This is a 56 hour chronometer that was probably made in the early 1870s. It has a later pattern Earnshaw spring detent escapement, and indications for hours, minutes, seconds, and up and down. George Blackie was a reputable chronometer maker who won first prize in the Greewich Trials of 1858. The U.S. Navy Bureau of Ships transferred this instrument to the Smithsonian in 1953.
Ref: Tony Mercer, Chronometer Makers of the World (Colchester, 1991), p. 109.