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New Acquisition: ‘La Planchette’

Spring, 1999

THE Museum has recently acquired, with the aid of a grant from the PRISM fund of the Museums and Galleries Commission, a French surveying instrument from around 1700. The instrument represents an ingenious solution to the problem of producing a map from measurements taken in the field, a process known to the English as ‘protraction’.

The use of angular measurements taken with a theodolite made protraction laborious and prone to error, so working surveyors looked for more direct, graphical methods. In England this meant using a plane table, which avoided protraction completely, since scaled measurements and lines of sight could be drawn directly onto a piece of paper as the survey proceeded, the paper itself becoming the map.

The French had a different solution, described in 1709 by Nicolas Bion as ‘la planchette’, a disc with a scale of degrees on the circumference and a centrally-pivoted sighting device for measuring horizontal angles. Bion mentions that the planchette could be fitted either with a telescope or simply with an alidade with plain sights. There was also a second telescope or a pair of fixed sights, for maintaining the alignment of the circle with respect to other observing stations.

Instruments of this type made by Bion are known, but the planchette recently acquired by the Museum, together with a larger example already in the collection, are both signed ‘Chapotot AParis’. There were two makers of that name, Louis and Jean, and while their family relationship is uncertain, both worked in Paris from the address ‘Quay de l’Horloge du Palais à la Sphère’. The register of French makers being assembled by Anthony Turner currently records working dates for Louis of 1670 to 1700, and 1676 to 1721 for Jean.

One Chapotot at least, generally assumed to be Louis, was a leading maker in late seventeenth-century Paris. He provided instruments for the Académie des Sciences and individually for Cassini, La Hire and Picard. Some of his work is described in the Journal des sçavans, where in 1684 he is acknowledged as a maker who contributes to both the improvement and the invention of mathematical instruments: ‘Le sieur Chapotot … travaille également à donner aux Instrumens de Mathématique déja inventés toutes la perfection que requiert la pratique, & à en imaginer tous les jours de nouveaux.’

Whether Chapotot invented or improved the instrument described by Bion, the examples in Oxford point to a developing design. The recent acquisition is 160 mm in diameter, with plain sights. The larger one (261 mm in diameter) has the two telescopic sights as illustrated by Bion, as well as something that Bion does not show: a very early example of a bubble level on the moving telescope. This planchette must also be one of the earliest surviving surveying instruments with telescopic sights. The optics of both telescopes are complete, including the cross-wires, their good condition owing much to the survival of the original, shaped, leather-covered case.

If the degree scale and sights, whether telescopic or plain, suggest a traditional method of protraction in which readings are taken from the degree scale on the circumference, it is the arrangement within the circular scale that offers a less demanding alternative.

Bion describes the use of removable discs of ‘carton’ (pasteboard) to be fitted inside the raised edge of the planchette, on which lines of sight could be drawn, indicated by the edge of the rule supporting the telescope or sights. These lines provided a graphic record of the survey. After a triangulation, for example, the process of protraction would reduce to setting a disc at either end of a scaled baseline on the map being drawn, marking off the appropriate lines of sight and continuing them to their intersections.

In both his instruments, Chapotot has added a bar at right angles to the sighting rule and continued it to the raised edge. This has not only given him a more elegant and integrated mounting for the magnetic compass than Bion’s rather ad hoc attachment to the rule, but it also lifts the rule above the card in a more stable manner for its motion round the horizon.

In the recently-acquired planchette, two further refinements are added. The central area has a disc of ivory, instead of cards or stiff papers. This can be marked with pencil, used for protraction, rubbed clean and used again. Also, the right-angle bar has a pivoted clasp that engages the central post on which the rule pivots, so that the whole assembly of sights and compass can be easily removed for access to the ivory disc. Evidently the maker sought to improve the commercial advantage of his design by easing the work of the practising surveyor.

J. A. B.