24 Co-tidal lines of the Pacific Coast of the United States. 
Co-tidal Hours. 
The cotidal hours thus far obtained between San Diego and 
Cape Disappointment, Columbia river, are contained between 
174 20 and 204 10™, increasing as a general rule, but with strik- 
ing exceptional cases, and not regularly in passing northward. 
The cotidal hour of 175 20™ characterizes the two stations in 
the southern reach referred to in the description of the coas 
185, 19>, 20% are found on the middle reach, and 204 gharectér- 
ises the northern. 
Co-tidal Groups. 
In discussing these results I have followed the same course as 
in the paper on the cotidal lines of the Atlantic, dividing the sta- 
tions into natural groups, end e epplying Lioyd’s mode of discus- 
sion of magnetic lines to t 
The northern group of stations. between Cape Disappointment 
and Cape Mendocino, (see plate) is composed of Cape Disappoint- 
ment, Port Orford, and Humboldt. The mean cotidal hour is 
194 58™, The mean of the longitudes of the stations is 124° 12’; 
the mean of latitudes 42° 15’. Calling the differences between 
the mean longitude and the longitude of each station when re- 
duced to nautical miles 2, the differences between the latitude of 
each station and the mean y, the difference between the cotidal 
hee at each station and the mean cotidal hour z, and assuming 
x as the sign of the algebraic sum of the numerical quantities 
obtained for the co-efficients of the equations furnished by each 
station, we form and solve the equations: 
M2 ¢°+Nrrcy=22z 
N22 yt+Ms y?=Zy z. 
In the case before us M gives for the co-efficient of the longi- 
tude 1-2 and N for that of latitude, 0-006. The tangent of the 
angle which the cotidal line makes with the meridian = =0°05 
and the angle is 2° 52’. The distance in ore miles perpen- 
dicular to the cotidal line corresponding to one minute of estab- 
lishment or V M?+N2? is 1-2 miles, and aicces the progress of 
of the tide wave in one hour, 50 miles. This is a velocity less 
than the depth would indicate to be correct, and from the small 
differences in the establishments of the stations, this must be an 
uncertain datum. We shall see however that in the next group 
where the establishment varies more considerably this datum is 
still less probable than the one here obtained. 
The direction of the line is nearly coincident with that of the 
trend of the coast, the cotidal angle being 2° 52’ and the gene- 
ral trend of the coast differing but two degrees from it. 
