be about 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. The Board of Ordnance theodolite 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