In a recently completed review of over 3,000 Curatorial accession files, funded by a grant awarded by the Institute of Museum and Library Services, Museum staff discovered that we have a rare navigation instrument, a “Townshend Double Reflecting and Repeating Circle.”
A reflecting circle is a device for measuring longitude that was more accurate than the sextant. It was developed in the 18th century in England and refined by French and German instrument makers in the first half of the 19th century. Henri-Prudence Gambey (1787-1847) was the foremost precision instrument maker in France and made devices for physicists, astronomers, surveyors and navigators, among others. While the reflecting circle was not generally favored in England, on the Continent (and French navy) it was used well into the 1800s to measure large angles with great accuracy. In the reflecting circle, mirrors were used to measure simultaneously the angular distance between two objects to establish longitude.
Our instrument was made by Charles Hervey Townshend (1833-1904), both a ships’ captain and an inventor. His “Townshend Double Reflecting and Repeating Circle”, for which he was granted a patent in 1888, was a valuable aid to maritime navigation with respect to longitude and latitude. With it one can sight three objects and measure two angular separations simultaneously, for example, two bright stars from the moon for the lunar longitude method, or three coastal landmarks for triangulation in coastal navigation. His experiments with oyster culture after he retired from the sea were of singular importance in the development of that industry.
Written by Dr. Paul Goodwin, IMLS Project Volunteer, 2024