24 
THE KEY. STEPHEN J. PEEEY ON THE MAGNETIC 
of position of the pole from p to pJ should cause a greater variation in the Dip at W 
than at E, W being nearer than E to the magnetic pole. On the other hand, since 
the line p p' is inclined at a greater angle to the meridian E P than to W P, the change 
of Declination due to the motion from p to p' should be less rapid at W than at E. 
With regard to the Intensity of the earth’s magnetic force, the laws of distribution are 
too complex and irregular to warrant any certain conclusion in a particular case, unless 
the conditions of local magnetism are taken fully into account. Comparing these con. 
elusions with the results derived from the observations discussed in this paper, we find a 
perfect agreement in the case of the Dip and Declination, and the observations of the 
Horizontal Force tend to show that greater nearness to the pole is combined with 
increased rate of variation in this element. 
Turning, now, our attention from the consideration of the difference between the two 
sets of values of the magnetic elements to examine the secular changes in the curves of 
equal Dip, Declination, and Intensity, we do not expect to find a very close agreement 
between theory and observation. The distribution of the Isoclinals and Isogonics, and 
still more that of the Isodynamics, is so irregular, that such a slight difference of position 
as the east and west of France would probably have scarcely any apparent effect upon the 
resulting values, any small inequality being at least partially veiled by accidental errors 
from locality or observations. Still, however, as the Isoclinals and Isodynamics are 
approximately at right angles to the magnetic meridians, we may be justified in the 
assumption that, as the pole’s path pp 1 approaches parallelism to WE, the difference 
of angle in east and west for both sets of lines will become much less marked. Here, 
again, we find that the results of the observations taken in France agree well with the 
assumption made. The Isogonics present a precisely similar coincidence, as might be 
expected from their position in relation to the pole’s actual path. 
Since, moreover, p p’ is more nearly parallel with the Isogonics than with the Isoclinals 
and Isedynamics, there is a greater fixity in the mean angle for the whole of France in 
the case of the former lines than in that of the latter. 
Lastly, the Isoclinals and Isodynamics are spreading out more quickly in the west 
than in the east, and there exists at present very little difference in the thickness of 
these lines in the two portions of the country, both of which conclusions would natu- 
rally follow from the fact that the pole is becoming more and more nearly equidistant 
from the east and west of France. The exceptional case of the Isogonics, which are 
spreading out in the east and drawing closer in the west, evidently arises mainly from 
inaccuracy of observation. 
A general Table may now be formed of all the magnetic elements, reduced to the 
epoch Jan. 1, 1869, for the stations in the east of France. 
