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Co-tidal lines of the Atlantic Coast of the United States. 19 
The motion perpendicular to the direction of the co-tidal line 
is twenty-four miles in half an hour, agreeing nearly with the ve- 
locity due to the depths, as will be seen by inspecting the chart 
of co-tidal lines which accompanies this paper, upon which the 
fifty and one hundred fathom curves are drawn from the best data 
we yet have. ‘The mean discrepancy of the computed establish- 
ments and of those observed for this group, is 16m. ; that of Beau- 
fort differing most, and for Cape Fear the least. 
If no correction had been applied for the positions of the tide- 
gauges within harbors, the results would have been as stated in 
gtoup @ bis, ‘Table No. 3, which, while the position of the co- 
tidal line is but little changed, gives a result for the movement of 
the tide-waves which is entirely too small, as must of course be 
the case. 
_ The next group north and east of this, 6, consists of Old Point 
Comfort, Delaware Breakwater, Cold Spring inlet, and Sandy 
Hook, embracing a part of the coast having the same general di- 
fection. The mean co-tidal hour is 12A. 18m. ; the angle of the 
co-tidal line with the meridian is 22° 21’, agreeing again with the 
general trend of the coast, which is about 26°, the true value of 
the motion being masked by the irregularities of the establish- 
ments. The compnted establishments agree well with the ob- 
- Served, differing but five minutes in the average, and six minutes 
at the greatest. 
In the next group, ¢, four stations are placed from Sandy Hook 
to Quick’s Hole, viz: Sandy Hook, Point Judith, Newport, ear- 
tied to the entrance of the bay by the depth, and Quick’s Hole ; 
the mean co-tidal hour is 12h. 6m., the direction of the co-tidal 
line 63° 27’ east of the meridian ; the distance gone over by the 
tide-wave in half an hour, forty miles; the mean difference in 
the computed and observed establishments is less than three min- 
Ules, and the greatest difference five minutes. Fire Island and 
ontank are omitted from this series as anomalous ; if, however, 
they are included, the result shown in ¢ bis, Table No. 3, is given, 
iN Which the direction of the co-tidal line is 58° 36’, and from 
Which a decidedly erroneous velocity results. 
Montank looks like a point of convergence, as termed by Mr. 
€well; but its result is uncertain, and the observations there 
and at Fire Island must obvious! y be repeated : 
€ group d consists of six stations between Cape Cod and 
Cape Ann, viz: Provincetown, Wellfleet, Boston, Nahant, Salem, 
and Gloucester. It gives for the mean co-tidal hour 15/. 15m., 
for the inclination of the co-tidal line to the meridian 31° 17, 
and for the distance gone over by the tide-wave in 3Um., thirty- 
two miles. The average difference between the compnted at 
observed co-tidal hour is less than four minutes, and the greatest 
difference Jess than ten. 
and 
