Edward Troughton introduced this type of reflecting circle in 1796, and by 1808 had produced almost 200 instrument of this sort. This example was probably made in 1804. It has a silver scale with three parts, each numbered from 0 to 180; this scale is graduated every 10' and read by three verniers and one magnifier. This example is on loan from John Stroud Kusner.
Ref: A. Stimson, "The Influence of the Royal Observatory at Greenwich upon the Design of 17th and 18th Century Angle-Measuring Instruments at Sea," Vistas in Astronomy 20 (1976): 123-130.