and using a terrestrial mark, the azimuth error by star observations.
The time was expressed in fractions of a second. He also constructed a
telescope with equatoreal mounting, to follow a star by one axial
motion. In 1728 his instruments and observation records were destroyed
by fire.

Hevelius had introduced the vernier and tangent screw in his
measurement of arc graduations. His observatory and records were burnt
to the ground in 1679. Though an old man, he started afresh, and left
behind him a catalogue of 1,500 stars.

Flamsteed began his duties at Greenwich Observatory, as first
Astronomer Royal, in 1676, with very poor instruments. In 1683 he put
up a mural arc of 140 degrees, and in 1689 a better one, seventy-nine
inches radius. He conducted his measurements with great skill, and
introduced new methods to attain accuracy, using certain stars for
determining the errors of his instruments; and he always reduced his
observations to a form in which they could be readily used. He
introduced new methods for determining the position of the equinox and
the right ascension of a fundamental star. He produced a catalogue of
2,935 stars. He supplied Sir Isaac Newton with results of observation
required in his theoretical calculations. He died in 1719.

Halley succeeded Flamsteed to find that the whole place had been
gutted by the latter's executors. In 1721 he got a transit instrument,
and in 1726 a mural quadrant by Graham. His successor in 1742,
Bradley, replaced this by a fine brass quadrant, eight feet radius, by
Bird; and Bradley's zenith sector was purchased for the observatory.
An instrument like this, specially designed for zenith stars, is
capable of greater rigidity than a more universal instrument; and