 which is set in the rock westerly from bench mark 3 and is 15 feet north of the 

 northeast corner of the boathouse. Elevation, 15.22 feet above sea-level datum; 

 20.09 feet above Boston low-water datum; 19.75 feet above local mean low 

 water. 



Bench mark 5 (established in 1899) is the top of an eyebolt, containing a large 

 ring, which is set in a rock near and northerly from bench mark 4. Elevation, 

 13.69 feet above sea-level datum; 18.56 feet above Boston low-water datum; 

 18.22 feet above local mean low water. 



Bench mark 8 (established in 1916) is a standard Coast and Geodetic Survey 

 bench mark set vertically in ledge in front of keeper's house and near it. Eleva- 

 tion, 21.05 feet above sea-level datum; 25.92 feet above Boston low-water 

 datum; 25.58 feet above local mean low water. 



Bench mark 9 (established in 1926) is a standard Coast and Geodetic Survey 

 bench mark cemented in the rock 20 feet from the keeper's house toward the 

 lighthouse. It is stamped "9/1926." Elevation, 18.64 feet above sea-level 

 datum; 23.51 feet above Boston low-water datum; 23.17 feet above local mean 

 low water. 



Bench mark 10 (established in 1926) is a standard Coast and Geodetic Survey 

 bench mark cemented into the rock on the point making out northwest from the 

 keeper's house. It is stamped "10/1926." Elevation, 11.44 feet above sea- 

 level datum; 16.31 feet above Boston law-water datum; 15.97 feet above local 

 mean low water. 



LOVELLS ISLAND 



The following bench mark is referred to the mean low water as 

 adopted by the United States Engineers; the mean low water, as 

 derived from the very short series of observations in 1912 by the Coast 



