since it entered service in 1859. The time is shown on four dials in diameter, which are made of milk glass and are lit from behind at night; the hour hand is long and the minute hand . The Clock Tower was designed by Augustus Pugin and built after his death. Charles Barry asked Pugin to design the clock tower because Pugin had previously helped Barry design the Palace. In a 2012 BBC Four documentary, Richard Taylor gives a description of Pugin's Clock Tower: It rises up from the ground in this stately rhythm, higher and higher, before you reach the clock face, picked out as a giant rose, its petals fringed with gold. There's some medieval windows above that and then you hits the grey [cast iron] roof, its greyness relieved by these delicate little windows, again picked out in gold leaf. And then it rises up again in this great jet of gold to the higher roof that curves gracefully upwards to a spire with a crown and flowers and a cross. It's elegant, it's grand, it's pretty and has this fairy-tale quality and it makes you proud to be British. Five bells hang in the belfry above the clock. The four quarter bells strike the Westminster Chimes every quarter-hour. The largest bell strikes the hours; officially called The Great Bell of Westminster, it is generally referred to as Big Ben, a nickname of uncertain origins which, over time, has been colloquially applied to the whole tower. The first bell to bear this name cracked during testing and was recast; the present bell later developed a crack of its own, which gives it a distinctive sound. It is the third-heaviest bell in Britain, weighing 13.8 tonnes. In the lantern at the top of Elizabeth Tower is the Ayrton Light, which is lit when either House of Parliament is sitting after dark. It was installed in 1885 at the request of Queen Victoria—so that she could see from Buckingham Palace whether the members were "at work"—and named after Acton Smee Ayrton, who was First Commissioner of Works in the 1870s. The shortest of the Palace's three principal towers (at ), the octagonal Central Tower stands over the middle of the building, immediately above the Central Lobby. It was added to the plans on the insistence of Dr. David Boswell Reid, who was in charge of the ventilation of the