y or
activity.

This impression is strengthened by the fact that the needle moves to the
east in the morning, when the solar rays increase magnetic activity in
that direction, and west again, as their influence increases there.

Now, these elements--the declination and horizontal and vertical
forces--all these periodical, regular, and irregular variations of
magnetic activity, are intimately connected with the variations of
atmospheric condition:

First, They show an increase of activity during certain hours of the day,
corresponding to, and obviously connected with, the diurnal atmospheric
changes.

Second, They show an increase of activity during the northern transit of
the atmospheric machinery--an _annual_ variation.

Third, They show an increase in that activity during the latter portion of
each decennial period, conforming to the occurrence of solar spots.

And, fourth, _Irregular variations_ of activity, corresponding with the
_irregular changes_ of atmospheric condition.

We will examine these results, and in doing so, take those of the element
of declination--one answering for all.

The magnetic needle moves to the west in summer, from about 8 A.M. till
about 2 P.M., and the extent of its progress, during that period,
constitutes the magnitude of its daily variation. It is found that this
variation differs in different months, and that it is normally greatest in
the summer months, and least in the winter, in the ratio of about two to
one. It is further found, that in different years the maximum activity
occurs in different months, and that the years differ also, and there is a
distinctly marked decennial period, corresponding most remarkably with the
decennial maxima of recurring solar spots, as observed by Schwabe. Dr.
Lamont, of Munich, gives us the following table of magnitude of
declination there, for the ten years preceding 1851, which clearly
exhibits this fact, and also the greater intensity during the northern
transit of the atmospheric machinery. He says:

"The magnitude of the variations of declination have a period of ten
years. For five years there is a uniform increase, and during the
following five years a uniform decrease in the variations. With us
the magnetic declination is a minimum at about eight o'clock in the
morning, and is greatest at two o'clock in the afternoon. Subtracting
the declination at eight o'clock from that at two o'clock, we obtain