The pendulum carries a small stack of old penny coins adding or subtracting coins has the effect of minutely altering the position of the bob's centre of mass, the effective length of the pendulum rod and hence the rate at which the pendulum swings.
The idiom of putting a penny on, with the meaning of slowing down, sprang from the method of fine-tuning the clock's pendulum. Together with an enclosed, wind-proof box sunk beneath the clockroom, the clock's pendulum is well isolated from external factors like snow, ice and pigeons on the clock hands, and keeps remarkably accurate time. This escapement provides the best separation between pendulum and clock mechanism. Instead of using the deadbeat escapement and remontoire as originally designed, Denison invented the double three-legged gravity escapement. As the clock mechanism, created to Denison's specification by clockmaker Edward John Dent, was completed before the tower itself was finished, Denison had time to experiment. This is due to the skill of its designer, the lawyer and amateur horologist Edmund Beckett Denison, later Lord Grimthorpe. The Elizabeth Tower at dusk, with The London Eye in the background. The clock became operational on 7 September 1859. At the base of each clock face in gilt letters is the Latin inscription DOMINE SALVAM FAC REGINAM NOSTRAM VICTORIAM PRIMAM, which means O Lord, keep safe our Queen Victoria the First. The surround of the dials is heavily gilded. Some of the glass pieces may be removed for inspection of the hands. The clock faces are set in an iron frame 7m in diameter, supporting 312 pieces of opal glass, rather like a stained-glass window. The clock and dials were designed by Augustus Pugin. The hour hands are 2.7 metres (9 ft) long and the minute hands are 4.3 metres (14 ft) long. The clock mechanism itself was completed by 1854, but the tower was not fully constructed until four years later, in 1858. The builders of the Allen-Bradley Clock Tower did not add chimes to the clock, so the Elizabeth Tower still holds the title of the "world's largest four-faced chiming clock". The clock faces are large enough to have once allowed the Elizabeth Tower to be the largest four-faced clock in the world, but have since been outdone by the Allen-Bradley Clock Tower in Milwaukee, Wisconsin. The interior volume of the tower is 4,650 cubic metres (164,200 cubic feet). The four clock faces are 55m above ground. The tower is founded on a 15m square raft, made of 3m thick concrete, at a depth of 4m below ground level. The first 61m of the structure is the Clock Tower, consisting of brickwork with stone cladding the remainder of the tower's height is a framed spire of cast iron. The design for the Clock Tower was Pugin's last design before his final descent into madness and death, and Pugin himself wrote, at the time of Barry's last visit to him to collect the drawings: "I never worked so hard in my life for Mr Barry for tomorrow I render all the designs for finishing his bell tower & it is beautiful." The tower is designed in Pugin's celebrated Gothic Revival style, and is 96.3m high. However, although Barry was the chief architect of the palace, he turned to Augustus Pugin for the design of the clock tower, which resembles earlier Pugin designs, including one for Scarisbrick Hall. The tower was raised as a part of Charles Barry's design for a new palace, after the old Palace of Westminster was destroyed by fire on the night of 22 October 1834. The Elizabeth Tower viewed from Westminster Bridge.
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