Plough The plough was the name given to an unusual instrument that existed for a short time. It was part cross-staff and part backstaff. In Figure 5, A is the transom that casts its shadow on the horizon vane at B. It functions in the same manner as the staff in Figure 1. C is the sighting vane. The navigator uses the sighting vane and the horizon vane to align the instrument horizontally. The sighting vane can be moved left to right along the staff. D is a transom just as one finds on a cross-staff. This transom has two vanes on it that can be moved closer or farther from the staff to emulate different-length transoms. The transom can be moved on the staff and used to measure angles. Almucantar staff The Almucantar staff is a device specifically used for measuring the altitude of the sun at low altitudes. Cross-staff The cross-staff was normally a direct observation instrument. However, in later years it was modified for use with back observations. Quadrant There was a variation of the quadrant – the Back observation quadrant – that was used for measuring the sun's altitude by observing the shadow cast on a horizon vane. Thomas Hood cross-staff Thomas Hood invented this cross-staff in 1590. It could be used for surveying, astronomy or other geometric problems. It consists of two components, a transom and a yard. The transom is the vertical component and is graduated from 0° at the top to 45° at the bottom. At the top of the transom, a vane is mounted to cast a shadow. The yard is horizontal and is graduated from 45° to 90°. The transom and yard are joined by a special fitting (the double socket in Figure 6) that permits independent adjustments of the transom vertically and the yard horizontally. It was possible to construct the instrument with the yard at the top of the transom rather than at the bottom. Initially, the transom and yard are set so that the two are joined at their respective 45° settings. The instrument is held so that the yard is horizontal (the navigator can view the horizon along the yard to assist in this). The socket is loosened so that the transom is moved vertically until the shadow of the vane is cast at the yard's 90° setting. If the movement of just the transom can accomplish