162 
CAPTAIN HAIG ON THE MAGNETIC ELEMENTS IN BRITISH 
It consists in referring all the results to the point of mean longitude and latitude 
among all the stations, and assuming the differences of longitude and latitude expressed 
in geographical miles to he plane rectangular coordinates of distance from the origin. 
It is also assumed that the three magnetic elements vary uniformly over the whole 
district surveyed. 
Dip. 
Table I. gives the individual results at each station, as well as the partial results from 
the needle when magnetized in opposite directions. Table II. shows the mean results, 
along with the most probable dip at each station computed by the method of minimum 
squares. By the method of minimum squares the variation of dip for one mile of 
longitude x = -J- 0'*272, and the variation for one mile of latitude ?/= + 0 f *776 ; from 
these values we get u= 70° 42' for the angle which an isoclinal line makes with the 
meridian measured from the north round by west, and - or . } =1-216 mile for 
the distance between isoclinals whose difference of dip is 1'. Column 5 contains the 
most probable dip at each station (^), obtained from equations i i=^-\-ax-\-hy . The 
probable error of the computed dip at each station is nearly equal to +5'. 
Intensity of the Magnetic Force. 
The stations where these observations were made are the same as the dip stations. 
At ten of them observations of deflection and vibration for the horizontal component 
were made with the unifilar, and observations by Lloyd’s statical method for variation of 
the total force ; at the remaining eleven stations the statical method only was employed. 
Observations at the first ten stations furnish values of the constant log A, of which a 
mean value might be adopted for use at the other eleven ; but from an examination of 
these values of log A, it appears that those belonging to 1861 are generally larger than 
preceding values, owing, I believe, to the weighted needle having become rusted. I 
have therefore adopted the mean of all values of log A previous to 1861 for statical 
observations up to that period, and the mean of those in 1861 for observations in that 
year. The first value is log A= 0-91931, and that for 1861 is log A= 0-92032. 
Table III. shows the unifilar observations, and the values of X, the horizontal com- 
ponent of the magnetic intensity, derived from them. Column 12 of this Table con- 
tains the dips at the several stations, and column 13 the total force <p=Xsec0. 
Panama, the first station in this Table, is not included in the general assemblage of 
results in Table IV., its distance from the other stations being too great. Table IY. 
shows the combination of all observations for force to determine the direction and 
distance apart of the isodynamic lines. Column 3 in this Table corresponds to column 
13 in Table III. We find (x) the variation in total force for one mile of longitude 
= +0-000925, and ( y ) the variation for one mile of latitude =+0-000896; ^ the 
distance between the isodynamic lines a unit of force apart=776-6 miles, or for a tenth 
