Ramsden surveying instruments
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The Ramsden surveying instruments discussed in this article are those constructed by Jesse Ramsden and used in high precision geodetic surveys carried out in the period 1784 to 1853. This includes the five great theodolites—great in name, great in size and great in accuracy—used in surveys of Britain and other parts of the world. Ramsden also provided the equipment used in the measurement of the many base lines of these surveys and also the zenith telescope used in latitude determinations.
The great theodolites
A total of eight such instruments were manufactured by Ramsden and others for use in Britain, India and Switzerland. Ramsden himself constructed three theodolites and a further two were completed to his design by Mathew Berge, his son-in-law and business successor, after Ramsden's death in 1805. Of the other instruments one was constructed by William Cary and the other two by the firm of Troughton and Simms.
Ramsden RS, the first theodolite
In 1783 the Royal Society of London reacted to French criticism of Greenwich Observatory by seeking Royal assent to undertake a high precision geodetic survey, the Anglo-French Survey (1784–1790), between Greenwich and the established French survey stations on the other side of the English Channel. Approval having been granted, General William Roy agreed to undertake the work and he immediately approached Ramsden to commission new instruments. Three years later the "great" theodolite was delivered after a delay attributable to Ramsden's tardiness, workshop accidents and his prediliction for continuous refinement—"this won't do, we must have at it again". The instrument was paid for by the Crown and the King immediately presented it to the Royal Society; for this reason the theodolite is designated as the Royal Society theodelite, or Ramsden RS in short.
There is a complete description of this theodolite in the final report of the Anglo-French Survey (1784–1790). The instrument was large, 36 inches (914 mm) across and it was normally mounted on a stand which placed the sighting telescope between 5 and 6 ft. high. It weighed about 200 lb (90 kilo) and the accessories and cases weighed as much again. It travelled around Britain for over sixty years, in its own sprung carriage, to locations where it was hauled up mountains, church towers and even scaffolded steeples.
The horizontal circular scale was divided very accurately with divisions at 15 minute (of arc) intervals using one of Ramden's own dividing engines; the marks on the 36 inch diameter scale would be about 1/6 inch (or 4mm) apart. The position of the telescope could therefore be read to the nearest quarter of a degree by eye but the exact position between the divisions was read with the aid of microscopes fitted with adjustable cross wires in the focal plane, as shown. The threads of the screws were such that fifteen full turns moved from from one scale mark to the next, i.e. 15 minutes, and since the scale on the adjusting knob allowed one sixtieth of a turn to be measured the resulting accuracy was within one arc second.
The instrument is also fitted with a vertical semi-circular scale to measure the elevations of distant stations and therefore a height difference. Cross wires similar to those used in the microscopes are fitted into the eyepiece; they are adjustable by a screw thread which allowed angles to be measured to within an arc second.
Typical distances in the Anglo-French survey were less than 20 miles (or 32 km) so that one second of arc corresponds to lateral or vertical displacements at the target station by approximately 7 inches (or 17 cm). No other theodolite could match this precision at that time. It was the first instrument to be able to measure the spherical excess of a survey triangle with sides upwards of 30 miles.
After completion of the Anglo-French survey this instrument was stored at the Royal Society but in 1799 the Board of Ordnance requested its use for the Principal Triangulation of Great Britain. On completion of the Survey the theodolite was stored in the headquarters of the Ordnance Survey at Southampton where it was destroyed in the bomb raids of 1941.
Ramsden BO, the second theodolite
. In his report to the Royal Society in 1875 William Roy had noted the suitability of India as a location for both meridian arc and parallel arc measurements. To his delight the East India Company were willing to undertake such a venture and ordered a second great theodolite from Ramsden. It was ready in 1791 but Ramsden felt obliged to increase the price because of problems in its manufacture. To his surprise the company rejected his price and refused to purchase the instrument. It was bought by the Duke of Richmond who, as Master of the Board of Ordnance, had provided most of the finance for Roy's Anglo-French Survey (1784–1790). The theodolite is designated as the Board of Ordnance theodolite, or Ramsden BO in short. Richmond's intention was to use the new theodolite on the extension of Roy's survey to the southern counties of Britain. The instrument was basically the same as the first with added refinements, mainly to the number and placement of the microscopes with their precision micrometer stages. It was in use until the completion of the Principal Triangulation of Great Britain in 1853 and it is now in the Science Museum in London.
There is a description of the improvements made to this theodolite in the account of the Trigonometrical Survey for the years 1791–1794 by Mudge, Williams and Dalby.
Other Ramsden theodolites
Ramsden made at least one other 3ft. theodolite of which parts were discovered in Switzerland. He may have made others. After his death his firm was inherited by Mathew Berge who is known to have constructed two more instruments to Ramsden's design.(Insley 2008)</ref>
Great theodolites by other builders
Chains and rods
The zenith telescope
- McConnell 2007Jesse Ramsden (1735–1800)
- Insley 2008 The Great Theodolites
- Ramsden the optician
- Roy 1790 pp135-160 The final report of the Anglo-French Survey (1784–1790).
- Ramsden 1877 The dividing engines.
- Ramsden's three foot geodetic theodolite, 1792, Science Museum, retrieved 2010-07-10
- Mudge, Dalby & Williams 1795, pp. 445–446 Account of the Trigonometrical Survey for the years 1791–1794.
- (Insley 2008)
- Insley, Jane (2008). The Tale of the Great Theodolites. FIG Working Week on Integrating the Generations. Sweden.
- Maskelyne, Nevil (1785). "Concerning the Latitude and Longitude of the Royal Observatory at Greenwich; With Remarks on a Memorial of the Late M. Cassini de Thury". Philosophical Transactions of the Royal Society of London 75: 385–480. doi:10.1098/rstl.1787.0018.
- McConnell, Anita (2007). Jesse Ramsden (1735–1800): London's leading scientific instrument maker. Aldershot: Ashgate Publishing Limited. ISBN 978-0-7546-6136-8. There is a substantial review by Richard Dunn.
- Mudge, William; Dalby, Isaac; Williams, Edward (1795). "An Account of the Trigonometrical Survey Carried on in the Years 1791, 1792, 1793, and 1794". Philosophical Transactions of the Royal Society 84: 414–622.
- "Ramsden the optician". The Mirror of Literature, Amusement and Instruction 10: 80. 1827.
- Ramsden, Jesse (1877). Description of an engine for dividing mathematical instruments. London: The Commissioners of Longitude. (The plates are incomplete.)
- Roy, William (1785). "An Account of the Measurement of a Base on Hounslow-Heath". Philosophical Transactions of the Royal Society of London 75: 385–480. doi:10.1098/rstl.1785.0024.
- Roy, William (1790). "An Account of the Trigonometrical Operation, Whereby the Distance between the Meridians of the Royal Observatories of Greenwich and Paris Has Been Determined". Philosophical Transactions of the Royal Society of London 80: 111–254. doi:10.1098/rstl.1790.0015.