Probably the 246-ton brig Una built on Prince Edward Island (Canada) in 1856 and owned by Coats & Co. of Shields who traded her to the Mediterranean ports. She was owned later by R. Putt of London who traded her to China.
Sailing close-hauled on the port tack, the brig is reducing sail in anticipation of boarding a pilot. This probably explains the approaching cutter under the tip of the bowsprit.
Identified by the name at the bow, recognition at a distance is possible by the Marryat Code hoist at the mainmast. Reading 2nd distinguishing pendant, 2,0,4,3, this correctly signifies Una.