1745 Visable through the clock's glazed door is George Graham's mercury compensation pendulum. In temperature change the pendulum the pendulum gets longer or shorter, changing the rate at which it swings. The mercury at the bottom compensates by expanding in the opposite direction. The blue hand shows mean solar time, which is constant. The gilt hand shows true solar time, as indicated by the sun. By George Graham, London
aperture=f2.8 focal length: 60.0 mm exposure 1/100 sec ISO: 3200