This is a marine sextant with a bubble level on the frame. It was owned the Royal Air Force, probably during World War I, examined at the Hezzanith Observatory in London in 1919, purchased from a hock shop in San Francisco in 1923, and donated to the Smithsonian in 1994. It was probably made by Heath & Co., an English firm that began making nautical instruments in 1835, adopted the Hezzanith trademark, and was still in business in the 1960s. The Sewill firm, established in Liverpool in 1835 and still in business in the 1990s, specialized in chronometers.
This design appears to be that of Heath's "Mark IV.E Patent 'Curve-Bar' Sextant." The frame is brass. The silvered scale is graduated every 10 minutes from -5o to +155o and read by vernier with tangent screw and swinging magnifier to 10 seconds of arc. A quick-release mechanism at the end of the arm engages with worm teeth cut on the underside of the limb. This mechanism, marked "‘HEZZANITH’ ENDLESS TANGENT SCREW AUTOMATIC CLAMP PATENT No. 17840," is designed to facilitate coarse adjustments. George Wilson Heath and Heath and Company, Ltd., both of Crayford, in Kent County, England, obtained a British patent for this feature in 1909. This sextant has two telescopes--one of high power and narrow field of view; the other of low power and wide field of view--as well as a pair of binoculars.
Ref: G. W. Heath and Heath & Co., "Improvements in Devices for the Adjustments of Sextants and other like Instruments," British patent #17,840.
Heath & Co., Catalogue (London, 1921-1922), p. 506.
