Gurley Micrometer Alidade

Manufactured by W. & L.E. Gurley, Troy, New York. The micrometer attachment us a modification of the tangent screw attached to the horizontal axis of the telescope and used for measuring vertical angles in terms of their tangents. Relatively few of these types of alidades are listed in early 1900s property records of instruments used by the U.S. Geological Survey.
Object ID: USGS-000159