Tracing Ramsden's ‘Plumbline Level’

A 18th-century controversy surrounding the accuracy of English-made astronomical quadrants, particularly those used at the Greenwich Observatory, revealed that their precision was indeed exceptional. Despite French astronomer Cesar-Francois Cassini de Thury's claims in 1785-87 that the latitude and longitude measurements were erroneous by up to 15 seconds, Astronomer Royal Nevil Maskelyne defended the accuracy of these instruments, supported by meticulous examinations conducted by instrument maker John Bird. This controversy prompted Duke of Marlborough at Blenheim Palace to commission Jesse Ramsden to develop a specialized apparatus to verify the arc of his own 6-ft quadrant, ensuring its reliability amid growing doubts about English quadrants' precision.

Chinnici, I., & Randazzo, D. (2011). Tracing Ramsden’s ‘Plumbline Level’. Journal for the History of Astronomy, 42(2), 161–176. https://doi.org/10.1177/002182861104200203