Jacobus Kley (*1716;†1791), Rotterdam.

Title page of Dierick Ruyter's book, published in 1631, on the use of the Pleynschael.
Title page of Dierick Ruyter's book, published in 1631, on the use of the Pleynschael.

Jacobus Kley, son of Pieter Kley and Willemina de Haas, baptised on 14 January 1716 and died 23 February 1791. Kley was a mathematical instrument maker in Rotterdam, where he probably was an apprentice to clock and scientific instrument maker Steven Hoogendijk. Kley made a wide variety of instruments, including astrolabes, octants and Davis quadrants. He made instruments for the Fundatie van Renswoude in The Hague (ca.1760) and provided instruments to Petrus van Musschenbroek (a 'spiegel octant', in 1754) in Leiden, and to Prof. Johann David Hahn in Utrecht. Kley provided Pieter Holm in Amsterdam with the octants that Holm retailed under his own name from 1752 to 1767.

In the Netherlands the 2-sided wooden 'pleinschaal' or navigational ruler as shown above, was preferred over the more common Gunter navigational rule. The Amsterdam Chamber prescribed the use of pleinschalen for use on VOC vessels. This example (329x33mm) signed by Kley, dating from the mid 18th century, is similar to the one by Gerard Hulst van Keulen in the Boerhaave Museum in Leiden. At one end there is an ancient repair with an iron strip and 2 rivets.

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[1] Otto E. van Poelje; Journal of the Oughtred Society: The Sliding Gunter-Versions for Navigation at Sea. 

[2] Also see Holm's sliding Gunter, and an unsigned pleinschaal.

[3] W. F. J. Morzer‐Bruyns; A history of the use and supply of the Pleynschael by instrument makers to the VOC. International Journal of Nautical Archaeology: 1982, Vol. 11, No. 4, p.293-296.

[4] Bert Boonman; Gunterschales en Pleinschalen, de functie en de maatvoering.