This building houses the Gill Reversible Transit Circle, built in 1905 to measure the position of stars as they pass the north-south meridian and to check the accuracy of clocks. It was also used to contribute to the Catalogue of Fundamental Stars (Fundamental Katalog FK4), and was in use until around 1980 - after which the use of space satellites became the norm in the 1990s. The building is in need of repair and apparently one of its chimneys is housing a bee hive.