The simple square lighthouse of Point Gellibrand in Battery Road, Williamstown, is one of Victoria's earliest. Its gently tapering walls of basalt were built by James Linacre to a design by Henry Ginn and completed between 1849 and 1851. It began giving time signals to ships in Hobsons Bay in 1854 by means of a light and shutter. The time ball, which had been located elsewhere at Point Gellibrand, was moved to the top of the lighthouse in 1861. Dropped at one o'clock each day, it allowed ships' chronometers, which were vital for navigation, to be corrected. This service ceased in 1926. In 1934 a 10-metre-high cylindrical brick tower was constructed to house a more powerful light and a navigation beacon. This extension and equipment were removed in 1987. The tower was subsequently returned to its original dimensions and a new time ball constructed. The sill of the lighthouse was used as the vertical datum plane for Victoria prior to metrication.