The Shepherd Gate Clock is mounted in the wall outside the Greenwich Observatory. It was the first clock to show Greenwich Mean Time to the public. It is a slave clock that keeps time by recieving electric (galvano-magnetic, as it was then called) pulses from a master clock in the Observatory. It has a twenty-four hour analogue dial with Roman Numerals and the addition of a zero (Romans did not use Zero)to mark midnight. With the coming of the railway, Sir George Airy wanted to standardise the many local times that were in use in the country at the time. In 1849, Charles Shepherd of 53 Leadenhall Street, London, had patented a system for controlling a network of Master and slave clocks using electricity. He had installed the Master, Slave Clocks and Time Ball at Greenwich by August 1852. The Slaves here worked with pulses received every second. Pulses were also sent to another Master at London Bridge which then passed them on at a lesser frequency to clocks all around the country. By 1866, time signals were being sent to Harvard University, USA, via transatlantic cable.