been measured at Fort Denison since 1857 on completion of the fort, initially using a bench mark cut into the stonework of the Martello Tower. From 1867 successive instruments were used as tide measuring technology developed. (The measurements since 1914 are graphed by Bureau of Meteorology). The Fort Denison photographs below show float activated tide gauge instruments in a cabinet and the system's stilling well. A wire connected to the upper drum mechanism passes out through the bottom right of the cabinet and runs over a pulley to drop down to the float system in the large pipe in the well. This system is now obsolete at Fort Denison but maintained as a museum exhibit. To the right of the large pipe in the well is an enclosed pipe which rises to the active modern system. Tide heights and times at Fort Denison are the primary base referent for published tide information for other places in the state of New South Wales. Gallery of guns See also Punishment in Australia References Sources Attribution Further reading External links , NSW National Parks & Wildlife Service [CC-By-SA] NSW Government: "Our Built Heritage Case Study: Fort Denison" Denison Islands of Sydney Sydney Harbour Buildings and structures in Sydney Museums in Sydney Military and war museums in Australia Infrastructure completed in 1857 Crimean War Martello towers Sea forts Towers in Australia Tourist attractions in Sydney 1857 establishments in Australia New South Wales places listed on the defunct Register of the National Estate New South Wales State Heritage Register Public executions Former military installations in New South Wales Time guns Oceanographic instrumentation Landmarks in Sydney Navigational aids Articles incorporating text from the New South Wales State Heritage Register Prison islands Sydney Harbour National Park