repurposed as a chimney, until around 1914. The 2017–2021 conservation works included the addition of a lift (or elevator) that was installed in the shaft. Its foundations rest on a layer of gravel, below which is London clay. Owing to this soft ground, the tower leans slightly to the north-west by roughly over 55 m height, giving an inclination of approximately . This includes a planned maximum of 22 mm increased tilt due to tunnelling for the Jubilee line extension. In the 1990s, thousands of tons of concrete were pumped into the ground underneath the tower to stabilise it during construction of the Westminster section of the Jubilee line. It leans by about at the finial. Experts believe the tower's lean will not be a problem for another 4,000 to 10,000 years. Name Journalists during Queen Victoria's reign called it St Stephen's Tower. As members of Parliament originally sat at St Stephen's Hall, these journalists referred to anything related to the House of Commons as "news from St Stephens" (the Palace does contain a feature called St Stephen's Tower, located above the public entrance). On 2 June 2012, the House of Commons voted in support of a proposal to change the name from the Clock Tower to Elizabeth Tower in commemoration of Elizabeth II in her Diamond Jubilee year, since the large west tower now known as Victoria Tower had been renamed in tribute to Queen Victoria on the occasion of her Diamond Jubilee. On 26 June 2012, the House of Commons confirmed that the name change could go ahead. David Cameron, then Prime Minister, officially announced the change of name on 12 September 2012. The change was marked by a naming ceremony in which John Bercow, then Speaker of the House of Commons, unveiled a plaque attached to the tower on the adjoining Speaker's Green. Prison Room Inside the tower is an oak-panelled Prison Room, which can only be accessed from the House of Commons, not via the tower entrance. It was last used in 1880 when atheist Charles Bradlaugh, newly elected Member of Parliament for Northampton, was imprisoned by the Serjeant at Arms after he protested against swearing a religious oath of allegiance to Queen Victoria. Officially, the Serjeant at Arms can still make arrests, as they have had the authority to do since 1415. The room, however, is currently occupied by the Petitions Committee, which oversees petitions