night in writing or drawing.

As part of his business, he undertook to survey land, and to
repair clocks and watches, besides carrying on his trade of a

carpenter. He soon obtained a considerable knowledge of what had
been done in clocks and watches, and was able to do not only what

the best professional workers had done, but to strike out
entirely new lights in the clock and watch-making business. He

found out a method of diminishing friction by adding a joint to
the pallets of the pendulum, whereby they were made to work in

the nature of rollers of a large radius, without any sliding, as
usual, upon the teeth of the wheel. He constructed a clock on

the recoiling principle, which went perfectly, and never lost a
minute within fourteen years. Sir Edmund Denison Beckett says

that he invented this method in order to save himself the trouble
of going so frequently to oil the escapement of a turret clock,

of which he had charge; though there were other influences at
work besides this.

But his most important invention, at this early period of his
life, was his compensationpendulum. Every one knows that metals

expand with heat and contract by cold. The pendulum of the clock
therefore expanded in summer and contracted in winter, thereby

interfering with the regular going of the clock. Huygens had by
his cylindrical checks removed the great irregularity arising

from the unequal lengths of the oscillations; but the pendulum
was affected by the tossing of a ship at sea, and was also

subject to a variation in weight, depending on the parallel of
latitude. Graham, the well-known clock-maker, invented the

mercurial compensationpendulum, consisting of a glass or iron
jar filled with quicksilver and fixed to the end of the pendulum

rod. When the rod was lengthened by heat, the quicksilver and
the jar which contained it were simultaneously expanded and

elevated, and the centre of oscillation was thus continued at the
same distance from the point of suspension.

But the difficulty, to a certain extent, remained unconquered
until Harrison took the matter in hand. He observed that all

rods of metal do not alter their lengths equally by heat, or, on
the contrary, become shorter by cold, but some more sensibly than

others. After innumerable experiments Harrison at length
composed 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 again appeared in London to present his
first chronometer.

The remarkable success which Harrison had achieved in his
compensating pendulum could not but urge him on to further

experiments. He was no doubt to a certain extent influenced by
the reward of 20,000L. which the English Government had offered

for an instrument that should enable the longitude to be more
accurately" target="_blank" title="ad.准确地；精密地">accurately determined by navigators at sea than was then

possible; and it was with the object of obtaining pecuniary
assistance to assist him in completing his chronometer that

Harrison had, in 1728, made his first visit to London to exhibit
his drawings.

The Act of Parliament offering this superbreward was passed in
1714, fourteen years before, but no attempt had been made to

claim it. It was right that England, then rapidly advancing to
the first position as a commercial nation, should make every

effort to render navigation less hazardous. Before correct
chronometers were invented, or good lunar tables were

prepared,[7] the ship, when fairly at sea, out of sight of land,
and battling with the winds and tides, was in a measure lost. No

method existed for accurately" target="_blank" title="ad.准确地；精密地">accurately ascertaining the longitude. The
ship might be out of its course for one or two hundred miles, for

anything that the navigator knew; and only the wreck of his ship
on some unknown coast told of the mistake that he had made in his

reckoning.
It may here be mentioned that it was comparatively easy to

determine the latitude of a ship at sea every day when the sun
was visible. The latitude--that is, the distance of any spot

from the equator and the pole--might be found by a simple
observation with the sextant. The altitude of the sun at noon is

found, and by a short calculation the position of the ship can be
ascertained.

The sextant, which is the instrumentuniversally used at sea, was
gradually evolved from similar instruments used from the earliest

times. The object of this instrument has always been to find the
angular distance between two bodies--that is to say, the angle

contained by two straight lines, drawn from those bodies to meet
in the observer's eye. The simplest instrument of this kind may

be well represented by a pair of compasses. If the hinge is held
to the eye, one leg pointed to the distant horizon, and the other

leg pointed to the sun, the position of the two legs will show
the angular distance of the sun from the horizon at the moment of

observation.
Until the end of the seventeenth century, the instrument used was

of this simple kind. It was generally a large quadrant, with one
or two bars moving on a hinge,--to all intents and purposes a

huge pair of compasses. The direction of the sight was fixed by
the use of a slit and a pointer, much as in the ordinary rifle.

This instrument was vastly improved by the use of a telescope,
which not only allowed fainter objects to be seen, but especially

enabled the sight to be accurately" target="_blank" title="ad.准确地；精密地">accurately directed to the object
observed.

The instruments of the pre-telescopic age reached their glory in
the hands of Tycho Brahe. He used magnificentinstruments of the

simple "pair of compasses" kind--circles, quadrants, and