Description

DUTCH PLEINSCHAAL NAVIGATION RULE, first half 19th century, signed “JMK & Zn” (for J.M. Kleman and Son), made of fine boxwood 13-1/2″ (34 cm) long. The rule is hand divided and punched with numerous scales, and has six little inset brass studs for use with dividers. One side has a straightforward linear scale of equal parts, running from 0 to 9 in units of approximately 2.2 cm, and with single and double transversal interpolation grids at each end. The other side has three groupings (for three chart scales) of gridded nonlinear scales of Sines and Chords (“H” and “C” in Dutch). Each grid is cut by transversals, and runs 0°-90° vertically, and 0°-10° horizontally. Thus one can pick off immediately with dividers the Sine (or Chord) value for any integral number of degrees. Alongside each grid are vertical axes of hours (“U,” running 0-6 and based on 24 hours as 360°) and of compass points or rhumbs (“S,” running 0-8 based on a 32-point compass rose). Condition is fine with minor nicks and stains, and a beautiful warm patina.

This is a form of the Plain-Scale or Plane-Scale, important for navigation and required equipment on Dutch East India Company ships. Its existence, design and use has been clarified recently by Otto Van Poelje (see Journal of the Oughtred Society, 2004), who finds only a handful have survived. In a 2011 paper Van Poelje treats, in detail, a rule sensibly identical to the present example, and by the same maker, the well-known instrument making firm of J.M. Kleman and Son, active under this name from 1809 – 1859.