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story:Measuring the Universe


Surveying was an important activity in the eighteenth century for several reasons. The enclosure of common land was proceeding rapidly, the military needed better maps for successful campaigns and finding longitude accurately required tables which depended on the relative position of the Greenwich and Paris observatories. Long-standing debates such as the shape of the Earth were tackled by the first large-scale international expeditions. The end of the century witnessed the beginning of the first ordnance survey of Great Britain.

Plane table outfit, 1745-1755. picture zoom © Science Museum/Science and Society Picture Library

While new and sophisticated surveying instruments were produced in the eighteenth century, many surveyors still used simple techniques. Often they were employed in constructing small local maps for gentlemen farmers and would only use the chain and cross head or other simple devices such as the waywiser. A standard chain, devised by Edmund Gunter, was 22 yards long, the length of a cricket pitch, and divided into 100 links. An acre was ten square chains. A cross head gave the direction perpendicular to that being measured. Another popular simple instrument was the plane table where the map could be drawn directly as the measurements were taken.

4.5 inch altazimuth theodolite, mid eighteenth century. picture zoom © Science Museum/Science and Society Picture

However the demands of some surveys, such as the mapping of the Highlands of Scotland after the Jacobite rebellion of 1745/6, required more advanced instruments and more accurate measurements. By the second quarter of the eighteenth century instrument makers were advertising altazimuth theodolites, which could measure vertically and horizontally. Levelling instruments were also much improved and incorporated a bubble. The map of the Highlands of Scotland, conceived to aid pacification of the Highlanders, set a pattern for map making in the colonies in the next 20 years: Canada and the St Lawrence, The East Coast of North America, Bengal, and Ireland.

Plan of the Principal Triangulation 1787-9. picture zoom © Science Museum/Science and Society Picture Library

Although there had been several maps of the colonies, it was not until 1783 (when peace was eventually declared with France) that Cassini de Thury of the Paris Observatory proposed the triangulation of the Southeast of England. This was in order to link with the French triangulation so that the exact relative position of the two observatories could be established. William Roy, who was placed in charge of the project, suggested that it was necessary to 'reinvent them all'. He commissioned Jesse Ramsden, the leading instrument maker of the time, to construct a 3' theodolite weighing 200lb.

While there was no innovation in the principle of the instrument, this theodolite was bigger and better than anything preceding it. Users were able to read a mark at 70 feet with an error of only two seconds. It was called ‘the father of accurate surveying instruments’. The same could be said of Ramsden's 100 foot chain, also commissioned and used on the project.

Resource Descriptions

Plane table outfit, 1745-1755.
4.5 inch altazimuth theodolite, mid eighteenth century.
Plan of the Principal Triangulation 1787-9.