This instrument is a modified version of the backstaff (see NAV0041), which was used to determine the height of the sun or a star above the horizon.

The backstaff is made from a lignum vitae frame with boxwood arcs, brass-capped rivets, and brass fittings. The sight vane and the horizon vane are made of brass, with a bubble level perpendicular to the frame. The shadow vane and lens, the bubble level that should be parallel to the frame, the bubble level for the label arm, and the oil lamp, are all missing. The oil lamp could be slotted into the frame to illuminate the spirit level tube when making observations at night.

The artificial horizon was designed by John Elton (patent no. 501 of 1728) and consists of two bubble levels that are used to keep the index arm horizontal, while the frame of the instrument rotates. As with a regular backstaff, the shadow or a light spot falls on the horizon vane, and the altitude is read from the scale. A third bubble level mounted to the label arm was used for the forward observation. The modifications, which John Elton first published in 1732, allowed the observer to measure the altitude of the sun when the horizon was not easily visible. Instead of lining up a vane with the horizon, the observer used the spirit levels attached to the instrument to determine when it was correctly orientated.

The scale on the thirty degree arc is from -2° to 32° by 10 arcminutes and reads to 1 arcminute by means of a vernier with an indicating line. There is no sixty degree arc, so that angles larger than 32° cannot be measured.

This is the earliest navigational instrument found with a vernier.