Eyepiece for the sextant used by Admiral Field aboard HMS DART
Datec 1884
Object number00003961
NameSextant part
MediumBrass
DimensionsOverall: 90 x 22 x 22 mm, 0.05 kg
ClassificationsTools and equipment
Credit LineANMM Collection
DescriptionAn eyepiece for the sextant (00003964) used by Sir Arthur Mostyn Field, Royal Navy, in surveying and charting Australian waters in HMS DART.
Between the years 1885 - 1889, Field surveyed the Louisiade Archipelago, off the southeast coast of New Guinea,
and waters off Queensland and Tasmania. His work in these areas were highly commended by the Admiralty and on his return to England in 1889, he was promoted to the rank of Commander.
HistoryAdmiral Sir Mostyn Field (1855 -1950 ) served on the Australia Station, on Admiralty surveys, between 1885 and 1899. As commander of HMS DART and later HMS PENGUIN, Field surveyed and charted much of the coasts of south eastern New Guinea and Tasmania, and various island groups in the South Pacific. in 1889 Field returned to England, was promoted to Commander and spent time in Irish and Scottish waters. Field went on to become Hydrographer of the Royal Navy from 1904 to 1909. He later recalled this time as being "an imporatnt period in the creation and development of the North Sea Fleet, and the selection and survey of suitable ancorages for it. In a letter to The Times in 1920, Field himself described the circumstances in which Scarpa Flow was decided upon as an ancorage for a large fleet, and how the surveying ship TRITON left Chatham in 1905 to begin a survey, lasting nearly five years, of what was destined in the war with Germany to be the principal base of the Grand Fleet." (The Times, Obituary, 2 July, 1950).
This sextant, made by Troughton and Sims, was used by Field on the Australia Station. It is a surveyor's/sextant ,equipped with a stand and two spirit levels so it could be used both on land and on shipboard to establish positions .
The 470-ton two-masted auxiliary schooner DART was built in 1877 by Barrow Shipbuilding Company, Barrow, North West England and fitted out as a surveying vessel for the Royal Navy's work on the Australia Station. It was employed in hydrographic surveys around the Great Barrier Reef, Tasmania, New Zealand and the Pacific Islands during the 1880s and 1890s.
In 1912 DART was sold to the Victorian Government and commenced service as a training ship for cadets, in which capacity it served until 1919.
The sextant was developed in 1757. It is an instrument of double reflection by means of two mirrors, and thus although its actual arc subtends an angle of 60 degrees (1/6th of a circle - hence the name sextant), it is capable of measuring angles up to 120 degrees. The sextant was an improvement on the earlier quadrant, an instrument capable for measuring angles up to 90 degrees (1/4th of a circle hence the name quadrant). The capacity of sextants to read angles greater than 90 degrees was an advantage when using the lunar distance method to determine longitude. This was also useful for taking horizontal angles.SignificanceSir Arthur Mostyn Field's career in the Royal Navy was extensive. He spent at least 23 years in foreign service as a nautical surveyer, including four years at the Australia Station. As Hydropgrapher to the Royal Navy, he was influencial in establishing Scarpa Flow as the base for the Great Fleet in WW1 and later WW2. Field wrote extensively on the technical aspects of maritmie surveying in publications such as Proceedings of the Royal Society, Encyclopedia Britannica and Hydrographical Surveying.
