This is a standard "U.S. Navy Surveying Sextant" with a bubble
level mounted above the telescope. Robert Wheeler Willson, a professor of
astronomy at Harvard University, introduced this form in the early years of the twentieth century, and made instruments available for experimental purposes during World War I. This example was made around 1920. P. V. H. Weems donated it to the Smithsonian in 1963. The frame is brass. The silvered scale is graduated every 20' from -5o to +185o and read by vernier with tangent screw and magnifier to 30" seconds of arc.
Ref: R. W. Willson, "Instrument Including Means for Determining the
Horizontal Direction," U.S. patent #1,705,146.