The Mystery Ship is HMS Mercury. She was laid down at Pembroke Dockyard on 16 March 1876, launched on 17 April 1878 and completed in September 1879. She differed from her earlier sistership Isis in having a straight stem instead of a clipper bow.
Initially classed as Despatch vessels, they were redesignated as Second Class Cruisers and were the first all-steel ships in the Navy. The photograph shows her after the original barque rig and almost all her armament had been removed, dating the photo between 1903 and 1905. She spent most of this time at Portsmouth as a Seagoing Navigation Training ship. After 1905 she was converted into a submarine depot ship, then in 1914 to a hulk at Chatham. She was sold for breaking in 1919.
Sandy McAuslan, Renton, Dumbarton