Tracing Ramsden's ‘Plumbline Level’

Abstract

Doubts on Accuracy of English QuadrantsIn eighteenth century practice of astronomy in Europe consolidated through establishment of several institutional observatories' and production of a suite of designs for observatory instruments2 such as mural quadrants (Figure 1), transit instruments, zenith and equatorial sectors - all instruments essentially related to main research programme of that time, namely astrometry.English makers became specialists in design and construction of observatory equipment, especially transit instruments and quadrants. The accuracy of their astrometric instruments3 allowed Astronomer Royal James Bradley to discover aberration of light and nutation, made known to astronomical community in 1729 and 1748 respectively. Following in footsteps of George Graham (1673-1751), his associates Jonathan Sisson (1690-1747) and John Bird (1709-76) became leaders in construction of such measuring instruments for practical astronomy; moreover, they succeeded in exporting them throughout Europe to equip important observatories.4The high reputation gained by English quadrants seriously questioned in years 1785-87, when controversies arose about accuracy of latitude and longitude measurements of Greenwich Observatory. During preliminary astronomical operations carried out to establish a geodetic link between Greenwich and Paris Observatories,5 Cesar-Francois Cassini de Thury (better known as Cassini HJ), director of Paris Observatory, cast doubts on precision of English measurements, asserting that latitude and longitude of Greenwich were in error by 15 and 11 respectively.6 In France, superior quality of English quadrants over French ones openly admitted by French community7 but this hard to accept for Cassini ??, who was painfully aware that French craftsmen were being eclipsed8 by their English colleagues: it not by chance that Paris Observatory had no English-made instruments at that time.9A few months after Cassini Ill's death, his on alleged errors in latitude and longitude of Greenwich Observatory passed to Astronomer Royal, Nevil Maskelyne, so that he might examine Cassini's claims.10 Two years later, in 1787, Maskelyne replied with a published memoir in which he defended results of measurements obtained by his predecessors, especially James Bradley, by whom the astronomical refractions and latitude were settled with greatest accuracy.12 The reports careful examinations of Greenwich 8-ft mural quadrant, made in 1750 by Bird, whose accuracy had been indirectly questioned. Bird himself, at Bradley's request, had twice examined arc of Greenwich quadrant, finding it half a second short in 1753 and devoid of errors in 1759.'3 Maskelyne concluded that determinations of latitude were accurate and unaffected by exceedingly large errors indicated by Cassini III, thus implying that it French measurements that were to be questioned.14Nonetheless, this controversy seems to have led some English astronomers to challenge reliability of astronomical quadrants. In early 1780s, Duke of Marlborough had equipped his private observatory at Blenheim Palace with a 6-ft quadrant built by Jesse Ramsden ( 1735-1 800).15 Because of recent doubts cast on accuracy of quadrants, in years 1787-88 he commissioned from Ramsden an apparatus for examining total arc of quadrant.16 By that time, Ramsden had also made 8-ft quadrants for Vilnius17 and Padua18 Observatories, delivered respectively in 1777 and 1779, which upon examination by Maskelyne had both shown an error not exceeding 2.5. 19 Moreover, as Ramsden in midst of constructing another 6-ft quadrant for Brera (Milan) Observatory,20 he understandably anxious to remove all doubts as to quality of his quadrants. …