. We can, however, imagine them represented
by a movement of the end of the needle if we suppose it made of elastic
material, so that it would lengthen when the force was greater and
contract slightly when the force was less. If a pencil were attached to
the end of such an elastic needle so as to make a mark on a sheet of
paper, and if for a moment we exclude the up and down movements, the
pencil would describe during the day a curve on the paper, as the end of
the needle swung backwards and forwards with the change in direction, and
moved across the direction of swing with the change in intensity. Now when
curves of this kind are described for a day in each month of the year,
there is a striking difference between the forms of them. During the
summer months they are, generally speaking, comparatively large and open,
and during the winter months they are small and close. This change in form
is seen by a glance at Plate XIII., which gives the curves throughout the
whole of one year. Let us now, instead of forming a curve of this kind for
each month, form a single average curve for the whole year; and let us
further do this for a series of years. (Plate XIV.) We see that the curves
change from year to year in a manner very similar to that in which they
change from month to month in any particular year, and the law of change
is such that in years when there are many sun-spots we get a large open
curve similar to those found in the summer, while for years when there are
few sun-spots we get small close curves very like those in the winter.
Hence we have two definite conclusions suggested: firstly, that the
changes of force are sympathetic with the changes in the sun-spots; and
secondly, that times of maximum sun-spots correspond to summer, and times
of minimum to winter. And here I must admit that this is about as far as
we have got at present in the investigation of this relationship. _Why_
the needle behaves in this way we have as yet only the very vaguest ideas;
suggestions of different kinds have certainly been put forward, but none
of them as yet can be said to have much evidence in favour of its being
the true one. For our present purpose, however, it is sufficient to note
that there is this very real connection, and that consequently Schwabe's
sun-spot period may have a very real importance with regard to changes in
our earth itself.

[Illustration:

GREENWICH MAGNETIC CURVES
1859-60
PLATE XIII.
GREENWICH