The main purpose of the observatory was a time service. The transits of stars through the meridian were observed and used to keep the observatory clock, a sidereal clock manufactured by Robert Bryson, accurate. Accurate time was important for navigation, and mariners would bring ships' chronometers from the port of Leith up to Calton Hill for adjustment. In 1854 the time ball was installed on Nelson's Monument next to the observatory and visible from the port. This was controlled by electrical pulses from the observatory clock. A few years later the One O'Clock Gun on Edinburgh Castle was added. This was also controlled through an electrical wire, spanning the city from Calton Hill to the Castle. Today the time ball and One O' Clock Gun are tourist attractions. They are no longer controlled from a state of the art clock, but are triggered by hand.