Bras quadrant, 1685-1724.
© Science Museum / Science & Society Picture Library
This astronomical quadrant on a wrought iron stand was made by the French instrument maker Michael Butterfield (1635-1724) in Paris. Astronomical quadrants were used to measure the angle of a celestial object from the zenith. Operated in conjunction with a transit instrument, they could be used to check the running rate of astronomical clocks known as regulators. The instrument's orientation in both the horizontal and vertical could be finely adjusted using both the plumb bob and spirit level. Unlike later instruments, the sighting telescope is fixed to the quadrant that pivots at the centre of its graduated radial scale which is marked in degrees.