The rim of this compass is graduated to 30 minutes. The variation arc on the south arm extends 20 degrees either way, and is read by folded vernier to 2 minutes. Two level vials are on the north arm, and an outkeeper is on the south arm. The side of one vertical sight is graduated to half degrees, for determining angles of elevation or depression. The machine-engraved "W. & L. E. Gurley Troy, N.Y." signature indicates a date of manufacture after 1876. The lack of serial number indicates a date before 1908.
Ref: W. & L. E. Gurley, A Manual of the Principal Instruments Used in American Engineering and Surveying (Troy, N.Y., 1904), pp. 108-120.
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