Ramsden surveying instruments

In 1783 the Royal Society of London reacted to (unfounded) 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".[3] 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).[4] The instrument was large, 36 inches (910 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 pounds (91 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.

Detail of the micrometer microscopes.

The horizontal circular scale was divided very accurately with divisions at 15 minute (of arc) intervals using one of Ramden's own dividing engines;[5] the marks on the 36-inch (910 mm) diameter scale would be about 1⁄6 inch (4.2 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 (18 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.[6]

Ramsden BO, the theodolite of 1791. Now in the Science Museum, London[7]

In his report to the Royal Society in 1775 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.[7]

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.[8]

Ramsden made at least one other 3-foot (0.91 m) theodolite of which parts were discovered in Switzerland.[2] After his death his firm was inherited by Mathew Berge who is known to have constructed two more large instruments to Ramsden's design.[2] Ramsden made many theodolites including an eighteen-inch instrument of the same general design as the great theodolites.[9] This instrument and his two great theodolites are described in the account of the Principal Triangulation by Clarke & James (1858).[10]

Although the East India company turned down Ramsden's second theodolite they commissioned a similar design from another London instrument maker, William Cary. This theodolite was in use in India from 1802 although there was a slight hiatus in 1808 when it was damaged when being hauled to the top of a building. It was repaired and in use for sixty years. A new theodolite to an improved design was made for the Indian Survey by Troughton and Simms in 1830 and the two together saw service until 1874 when the Cary instrument was replaced by another by Troughton and Sims. This last instrument was a monster weighing 1,455 lb (660 kg) when in its travelling cases: it was no surprise that it was deemed too heavy for transport up mountains and it passed to South Africa in 1882.[2] No more great instruments were made after 1874 for continuing advances in precision instruments eclipsed their performance: by the end of the nineteenth century an accuracy of 1 arc second could be obtained with a 12-inch (300 mm) instrument.[11]