In 1800 he settled at Westbury near Bristol, and began to determine star-places with a fine altitude and azimuth circle of 22 ft.
In fact the modern equatorial, and the altitude and azimuth circle are astrolabes in the strictest and oldest meaning of the term; and Tycho in one of his astrolabes came so near the modern equatorial that it may be taken as the first of the kind.
The two verniers on the azimuth circle having been read, the magnet is then inverted, i.e.
The mean of all the readings of the verniers gives the reading on the azimuth circle corresponding to the magnetic meridian.
Hence if the readings of the verniers on the azimuth circle are made when the transit is observed we can deduce the reading corresponding to the geographical meridian.