The following details are taken from the Introduction to the 1837 volume of Greenwich Observations.
‘The focal length of the vertical telescope is about 25 feet, and its aperture 5 inches. It turns horizontally [freely in azimuth] upon a conical pivot at its lower end, and in a collar with steadying springs at nearly 5 feet from its upper end. This collar is carried by the upper extremity of a very large iron tube, within which the tube of the telescope revolves without touching it: the iron tube is supported by four curved iron legs diverging from its lower extremity, which is 4 feet 8 inches above the lower extremity of the telescope: the iron tube has no other support; and neither the iron tube nor its legs is connected with the walls of the building. At 3½ inches below the object glass, the wire of the plumb-line is attached to a reel exterior to the telescope-tube (carried by the tube), and, passing over a screw, depends in the interior of the tube. The use of the screw is, to enable the observer to move the point of suspension laterally, either for the convenience of bringing the wire to the center of the field of the microscopes, or for the estimation of the relative values of the micrometers (to be mentioned hereafter). The wire in the greater part of its length passes within a small tube fixed within the telescope-tube: the plumb-bob hangs in a pot of water attached below the plate closing the lower end of the telescope-tube. At 5½ inches below the screw, a micrometer-microscope is fixed on the same side of the tube, for observing the upper part of the wire: at the opposite side of the tube is an aperture with a reflector, for illuminating the microscope. At 1½ inches above the lower end of the telescope, another micrometer-microscope is fixed on the opposite side of the tube, for observing the lower part of the wire. In the eye-piece of the telescope there are, carried by a fixed plate, ten pairs of wires arranged as acute crosses; and, carried by the moveable micrometer-plate, eleven single wires, whose intervals are not very different from those of the fixed crosses. This moveable plate is made to slide by a micrometer-screw acting in opposition to a spring in the usual way: its range is such as will carry any one of the moveable wires to touch the adjacent fixed wires on each side. The eye-piece is a four-glass eye-piece with a diagonal reflector: it is carried by a separate slider.
In the daily use of the Zenith Tube, it is reversed in azimuth after each observation.’
Later volumes of Greenwich Observations carried a slightly extended version as a result of changes subsequently made to the instrument. Click here to read the final version to be published before the instrument was decommissioned.
The extension of Flamsteed House for Airy in 1835 had a negative effect on the seeing particlulaly as a result of a stove installed in the most northerly of the new bedrooms whose flue failed to clear the top of the Octagon Room. With the prevailing wind from the southwest, the smoke must frequently have blown across the top of the Zenith Tube, and towards the Octagon Room windows. Airy told the Visitors in 1836 that he had ‘reason to think that much of the difficulty of seeing stars well defined, &c. arose from the passage of currents of unequally-heated air near to the object-glass’. It took some time for the flue to be rerouted. The Zenith Tube Room was incorporated into the living accommodation in 1848. The1846 plans deposited in the National Archives (ADM140/426) include an east-west sectional view though the Zenith Tube and Zenith Tube Room (Section 6).
No observations were published in 1835 & 1836
Main, R.: On the Values of the Constants of Nutation and Aberration, and of the Parallax of γ Draconis, as deduced from the Observations made with the Twenty-five-foot Zenith Tube at the Royal Observatory, Greenwich (Memoirs of the Royal Astronomical Society, Vol. 24, p.147–187