Dividing engines allowed eighteenth- and nineteenth-century instrument makers to more precisely and quickly mark the divisions on mathematical instrument scales - like the lines on a rule or protractor - and to make smaller and more affordable instruments. The technology was partially inspired by clock making tools. The ability to make sextants in this manner helped to revolutionise navigation and the search for the longitude at sea. The Board of Longitude dealt a number of times with the British experts at the early engines' use, Jesse Ramsden and Edward Troughton.
The Commissioners relied upon these men's wares and expertise, and also rewarded Ramsden and helped to spread his dividing technology. The instrument maker developed his first circular engine in 1767, an improved version in about 1775, and a straight line dividing engine by 1777. John Troughton constructed an engine after Ramsden's instructions from 1775 to 1778, which also inspired his brother Edward. Edward Troughton became renowned for his own dividing skills, and produced an improved engine in 1793.
Ramsden's circular dividing engine had a 45-inch horizontal wheel with 2160 precision teeth, which were engaged by an accurate lead-screw. The lead-screw was turned by a treadle and cord to rotate the wheel through a specific, exact angle. A workman would center a sextant on the wheel and divide the 120° of its scale by repeatedly depressing the treadle and then moving a radially sliding cutter. This process now took just 30 minutes rather than hours, and sextants as small as eight inches in radius could be divided this way.
See the essays on Jesse Ramsden and Edward Troughton for more information about their careers, dealings with the Board, and some of their surviving instruments.
History and Philosophy of Science
University of Cambridge