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ISSN 1039-3498 | no. 47| October 2000
Re-assembling Mitchells terrestrial sextant
Artist, poet, surveyor and explorer, Thomas Livingston Mitchell, Australias first Surveyor-General arrived in Sydney in September 1827, having emigrated from England with a young family.
Following the return of John Oxley to England in 1828, Mitchell found himself appointed Surveyor-General of New South Wales on 28 May 1828. He proved to be a tough and demanding leader, expecting from his staff the same degree of enthusiasm, commitment and efficiency which he applied to his own life.
Beginning with the first triangulation survey of the colony in 1828, Mitchell went on to conduct around 900 surveys leading to the production of the first of his significant maps produced in Australia: Map of the Colony of New South Wales, published in 1834.
Vital to the compilation and subsequent publication of his surveys were various instruments, which were in use by surveyors and explorers of the nineteenth century. Mitchells theodolite, now in the possession of the New South Wales Department of Lands, has been well documented for the role it played in his triangulation survey in 1828.
Little is known of the use he made of his large and impressive terrestrial sextant, acquired by the National Library of Australia in the 1960s. Housed in a slightly battered and cracked cedar box, the instrument, constructed from brass, base metal (stand) and glass (lenses), was made by the British mathematical instrument firm, R.B. Bate.
It is not known when Mitchell acquired the instrument, which weighs, with box, a hefty 26 kilograms. However, given that it can be operated in two positions, it would have been an essential instrument for all his surveys and it is presumed it was acquired around the same time as the theodolite, in 1828.
Since its acquisition by the Library nearly 30 years ago, the instrument has lain in its box, in three sections until recently when a local surveyor with an interest in Mitchells surveys demonstrated to staff the simple method of assemblage! The heavy, ribbed, base metal stand provides support to the main body of the instrument while the large observation lens is screwed into its support at the top of the structure. An accompanying mercury bath, which has not survived, would have provided the reflecting surface when the instrument was used for levelling purposes.
The instrument operates in two positions, the vertical position, used for trigonometric levelling, and the planar or horizontal plane position (as shown in the illustration) for measuring the sextant angle as required in plane table surveying.
In addition to the sextant, which is one of the treasures of the Librarys
collection, manuscript letters, diaries and other published maps created
by Mitchell form a lasting memorial to one of the greatest surveyors and
explorers in Australias European history.
Maura OConnor
Above left: Mitchells sextant is one of the treasures of the Librarys collection. Photograph: Damian McDonald
Please address any comments and enquiries to The Editor, Gateways: gateways@nla.gov.au