work. Like George Graham, he observed that the chief cause of
irregularity in a well-made clock was the varying length of the
pendulum, which in warm weather expanded and became a little longer, and
in cold weather became shorter. He remedied this by the invention of
what is often called the gridiron pendulum, made of several bars of
steel and brass, and so arranged as to neutralize and correct the
tendency of the pendulum to vary in length. Brass is very sensitive to
changes of temperature, steel much less so; and hence it is not
difficult to arrange the pendulum so that the long exterior bars of
steel shall very nearly curb the expansion and contraction of the
shorter brass ones.

While he was thus perfecting himself in obscurity, the great world was
in movement also, and it was even stimulating his labors, as well as
giving them their direction.

The navigation of the ocean was increasing every year in importance,
chiefly through the growth of the American colonies and the taste for
the rich products of India. The art of navigation was still imperfect.
In order that the captain of a ship at sea may know precisely where he
is, he must know two things: how far he is from the equator, and how far
he is from a certain known place, say Greenwich, Paris, Washington.
Being sure of those two things, he can take his chart and mark upon it
the precise spot where his ship is at a given moment. Then he knows how
to steer, and all else that he needs to know in order to pursue his
course with confidence.

When John Harrison was a young man, the art of navigation had so far
advanced that the distance from the equator, or the latitude, could be
ascertained with certainty by observation of the heavenly bodies. One
great difficulty remained to be overcome--the finding of the longitude.
This was done imperfectly by means of a watch which kept Greenwich time
as near as possible. Every fine day the captain could ascertain by an
observation of the sun just when it was twelve o'clock. If, on looking
at this chronometer, he found that by Greenwich time it was quarter past
two, he could at once ascertain his distance from Greenwich, or in other
words, his longitude.

But the terrible question was, how near right is the chronometer? A
variation of a very few minutes would make a difference of more than a
hundred miles.

To this day, no perfect time-keeper has ever been made. From an early
period, the governments of commercial nations we