former. Vanes were added to the ends of the longest cross-piece and another to the end of the main staff. The instrument was reversed so that the shadow of the upper vane on the cross piece fell on the vane at the end of the staff. The navigator held the instrument so that he would view the horizon lined up with the lower vane and the vane at the end of the staff. By aligning the horizon with the shadow of the sun on the vane at the end of the staff, the elevation of the sun could be determined. This actually increased the accuracy of the instrument, as the navigator no longer had to position the end of the staff precisely on his cheek. Another variant of the cross-staff was a spiegelboog, invented in 1660 by the Dutchman, Joost van Breen. Ultimately, the cross-staff could not compete with the backstaff in many countries. In terms of handling, the backstaff was found to be more easy to use. However, it has been proven by several authors that in terms of accuracy, the cross-staff was superior to the backstaff. Backstaves were no longer allowed on board Dutch East India Company vessels as per 1731, with octants not permitted until 1748. In surveying In surveying, the term jacob staff refers to a monopod, a single straight rod or staff made of nonferrous material, pointed and metal-clad at the bottom for penetrating the ground. It also has a screw base and occasionally a ball joint on the mount, and is used for supporting a compass, transit, or other instrument. The term cross-staff may also have a different meaning in the history of surveying. While the astronomical cross-staff was used in surveying for measuring angles, two other devices referred to as a cross-staff were also employed. Cross-head, cross-sight, surveyor's cross or cross - a drum or box shaped device mounted on a pole. It had two sets of mutually perpendicular sights. This device was used by surveyors to measure offsets. Sophisticated versions had a compass and spirit levels on the top. The French versions were frequently eight-sided rather than round. Optical square - an improved version of the cross-head, the optical square used two silvered mirrors at 45° to each other. This permitted the surveyor to see along both axes of the instrument at once. In the past, many surveyor's instruments were used on