A circular dividing engine graduates arcs of navigational and other
mathematical instruments. Before its invention, arcs of this sort had to be
divided by hand. This method could be quite accurate, but it was extremely
tedious and only a few experts were qualified to do it. Jesse Ramsden began
to design a dividing engine around 1760. His first example was more successful
than the clockmakers’ dividing engines that had come before it, but it was not
yet accurate enough for instruments that would be used at sea.
Ramsden completed this diving engine in 1775. It earned him the British Board of Longitude Prize, on the condition that he publish instructions and offer training on how the dividing engine was made, and use it to divide the arcs of octants and sextants for other instrument makers, essentially at cost. The technology and design of Ramsden's instrument spread rapidly, and so did the instrument's influence. The dividing engine drastically changed the navigational instrument enterprise by making the construction and use of octants and sextants possible on a much larger and more affordable scale, while maintaining, or perhaps even improving upon, the accuracy of the instruments.
This dividing engine has a braced mahogany frame with three legs and three frictionless wheels. The wheels support a heavy bronze wheel that has 10 radial arms, an inner ring, and a central pivot. The outer edge of the bronze wheel is covered with a brass ring that is cut with 2160 gear teeth and attached to the outer edge of the wheel. These teeth engage a worm screw on one side of the machine. Turning the screw 6 times rotates the carriage for the stylus one degree. The accuracy of the dividing engine relied on the accuracy of the screw, which Ramsden made using a specialized lathe. An arc to be graduated was clamped to the arms of the bronze wheel with the cutting mechanism above it. A brass ring resting on the arms suggests how this may have been done.
The dividing engine eventually became the property of Ramsden’s successor, Matthew Berge, and Berge's successors, Nathaniel Worthington and James Allan. By the mid-nineteenth century American instrument-making firms had begun to buy dividing engines, many of which were made by those Ramsden trained. The Philadelphia firm of Knox and Shain purchased Ramsden's dividing engine from his successors for their use. Henry Morton, president of the Stevens Institute of Technology, brought it to the Smithsonian in 1890.
Ref: Jesse Ramsden, Description of an Engine for Dividing
Mathematical Instruments (London, 1777).
J. Elfreth Watkins, "The Ramsden Dividing Engine" (Washington, 1891), pp. 721-739.
Chris Evans, Precision Engineering: An Evolutionary View (Bedford, 1987) 69-73.