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Plan and Section of Jesse Ramsden's 'Great Theodolite', from "AN ACCOUNT of the Trigonometrical operation for determining the difference of Longitude"

Plan and Section of Jesse Ramsden's 'Great Theodolite', from

Artist and engraver: Reynold, Thomas

Medium: Engraving

Date: 1790

Shelfmark: King's MS 272

Item number: f.37

Length: 401

Width: 716

Scale: Millimetres

Genre: Drawing

Thomas Reynold's plan and sections of Ramsden's theodolite reveal its elegant construction. As the telescope rotates, its angle of view is read off the circular scale through a microscope-like arrangement of lenses mounted at each end of a bar set at right angles to the telescope. Averaging the measurements taken from both ends gives a more accurate observation. If necessary, the scale is illuminated by a candle, set in a housing to protect it from the wind. The accuracy of the measuring scale was of the utmost importance. In 1753, Ramsden had invented a device called a circular dividing engine, which revolutionised scientific instruments. Until then, all scales had been engraved by hand, a process that was time consuming, expensive and limited in accuracy. Dividing a circular scale into degrees and fractions of a degree was a particularly difficult task. Ramsden's circular dividing engine mechanised the procedure and virtually eliminated human error. The scale to be engraved was mounted on a circular platform with a toothed edge. The evenly spaced teeth engaged a horizontal screw to form a 'worm gear'. This gearing transferred a turn of the screw into a much reduced, and therefore very controlled, rotation of the platform. The platform could be moved through such small angles that it became possible to engrave lines on the scale to an accuracy of one sixtieth of a degree, less that a 20,000th part of the full circle.
Ramsden was a Yorkshire man, the son of an inn keeper. At the age of 23, he came to London as an apprentice to the instrument maker, J Burton. In 1762, he established his own business in Piccadilly. During the following decades, he built many of the most important astronomical and surveying instruments in Britain and Europe. Such was his skill that customers accepted the long delays in delivery, for which Ramsden became notorious. The king himself once observed that Ramsden had arrived at court on the right day and at the right time - but a year late.

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