CHANGE IN DIRECTION OP EARTH’S MAGNETIC FIELD. 275 
sphere, the line of intersection will be a serpentine curve 
which represents the actual secular motion of the needle. 
At the present time we know, with moderate accuracy, the 
values of the three magnetic elements for the inhabited por¬ 
tions of the world, and also, with a lesser accuracy, the rates 
of secular change in the elements; but we have no knowl¬ 
edge as to whether the needle, when it points in a certain 
direction at a given place, will ever return to the same posi¬ 
tion again, or whether it will at the end of a certain period 
assume the same direction, and again sweep over the same 
path in the same period. Nor do we know that the secular- 
variation period, if there shall hereafter be found to be one, 
will be the same in all parts of the world. 
Following the observations and results for the different 
stations is a condensed Statement of the empirical equations 
for declination and inclination and of the decennial values 
of both elements from which the curves of the secular mo¬ 
tion of the needle have been traced in each case upon a 
gnomonic projection of a globe twelve inches in radius, 
whose point of tangency has for its latitude the mean of the 
extreme values of the inclination during the epoch for which 
the curve is constructed, and for its longitude the mean of 
the extreme declinations. 
