, it gives us a good general idea of the nature of terrestrial
magnetism. One result that follows from the law of poles already
mentioned is that the magnetism which seems to belong to the north pole
of the earth is what we call south on the magnet, and vice versa.

Careful experiment shows us that the region around every magnet is
filled with magnetic force, strongest near the poles of the magnet, but
diminishing as the inverse square of the distance from the pole. This
force, at each point, acts along a certain line, called a line of
force. These lines are very prettily shown by the familiar experiment
of placing a sheet of paper over a magnet, and then scattering iron
filings on the surface of the paper. It will be noticed that the
filings arrange themselves along a series of curved lines, diverging in
every direction from each pole, but always passing from one pole to the
other. It is a universal law that whenever a magnet is brought into a
region where this force acts, it is attracted into such a position that
it shall have the same direction as the lines of force. Its north pole
will take the direction of the curve leading to the south pole of the
other magnet, and its south pole the opposite one.

The fact of terrestrial magnetism may be expressed by saying that the
space within and around the whole earth is filled by lines of magnetic
force, which we know nothing about until we suspend a magnet so
perfectly balanced that it may point in any direction whatever. Then it
turns and points in the direction of the lines of force, which may thus
be mapped out for all points of the earth.

We commonly say that the pole of the needle points towards the north.
The poets tell us how the needle is true to the pole. Every reader,
however, is now familiar with the general fact of a variation of the
compass. On our eastern seaboard, and all the way across the Atlantic,
the north pointing of the compass varies so far to the west that a ship
going to Europe and making no allowance for this deviation would find
herself making more nearly for the North Cape than for her destination.
The "declination," as it is termed in scientific language, varies from
one region of the earth to another. In some places it is towards the
west, in others towards the east.

The pointing of the needle in various regions of the world is shown by
means of magnetic maps. Such maps are published by the United States
Coast Survey, whose experts make a c