MEMOIR ON BOSTON HARBOR. 97 



vicinity agree with those on the Commissioners' chart, with which the comparisons have 

 been made, the difference in the reduction of the two charts being taken into account. 



The plane of reference adopted in the general chart of Boston Harbor, executed by 

 the Coast Survey, but not yet published, is mean low water. The depth of water on 

 some rocks in the lower harbor given in Wadsworth's chart coincides with the Coast 

 Survey determinations. Finally, the mean rise and fall of common and spring tides, ac- 

 cording to Wadsworth, harmonize better with the reduction to mean low water than to 

 any other plane. This question of the reduction of the soundings is thus carefully con- 

 sidered on account of its important bearing upon the preceding measurements. Their 

 strict accuracy depends upon the standard being correctly ascertained. If, however, 

 contrary to all the evidence in the case (and the comparison of the depths on the rocks 

 leaves but litde room for doubt), the very lowest observed or recorded tide should have 

 been used by the Commodore, the loss of capacity in the main ship-channel, though less 

 than above stated, would still be alarmingly great. 



The transfer of the sections of the Coast Survey chart to that of 1817 has been 

 made by means of rectangular coordinates, the axis of abscissas being drawn through 

 two distant points that had remained unchanged in the interval, and the centre being 

 the draw of Charlestown Old Bridge. The measures taken on these sections harmonize 

 with each other. Others might have been added, but these are abundantly sufficient to 

 answer the purpose in view. 



To the foregoing facts is to be added another more conclusive proof of the deterio- 

 ration of this part of the harbor, which results from a comparison of the chart of the 

 Commissioners of 1835 with that of the Commissioners of 1846. 



It is well known that the former survey was made with extraordinary minuteness 

 and exactness. The precise form of the bottom is laid down in measures of depth below 

 the coping of the dry dock, and the same plane of reference was adopted for the latter sur- 

 vey. The means of comparison, therefore, are strictly accurate, and the results are relia- 

 ble. The survey of the Commissioners of 1835 extended so far below the wharves as 

 to include the cross-section No. 1 of the Commissioners' survey of 1846. 



The area of this section (at low water) is on the former seventy four thousand eight 

 hundred square feet; on the latter, sixty two thousand one hundred feet; showing a loss 

 of twelve thousand seven hundred feet in the water capacity of this place. The breadth 

 of this section on the former is three thousand nine hundred and sixty feet ; on the latter, 

 three thousand five hundred and forty-six feet ; showing a decrease of four hundred and 

 fourteen feet. The profiles of these sections here presented exhibit the change in the 

 form of the bottom. 



