Bird, John (1709 - 1776)

In 1749 he was contracted by the Royal Observatory to build a new mural quadrant which had a radius of 8 feet (2.43 metres) and two carefully engraved arc plates. Bird's major acheivement was to devise a reliable way to mark the arc plates on the quadrant to a high degree of accuracy by the method of continual bisection devised by the astronomer George Graham (1673-1751). Essentially, this method involved determining half the distance of a circular arc by calculating the exact middle point from either end and then bisecting the minute disparity that invaribly occurred. The same process was repeated with each half of the circular arc, then each quarter, eighth and so on until the divisions on the arc could register minute differences in measurement.

The accuracy of measurements obtained by the quadrant resulted in Bird building similar instruments for observatories in Russia, France Spain and Hanover. He was also paid 500 pounds by the Board of Longitude to teach an apprentice his methods and to provide the board with a full account of them, which he did by the publication of two pamphlets: The Method of dividing Astronomical Instruments(1767), and The Method of constructing Mural Quadrants exemplified by a Description of the Brass Mural Quadrant in the Royal Observatory at Greenwich (1768). 142).

Among other instruments constructed by Bird was the astronomical quadrant used by James Cook at Tahiti to observe the Transit of Venus in June 1769.