96 MEMOIR ON BOSTON HARBOR. 



precision and minuteness in details of the present mode of construction were not then 

 practised. The six-foot curves have been traced on Wadsvvorth's chart from the sound- 

 ings by myself, but the edge of the dotted surface is described on the chart itself as 

 the fourteen-foot curve, and this last limit, therefore, is strictly exact. The following 

 tables exhibit the breadths of the sections on both charts. 

 Between the six-foot curves : — 



Wadsworth. Coast San-ey. 



Cross-section No. 1 ... 4350 . . . 3333 



" " " 2 . . . 3000 . . . 3104 



« " "3 ... 3600 . . . 3104 



" " " 4 . . . 2700 . . . 2167 



Between the fourteen-foot curves : — 



Cross-section No. 1 . . .3150 . . . 2500 



" " " 2 . . . 2700 . . . 2650 



» " "3 ... 3000 . . . 1833 



« " " 4 . . . 1200 ... 708 



If these figures are summed up, it will be found that, in the first case, the mean loss 

 of breadth in this part of the channel has been four hundred and eighty-five feet ; and 

 in the second case, five hundred and eighty-nine feet. The average of the two is five 

 hundred and thirty-seven feet. 



Section No. 4 crosses the harbor at the narrowest part of the entrance above Castle 

 Island, that is, at the buoy of the Upper Middle, and here the loss is four hundred and 

 ninety-two feet. Again, there is a point on the northwest part of South Boston Flats, 

 where fourteen feet is marked on Wadsvvorth's chart ; on the same spot four feet only is 

 given by the Coast Survey chart, showing a loss of ten feet in thirty years. 



The fourteen foot-depth has been carried out by the encroachments on the channel, 

 if taken in the nearest direction, four hundred and fifty-eight feet ; but much farther if 

 measured on the line of the section, that being the direction in which the Flats have 

 gained most rapidly at this point. On the opposite side, there was apparently nine feet 

 in Wadsworth's time, where there is now four ; the loss of breadth between these two 

 depths appears by the same comparison to be three hundred feet. 



The zero of reduction of the soundings on Commodore Wadsworth's chart, or the 

 plane of reference, is said to be low water, by which is evidently to be understood mean 

 low water. This is the common plane of reduction, and any departure from it would 

 have been specially mentioned. The depths at Charlestown Bridge are known to be 

 the same now that they were when the bridge was built ; and the soundings in this 



