by Nelson at the Battle of Cape St Vincent in 1797. Time ball On top of the tower is a time ball, a large ball which is raised and lowered to mark the time. It was installed in 1853 and became operational in March 1854 to act as a time signal to the ships in Edinburgh's port of Leith, and to ships at the anchorage in the Firth of Forth known as Leith Roads, allowing the ships to set their chronometers. The time ball was the idea of Charles Piazzi Smyth, the Astronomer Royal for Scotland, and was originally triggered by a clock in the adjacent City Observatory, to which it was connected by an underground wire. The mechanism was the work of Maudslay, Sons & Field of Lambeth, who had previously constructed the time ball mechanism for Greenwich Observatory. The installation was carried out by James Ritchie & Son who are still retained by City of Edinburgh Council to maintain and operate the time ball. The ball, constructed of wood and covered in zinc, and originally weighing about 90 kg, is raised just before 1 pm, and at precisely 1 pm, is dropped from atop the mast. The commonly stated mass of 15 cwt (762 kg) is a myth stemming from an exaggeration by Piazzi Smyth in 1853. Later, in 1861, the One O'Clock Gun was established at Edinburgh Castle to provide an audible signal when fog obscured the time ball. The time ball was operated for over 150 years, until it was damaged by a storm in 2007. In 2009, as part of the restoration of the monument, the time ball was removed, and the mechanism repaired. The time ball was brought back into service on 24 September 2009. The mechanism is now operated manually, based on the firing of the One O'Clock Gun. See also Monuments and memorials to Horatio Nelson, 1st Viscount Nelson, for other monuments to Lord Nelson References External links Nelson Monument conservation, Edinburgh World Heritage Monuments and memorials in Edinburgh Category A listed buildings in Edinburgh Tourist attractions in Edinburgh Monuments and memorials to Horatio Nelson Calton Hill Towers completed in 1815 1815 establishments in Scotland Time balls Observation towers in the United Kingdom