1. A study of the accuracy of scale graduations on a group of European astrolabes
2. This information is contained in a letter from Joseph Crosthwait to Abraham Sharp, January 27, 1721–2. It is bound in date order in a manuscript volume entitled ‘Sharp Letters’, in the Royal Society Library, ref. FlSh xxiv d. Part of the letter is also printed in
Baily's Francis Account of the Revd. John Flamsteed London 1835 346 346 The Mural Arc, which encompassed 140°, came into use in 1688, and remained the principal instrument until 1719. It was sold following Flamsteed's death, and its whereabouts unknown after about 1722.
3. The quadrant's current accession no. is 1975.54. I am indebted to Dr. A. D. C. Simpson, of the Royal Scottish Museum, for further information about this instrument. It was included in 1876, in the exhibition of scientific apparatus at South Kensington as Item 1773. See Catalogue , third edition 398 398 See also David Bryden's “Britain's First Observatory”, Journal for the History of Astronomy, 3 (1972), 205.