must reserve for its proper place in our course any detailed
description of the disturbances to which the magnetism of our planet
is found to be subject. Some of these disturbances are periodic,
following the regular courses of the sun and moon. Others are sudden,
and are called magnetic storms, but, like the storms of the
atmosphere, they have their known seasons of frequency. The last and
the most mysterious of these magnetic changes is that secular
variation by which the whole character of the earth, as a great
magnet, is being slowly modified, while the magnetic poles creep on,
from century to century, along their winding track in the polar
regions.

We have thus learned that the interior of the earth is subject to the
influences of the heavenly bodies, but that besides this there is a
constantly progressive change going on, the cause of which is entirely
unknown. In each of the magnetic observatories throughout the world
an arrangement is at work, by means of which a suspended magnet
directs a ray of light on a preparred sheet of paper moved by
clockwork. On that paper the never-resting heart of the earth is now
tracing, in telegraphic symbols which will one day be interpreted, a
record of its pulsations and its flutterings, as well as of that slow
but mighty working which warns us that we must not suppose that the
inner history of our planet is ended.

But this great experimental research on Terrestrial Magnetism produced
lasting effects on the progress of science in general. I need only
mention one or two instances. The new methods of measuring forces
were successfully applied by Weber to the numerical determination of
all the phenomena of electricity, and very soon afterwards the
electric telegraph, by conferring a commercial value on exact