25O PHYSICAL GEOGRAPHY. 



which these altitudes have been determined, with our reasons for decision 

 in cases of disagreement, will enable those who have occasion to use our 

 figures to do so understandingly. 



In accordance with the example of eminent physicists, the standard to 

 which all our altitudes are referred is the level of the sea at mean tide. 

 Heights along railroads, unless otherwise specified, are taken on top of 

 the rail in front of passenger stations. 



Reference Line from Portsmouth, through Concord and White 

 River Junction, to Connecticut Lake. 



Several prominent lines of reference have been determined, which have 

 served as a basis for aneroid measurements and estimates. The first 

 commenced with mean tide water at Great bay, below the railroad bridge 

 between Newmarket Junction and Stratham, May 2, 1870. Messrs. 

 Frank and H. D. Woodbridge, then members of Dartmouth college, 

 levelled from here to Manchester depot, over the Concord & Portsmouth 

 Railroad. They found the mean tide water to be 10.7 feet below the 

 bottom of the rail. The centre of Manchester depot they found to be 

 180.832 feet above this mean tide water. 



The difference between Manchester and Concord depots was derived 

 from a comparison of several measurements, as follows : 



Between Manchester depot and Hooksett bridge, 24.565 



Hooksett bridge to Carter's bridge, Concord (J. A. Weston), . . 40. 



Carter's bridge to Concord depot (railroad survey) , . . . . 7. 



71.565 



This places the height of track at Concord depot 252.397 feet above 

 mean tide, 16 feet higher than previously supposed. The correctness of 

 this change will appear by the comparison of levels from this base over 

 the Northern and Concord & Claremont railroads, with lines of levels 

 from the sea by way of the Fitchburg and Cheshire railroads, and the 

 recent surveys for the Portland & Ogdensburg Railroad. A strong con- 

 firmation of this is further supplied by the Concord & Rochester Rail- 

 road survey, by Chas. C. Lund, hereafter given, the profiles of which 

 agree exactly with the corrected height of Concord. 



