become more or less magnetic, and when the ship is built of steel,
as modern ones are, this magnetism will be more or less permanent.

We have already said that a magnet has the property of making
steel or iron in its neighborhood into another magnet, with its
poles pointing in the opposite direction. The consequence is that
the magnetism of the earth itself will make iron or steel more or
less magnetic. As a ship is built she thus becomes a great
repository of magnetism, the direction of the force of which will
depend upon the position in which she lay while building. If
erected on the bank of an east and west stream, the north end of
the ship will become the north pole of a magnet and the south end
the south pole. Accordingly, when she is launched and proceeds to
sea, the compass points not exactly according to the magnetism of
the earth, but partly according to that of the ship also.

The methods of obviating this difficulty have exercised the
ingenuity of the ablest physicists from the beginning of iron ship
building. One method is to place in the neighborhood of the
compass, but not too near it, a steel bar magnetized in the
opposite direction from that of the ship, so that the action of
the latter shall be neutralized. But a perfect neutralization
cannot be thus effected. It is all the more difficult to effect it
because the magnetism of a ship is liable to change.

The practical method therefore adopted is called "swinging the
ship," an operation which passengers on ocean liners may have
frequently noticed when approaching land. The ship is swung around
so that her bow shall point in various directions. At each
pointing the direction of the ship is noticed by sighting on the