A what? A Shortt-Synchronome clock. Not just any clock, but a very special type of clock. The most accurate mechanical clocks ever built.
Instead of the normal single-pendulum grandfather clock, it's got two pendulums. One is normal, one resides in a near-vacuum in a specially designed metal tank. The near-vacuum eliminates aerodynamic drag on the pendulum and prevents it from being affected by atmospheric pressure variations. The pendulum itself is designed of low-thermal-expansion metals and has a special section with special thermal properties so that it remains exactly the same length regardless of the temperature. Since a pendulum's period depends on local gravity (not changing) and length, this means that the Shortt-Synchronome free pendulum clock keeps extremely accurate time.
How accurate? More accurate than the earth itself.
That's right. These clocks, which are not small, but not spectacularly large, were the first human-created objects to keep time more accurately than the Earth itself. And that's one impressive achievement - to keep time more accurately than a six-times-10-to-the-24st-kilogram spinning ball of iron and rock.
Said spinning ball is not perfect. In a process called nutation, the Earth's rotation varies, ever so slightly. The Shortt clock was the first artificial object that kept time more accurately than the earth. It looses just 200 microseconds per day - one second every twelve years. In fact, it's so sensitive that it can measure the change in local gravity due to the moon passing overhead.
And mind you, the first one was built in 1921 (by railway engineer William Hamilton Shortt and horologist (what a cool word; it means 'clock scientist') Frank Hope-Jones). That's before quartz clocks, that's before atomic clocks, that's before we could measure the blips from millisecond pulsars.
And that is engineering. Making something work.
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