osed a frame somewhat resembling a
gridiron, in which the alternate bars were of steel and of brass, and
so arranged that those which expanded the most were counteracted by
those which expanded the least. By this means the pendulum contained
the power of equalising its own action, and the centre of oscillation
continued at the same absolute distance from the point of suspension
through all the variations of heat and cold during the year.[5]

Thus by the year 1726, when he was only thirty-three years old,
Harrison had furnished himself with two compensation clocks, in which
all the irregularities to which these machines were subject, were
either removed or so happily balanced, one metal against the other,
that the two clocks kept time together in different parts of his house,
without the variation of more than a single second in the month. One
of them, indeed, which he kept by him for his own use, and constantly
compared with a fixed star, did not vary so much as one whole minute
during the ten years that he continued in the country after finishing
the machine.[6]

Living, as he did, not far from the sea, Harrison next endeavoured to
arrange his timekeeper for purposes of navigation.

He tried his clock in a vessel belonging to Barton-on-Humber; but his
compensating pendulum could there be of comparatively little use; for
it was liable to be tossed hither or thither by the sudden motions of
the ship. He found it necessary, therefore, to mount a chronometer, or
portable timekeeper, which might be taken from place to place, and
subjected to the violent and irregular motion of a ship at sea, without
affecting its rate of going. It was evident to him that the first
mover must be changed from a weight and pendulum to a spring wound up
and a compensating balance.

He now applied his genius in this direction. After pondering over the
subject, he proceeded to London in 1728, and exhibited his drawings to
Dr. Halley, then Astronomer-Royal. The Doctor referred him to Mr.
George Graham, the distinguished horologer, inventor of the dead-beat
escapement and the mercurial pendulum. After examining the drawings and
holding some converse with Harrison, Graham perceived him to be a man
of uncommon merit, and gave him every encouragement. He recommended
him, however, to make his machine before again applying to the Board of
Longitude.

Harrison returned home to Barrow to complete his task, and many years
elapsed before he aga