130 AKSEL S. STEEN. TERRESTRIAL MAGNETISM. — [NORW. POL. EXP. 
The material is thus insufficient for a direct calculation of the three 
constants, and I therefore hesitated at first in considering the entire index- 
error as approximately constant, with the mean value — 17’ for needle B, 
and — 23’ for needle B'. This is a serious matter, however, when it 
is a question of observations in the polar regions where the inclination is 
not far from 90°, and the total intensity, or the horizontal intensity, has to 
be calculated by the formula 
sige leid) 
W cos: 
(2) 
It therefore occurred to me that it ought perhaps to be possible to make 
use of some of the observations made during the expedition with the Fox 
apparatus for the determination of the total intensity, as a check upon, or 
for the eventual improvement of, the value of the index-error, if we had 
simultaneous, reliable determinations of the horizontal intensity. 
The apparatus was accompanied by the two already-mentioned cylin- 
drical deflectors for the determination of the total intensity, as also by a set 
of accurately corrected weights. On only one occasion, however, was an 
attempt made to use these weights; and the observation-result obtained does 
not admit of criticism, as there is no material for the calculation of a table 
for equivalent weights. The employment of weights, moreover, is not very 
practical in the severe cold of the polar regions, on account of the repeated 
opening of the door of the apparatus. The deflectors, on the contrary, were 
regularly used for intensity determinations in connection with the inclination 
observations, generally, however, only one deflector, both deflectors together 
having been used only 5 times. 
If we call the inclination-needle’s angle of deflection produced by the 
employment of both deflectors simultaneously, w., and the total intensity W, 
we have the following condition for equilibrium: 
Wsin yp, = #, (1+ C4), (3) 
where R, is a constant quantity dependent upon the magnetic moment of 
both deflectors and the needle employed, ¢ is the temperature-coefficient, and 
¢ the temperature observed during the deflection-observations. A, and ¢ may 
then be determined by taking a series of observations of w, under the 
greatest possible differences of temperature, at one or more places where 
