page_title: Ramsden surveying instruments
section_title: The Royal Society theodolite
hierarchical_section_title: Ramsden surveying instruments / The great theodolites / The Royal Society theodolite
caption_attribution_description: English: Detail of the microscopes of the three foot theodelites manufactured by Jesse Ramsden. Image published in "An Account of the Trigonometrical Operation, Whereby the Distance between the Meridians of the Royal Observatories of Greenwich and Paris Has Been Determined. By Major-General William Roy" (January 1, 1790). Now available at archive.org. The illustration is from Plate VI. (The plates are at the end of the bound volume but Plate VI is at page 214 of the PDF file.) The full text description is on true pages 135-160 corresponding to pages 28-53 of the PDF file
context_page_description: The Ramsden surveying instruments 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.
context_section_description: 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 predilection 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 theodolite, 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 kg) 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 (910 mm) diameter scale would be about ​¹⁄₆ inch (4 mm) 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 micrometer 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 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 five arc seconds.
  Typical distances in the Anglo-French survey were less than 20 miles (32 km): at that distance one second of arc corresponds to lateral or vertical displacements at the target station of approximately 7 in (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 large survey triangles.
  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 bombing raids of 1941.
caption: Detail of the micrometer microscopes.
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