Autograph letter dated 4th March 1840 to Thomas Maclear Esq, The Observatory, Cape of Good Hope:
Sir,
Captain Henderson, the Officer of Engineers appointed by the Master General of the Ordnance to assist you in the Survey at the Cape of Good Hope, being about to embark with his party and stores, I am commanded by my Lords Commissioners of the Admiralty to send you herewith a draft of Instructions, for your guidance.
I am Sir, Your very humble servant,
(Signed) John Barrow
2 pages on a sheet of laid water marked paper, foolscap (320 x 200 mm).
With:
The Instructions in another hand, 12 pages, foolscap (320 x 200 mm).
Official Instructions to Thos. Maclear:
I. Mr. Maclear being now furnished with standard bars & compensation bars for effecting a base measure of the most accurate kind & with an excellent Theodolite for the observation of angles in triangulation & having also the assistance of an Engineer Officer of great experience in the Ordnance Survey of Britain, with a party of Sappers together with all necessary stores & the Commander of the Forces in Cape Town having been directed to supply to Mr. Maclear upon his request such further assistance as can be conveniently spared from the duties of the Garrison — His Lords Commissioners of the Admiralty consider that Mr. Maclear is now amply supplied with every requisite for carrying on the survey connected with the repetition of Lacaille's Measure of an Arc of Meridian contemplated in the Xth Article of the Instructions transmitted by Their Lordships to Mr Maclear on the occasion of sending Bradley's Sector to the Cape of Good Hope & they desire therefore that Mr Maclear will proceed as soon as practicable to the execution of that Survey.
II. Their Lordships desire in the first place that it may be clearly understood that the Officer of Engineers is to be held responsible for nothing more than giving to Mr Maclear every assistance which his experience in the use of the Instruments enables him to afford, managing the assisting persons to the best advantage, taking charge of the stores & accounts of the party and providing for their security. For the Scientific accuracy of the Survey Mr Maclear is to be entirely responsible & Their Lordships are confident that with the means at his command Mr Maclear will conduct the survey in such a manner as to make it valuable to Science and creditable to the Nation.
III. The objects proposed in this survey are the following: First: the verification of LaCaille's survey. Secondly: the extension of LaCaille's Arc of Meridian, if it should be found easy to extend it. Thirdly: the fixing geographically such points as will hereafter be useful in any survey that may be undertaken for accurately mapping the Cape Colony.
IV. With regard to the measure of the base their Lordships think it proper that the following rules should be followed.
- Before commencing the field work, the party should be trained to the use of the compensation bars on the ground in the neighbourhood of the Observatory.
- In the selection of a line for the base measure, LaCaille's line, if it can be identified, is to be preferred.
- The two iron bars must be exposed most carefully before commencing……
IX. It will be Mr. Maclear's duty as early as possible to transmit to the Secretary of the Admiralty a complete copy of the observations of every kind made on the survey &, as soon as the reduction of the observations shall be terminated, to transmit to the Secretary a full statement of the results obtained & the method by which they have been obtained.
This letter initiated the most important geodetic event in southern hemisphere history in three distinct areas: the scientific shape of the Earth, the practicality of global navigation, and the colonial mapping of South Africa.
S2A3 Biographical Database of Southern African Science (https://www.s2a3.org.za/bio/Biograph_final.php?serial=1791)
Thomas Maclear, astronomer at the Royal Observatory, Cape of Good Hope (1794-1879),.....One of Maclear's tasks was to re-measure and extend the arc of meridian measured some 80 years earlier by Abbe N.L. De la Caille and at the same time determine the geographical positions of points for later use in mapping the colony. From 1837 to 1847 Maclear and his assistants spent a large proportion of their time on the field work of this endeavour. Despite intensive efforts to identify De la Caille's survey points (Maclear, 1838) he could not find the exact site of the northern end point of the earlier arc and therefore conducted a new survey. Six months were spent in accurately measuring a baseline some 13 km long in the Zwartland; Maclear estimated the probable error in its length to be about 8 mm - less than one part per million. The angular measurements were made with the best available theodolites and were often repeated 100 times or more. The arc stretched from the Royal Observatory in Cape Town to a point beyond the Kamiesberg in Bushmanland (a north-south distance of some 500 km), while the triangulation was extended eastwards from Cape Town to Cape Agulhas. The latitudes at the north and south points of the arc, and at several other stations, were accurately measured with an unwieldy zenith sector. The results were published in two volumes entitled Verification and extension of La Caille's arc of meridian at the Cape of Good Hope (London, 1866).
Oxford Dictionary of National Biography, Cameron (J. M. R.) (https://www.oxforddnb.com/display/10.1093/ref:odnb/9780198614128.001.0001/odnb-9780198614128-e-1544)
Barrow, Sir John, first baronet (1764–1848), promoter of exploration and author, was born on 19 June 1764 at Dragley Beck, Ulverston, Lancashire, the only child of Roger Barrow, journeyman tanner, and his wife, Mary. Educated at the local Town Bank grammar school, which he left at the age of thirteen, Barrow worked successively as a clerk in a Liverpool iron foundry, as a landsman on a Greenland whaler, and as a mathematics teacher in a Greenwich academy preparing young men for a naval career, until offered the position of comptroller of household to Lord Macartney's embassy to China (1792–4). He served with distinction during this embassy and Macartney's governorship of the Cape of Good Hope (1797–9), collecting much of the commercial and strategic intelligence about the eastern seas and southern Africa that Macartney forwarded to Henry Dundas, president of the Board of Control and secretary of state at war. Barrow was promoted to the post of auditor general to Cape Colony in September 1798, and married Anna Maria Trüter (1777–1857) at Stellenbosch on 26 August 1799. His intention to settle at the Cape was frustrated by its return to the Dutch in 1803. He was offered the second secretaryship of the Admiralty by Dundas (by then Lord Melville) on 5 May 1804, and this he held, except for the period between 10 February 1806 and 7 April 1807, until 28 January 1845.
Reserve: $3000
Estimate: $4000/5000
