20 Co-tidal lines of the Atlantic Coast of the United States. 
Group econtains Newburyport, Portsmouth, Portland, Fourchue 
Island, Ellenwood’s Island, and Cape Sable. ‘The mean co-tidal 
hour is 14h. 37m., the angle of the co-tidal line with the meridian 
44° 04’, and the motion of the tide-wave thirty miles in 30 minutes. 
The average difference of the computed and observed co-tidal 
hour is rather more than ten minutes, and ihe greatest difference 
in the case of Cape Sable Island amounts to 23m. Newburyport, 
which is bronght to the sea establishment by the depth, differs 
but six minutes in the computed and observed numbers 
The twelve stations of the last two groups combined in one 
group, give 14h. 56m., for the mean co-tidal hour, 38° 46’ for the 
inclination of the co-tidal line, and thirty-three miles in half an 
hour for the motion. 
etween ¢ and d is the anomalous group which occupies 
Martha’s Vineyard and Nantucket sounds, and includes the sea of 
the southern and eastern shores of these islands, and the passage 
between them. A side sketch shows the anomalous co-tidal 
yogi varying from 12h. L9m. at Menamsha Bight, and 12A. 3820. 
Wood's Hole, to 164. 02mm. at Holmes’ hole. The observations — 
“i collected are not sufficient to trace with precision the details — 
of these changes, though abundantly so to establish the general 
phenomena. They show, conclusively, that this is a case of in-_ 
terference, and point to the nature and amount of it. I prefer to | 
obtain further observations necessary to give the particulars of @ 
this interference, before entering upon a discussion of this curious © 
series. he heights concur with the times in giving the same 
solution to these cases, a mean rise and fall of one foot at Nan- 
tucket island being placed, as it were between a rise and fall of 
3:3 feet at Menamsha Bight, and of 11-5 feet at Provincetown. 
The shoal ground off Nantucket and its influence in the direction 
of the co-tidal lines, are roughly traced upon the chart. 
Group f is at the extreme southern portion of the series where 
the tide-wave is turned by the Bahama banks, and makes its way 
through the straits of Fiorida. The three gee: in that group 
are Cape Florida, Cape Canaveral, and St. John’s entrance; of 
this Cape Canaveral affords but a vague Rae The co-tidal line 
makes an angle of 117° 12’ with the meridian, and the motion in 
half an tie: is twenty-nine miles. These results are collected in 
Table No. 3. 
Fro i the general indications of these groups, and with the ap- 
proximate form of the curves of depths, it is not difficult to trace 
the probable forms of the co-tidal lines. With the assistance of 
Mr. Pourtales the chart now presented has been ub 
shows the co-tidal line of 12., aves the trend of the southern 
coast from Tybee towards ras, eth close to the shore 
as it passes southward into ra aisaice of Florida, oven ae be- 
tween Cape Lookout and Hatteras in passing north’ and east- 
