This wooden rule is known as a Gunter scale or Gunter rule and has scales on each face to help with navigational calculations. The scales on the front include one in inches, diagonal scales that allow a pair of dividers to be set to an exact length, and a gunnery scale. Those on the back include sines and tangents. The maker's name is inscribed above the scale of inches as 'ACKERMANN & Co 96 STRAND'. The firm was working from this address in about 1850. A second inscription on the front reads 'NAVIGATION SCALE BY B. DONN', indicating that this was Benjamin Donn's improved version of the Gunter scale, which he described in a pamphlet in 1772. Donn (1729–98) was a teacher at a Mathematical School set up by his father in Bideford, Devon, before establishing his own Mathematical Academy in Bristol.