Graham, the well-known clock-maker, invented the
mercurial compensation pendulum, 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.
Pages:
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120