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The details of the clock and its performance will be published and I'm not the one to go into them, but I can confirm that by 'sealed', what is meant is that a perspex cover was put over the clock so that the clock itself could not be tampered with and to keep dust off. Representatives of the National Physical Laboratory and the Worshipful Company of Clockmakers then put seals on as an independent verification. The clock is, therefore, subject to changes in pressure, temperature etc (i.e. it's a pendulum in free air). The mechanical compensation for atmospheric pressure changes is now so good that the clock responds primarily to temperature.