to bring to their
present state, and which are still far from perfect. The measurement of
the distance of a satellite is not a job to be done in an evening; it
requires patient labor extending through months and years, and then is
not as exact as the astronomer would wish. He does the best he can, and
must be satisfied with that.

IX

THE MARINER'S COMPASS

Among those provisions of Nature which seem to us as especially
designed for the use of man, none is more striking than the seeming
magnetism of the earth. What would our civilization have been if the
mariner's compass had never been known? That Columbus could never have
crossed the Atlantic is certain; in what generation since his time our
continent would have been discovered is doubtful. Did the reader ever
reflect what a problem the captain of the finest ocean liner of our day
would face if he had to cross the ocean without this little instrument?
With the aid of a pilot he gets his ship outside of Sandy Hook without
much difficulty. Even later, so long as the sun is visible and the air
is clear, he will have some apparatus for sailing by the direction of
the sun. But after a few hours clouds cover the sky. From that moment
he has not the slightest idea of east, west, north, or south, except so
far as he may infer it from the direction in which he notices the wind
to blow. For a few hours he may be guided by the wind, provided he is
sure he is not going ashore on Long Island. Thus, in time, he feels his
way out into the open sea. By day he has some idea of direction with
the aid of the sun; by night, when the sky is clear he can steer by the
Great Bear, or "Cynosure," the compass of his ancient predecessors on
the Mediterranean. But when it is cloudy, if he persists in steaming
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