ahead, he may be running towards the Azores or towards Greenland,
or he may be making his way back to New York without knowing it.
So, keeping up steam only when sun or star is visible, he at
length finds that he is approaching the coast of Ireland. Then he
has to grope along much like a blind man with his staff, feeling
his way along the edge of a precipice. He can determine the
latitude at noon if the sky is clear, and his longitude in the
morning or evening in the same conditions. In this way he will get
a general idea of his whereabouts. But if he ventures to make
headway in a fog, he may find himself on the rocks at any moment.
He reaches his haven only after many spells of patient waiting for
favoring skies.

The fact that the earth acts like a magnet, that the needle points
to the north, has been generally known to navigators for nearly a
thousand years, and is said to have been known to the Chinese at a
yet earlier period. And yet, to-day, if any professor of physical
science is asked to explain the magnetic property of the earth, he
will acknowledge his inability to do so to his own satisfaction.
Happily this does not hinder us from finding out by what law these
forces act, and how they enable us to navigate the ocean. I
therefore hope the reader will be interested in a short exposition
of the very curious and interesting laws on which the science of
magnetism is based, and which are applied in the use of the
compass.

The force known as magnetic, on which the compass depends, is
different from all other natural forces with which we are
familiar. It is very remarkable that iron is the only substance
which can become magnetic in any considerable degree. Nickel and