258 
MAGNETIC SURVEY IN NORTH AMERICA. 
corrections are made in the tracing of the lines as may appear to be required. Ob- 
servations which, from station error or otherwise, cause their mark to fall amongst 
those of a different degree from themselves, are given a distinctive sign, so that they 
do not mislead, whilst at the same time they are kept in view. The number of 
stations where local influences of this magnitude were found to prevail, amounts to 
eleven in the 335 stations at which the Inclination is determined in this memoir. 
By this process each isoclinal line is deduced independently of those on either 
side of it, and distinctive features in each are shown, whether resulting from the 
general system or from district anomalies. 
The northern observations of Lieut. Lefroy extend so far to the westward that 
they overlap the meridians of some of the stations of Sir Edward Belcher, re- 
ported in the IVth Number of these Contributions. I have therefore included the 
four following stations of Sir Edward Belcher in this Map, and the isoclinal lines 
of 70° and 76° are thus carried across the whole continent of America from sea to sea. 
Latitude. 
Longitude. 
Inclination. 
Port Etches . . . 
. 60 21 
213 19 
76° 02-9 
Sitka 
. 57 03 
224 36 
75 49T 
Baker’s Bay . . . 
. 46 17 
235 58 
69 267 
Fort Vancouver . . 
. 45 37 
23 7 24 
69 22-2 
The lines drawn on the Mercator’s map have been transferred to the map on the 
polar projection which accompanies this memoir ; and for the purpose of more readily 
computing the inclination at any geographical position within the limits of the survey, 
corresponding to the observations combined in drawing the lines, the following 
Tables have been formed, which show the value of the Inclination at the intersection 
of every degree of latitude, with every fifth degree of longitude between the longi- 
tudes of 231° and 261°, and with every 2^ degrees between those of 261° and 291°. 
The use of these Tables seems too obvious to need an explanation ; by the usual pro- 
cess of interpolation, the Inclination due to any geographical locality within the 
bounds of the survey may be obtained by a very light calculation : a formula which 
should give the same result would necessarily consist of a considerable number of 
terms, and would therefore be of less practical utility. 
