A good early instrument in its mahogany and boxwood case. Engraved is the maker's name of the side of the viewing prism, with the address "62 Charing Cross".This is used when a natural horizon line at sea or on land is unable to be observed, so a true "horizontal" horizon for sighting a sextant or other navigation instrument. Of interest is that this instrument doesn't employ a mercury bubble or level, but a gravitational plum bob-like tool.
