Troughton's repeating circle without reflection, 1820.
© Science Museum / Science & Society Picture Library
Print of 'Troughton's Repeating circle' with diagram, an instrument used for astronomical measurement and large-scale surveying work. The engraving, taken from 'Rees Cyclopedia' published 1812-1820, shows a portable instrument that was used to precisely measure angles in both the heavens and in trigonometrically surveys on the Earth. Having two circular scales and telescopic sights the instrument's accuracy was achieved by schematic repeated observations. The French astronomer Pierre-Francois-Andre Mechain (1744-1805) and Joseph Delambre (1749-1822) used Instruments of this type were used by to survey a north-south meridian arc acros France. From these measurements the length of the metre was defined as one ten-millionth of the Earth's quadrant measured from its poles.