‘Een extra fraai Octant in zyn kas’ uit de verzameling van een achttiende-eeuwse Amsterdamse regent
Authors: Bruyns W.F.J. Mörzer 
Published: October 2015

‘A very fine Octant’ from the collection of an eighteenth-century Amsterdam regent The Utrecht University Museum owns an octant made by Jonathan Sisson, an eighteenth-century London scientific instrument maker. The device was acquired in 1776 by Utrecht University at the auction sale of the instrument collection of Gerard Aernout Hasselaer, an Amsterdam regent, who held important posts at Dutch admiralties and the Amsterdam Chamber of the Dutch East-India Company (VOC). The octant had been invented in 1731 by John Hadley, a vice-president of the Royal Society of London. Sisson made one of the first copies, which was put at the test at sea the following year. This try out proved to be a success, and octants soon replaced older instruments for measurements of the altitude of celestial bodies above the horizon at sea. The Dutch Republic was one of the first countries outside Britain where the octant was introduced. Among others, this introduction was stimulated by contacts between scientific instrument makers in both countries. In 1735 for instance, the Leiden instrument maker Jan van Musschenbroek was the first Dutchman to sell octants. It is known that he had business contacts with Sisson. In 1737 the octant was used for the first time on the Dutch vessel Teijlingen of the Amsterdam Admiralty. These developments occurred at a time that officials in the Dutch Republic became increasingly aware that the knowledge and practice of navigation on Dutch ships lay well behind those used at British ships. Dutch seamen and scholars drew up reports, pointing at the lagging of knowledge, and among others recommended the use of octants. These recommendations soon led to improvements, such as the founding of a navigational school in Amsterdam and the publication of a Dutch manual on its ships. Then these instruments were bought from Van Musschenbroek. Three years later the Company purchased octants from Benjamin Ayres, a former apprentice and brother-in- law of Sisson, working in Amsterdam since 1743. VOC governor Hasselaer played a stimulating role in this process of navigational improvements, being also the official representative of Prince William IV in three Dutch Admiralties. Hasselaer’s instrument collection reflects his interest, containing octants by both Sisson and Ayres. Sisson’s copy is particularly interesting because it has a number of technical devices, which are not found on contemporary octants. It has a brass frame with a wooden handle on the back, and two on either side for steadying the heavy instrument, a rack and pinion for slow-motion movement of the index arm, different types of adjustment for the mirrors, and a sight tube as well as a sight-vane. These devices are only found on octants by Sissons’s pupil Ayres, and on instruments made by others from the 1750s onwards. It is unlikely that Sisson’s example was intended for use at sea, for it was too heavy and probably also too expensive. Technically the instrument was well ahead of its time, suggesting that it was made specially for Hasselaer, perhaps at a time when the Company was contemplating the adoption of octants for its ships.