The law of gravitation
is at the bottom of all his work. The effects of this law require
mathematical processes which it has taken two hundred years 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.
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