<P> Since the tower was not yet finished, the bell was mounted in New Palace Yard . The first bell was transported to the tower on a trolley drawn by sixteen horses, with crowds cheering its progress . During the bell's testing, it cracked beyond repair and a replacement had to be made . The bell was recast on 10 April 1858 at the Whitechapel Bell Foundry as a 131⁄2 ton (13.76 - tonne) bell . This was pulled 200 ft (61.0 m) up to the Clock Tower's belfry, a feat that took 18 hours . It is 7 feet 6 inches (2.29 m) tall and 9 feet (2.74 m) diameter . This new bell first chimed in July 1859; in September it too cracked under the hammer . According to the foundry's manager, George Mears, the horologist Denison had used a hammer more than twice the maximum weight specified . For three years Big Ben was taken out of commission and the hours were struck on the lowest of the quarter bells until it was repaired . To make the repair, a square piece of metal was chipped out from the rim around the crack, and the bell given an eighth of a turn so the new hammer struck in a different place . Big Ben has chimed with a slightly different tone ever since, and is still in use today with the crack unrepaired . Big Ben was the largest bell in the British Isles until "Great Paul", a 163⁄4 ton (17 tonne) bell currently hung in St Paul's Cathedral, was cast in 1881 . </P> <P> Along with the Great Bell, the belfry houses four quarter bells which play the Westminster Quarters on the quarter hours . The four quarter bells sound G ♯, F ♯, E, and B. They were cast by John Warner & Sons at their Crescent Foundry in 1857 (G ♯, F ♯ and B) and 1858 (E). The Foundry was in Jewin Crescent, in what is now known as The Barbican, in the City of London . The bells are sounded by hammers pulled by cables coming from the link room--a low - ceiling space between the clock room and the belfry--where mechanisms translate the movement of the quarter train into the sounding of the individual bells . </P> <P> The quarter bells play a once - repeating, 20 - note sequence of rounds and four changes in the key of E major: 1--4 at quarter past, 5--12 at half past, 13--20 and 1--4 at quarter to, and 5--20 on the hour (which sounds 25 seconds before the main bell tolls the hour). Because the low bell (B) is struck twice in quick succession, there is not enough time to pull a hammer back, and it is supplied with two wrench hammers on opposite sides of the bell . The tune is that of the Cambridge Chimes, first used for the chimes of Great St Mary's church, Cambridge, and supposedly a variation, attributed to William Crotch, based on violin phrases from the air "I know that my Redeemer liveth" in Handel's Messiah . The notional words of the chime, again derived from Great St Mary's and in turn an allusion to Psalm 37: 23--24, are: "All through this hour / Lord be my guide / And by Thy power / No foot shall slide". They are written on a plaque on the wall of the clock room . </P> <P> One of the requirements for the clock was that the first stroke of the hour bell should be correct to within one second per day . The tolerance is with reference to Greenwich Mean Time (BST in summer). So, at twelve o'clock, for example, it is the first of the twelve hour - bell strikes that signifies the hour (the New Year on New Year's Day at midnight). The time signalled by the last of the "six pips" (UTC) may be fractionally different . </P>

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