This was one of 5 altazimuth instruments commissioned from Troughton and Simms for the British Transit of Venus expeditions of 1874.

An altazimuth instrument is a telescope with scales and mounting that allow both local coordinates - the altitude and the azimuth - of a star to be measured without the need of any additional apparatus.

The telescope and mount each have their own fitted wooden box. The telescope is made of brass and has a focal length of 550 mm, a lens aperture of 50 mm , and an overall tube length of 250 mm. The diameter of the setting circles is 400 mm. In addition to the telescope and mount there are 2 eyepieces, 4 fine reading microscopes, 2 course reading microscopes, 2 300 mm diameter glass plates (1 broken), 2 enlarging lenses to read the altazimuth scale (1 missing) and 2 spirit levels.