were switched in favour of just one with pegs to indicate the ends. These pegs were mounted in one of several pairs of holes symmetrically located on either side of the transom. This provided the same capability with fewer parts. The transom on Frisius' version had a sliding vane on the transom as an end point. Usage The user places one end of the main staff against their cheek, just below the eye. By sighting the horizon at the end of the lower part of the transom (or through the hole in the brass fitting) [B], then adjusting the cross arm on the main arm until the sun is at the other end of the transom [C], the altitude can be determined by reading the position of the cross arm on the scale on the main staff. This value was converted to an angular measurement by looking up the value in a table. Cross-staff for navigation The original version was not reported to be used at sea, until the Age of Discoveries. Its use was reported by João de Lisboa in his Treatise on the Nautical Needle of 1514. Johannes Werner suggested the cross-staff be used at sea in 1514 and improved instruments were introduced for use in navigation. John Dee introduced it to England in the 1550s. In the improved versions, the rod was graduated directly in degrees. This variant of the instrument is not correctly termed a Jacob's staff but is a cross-staff. The cross-staff was difficult to use. In order to get consistent results, the observer had to position the end of the pole precisely against his cheek. He had to observe the horizon and a star in two different directions while not moving the instrument when he shifted his gaze from one to the other. In addition, observations of the sun required the navigator to look directly at the sun. This could be a uncomfortable exercise and made it difficult to obtain an accurate altitude for the sun. Mariners took to mounting smoked-glass to the ends of the transoms to reduce the glare of the sun. As a navigational tool, this instrument was eventually replaced, first by the backstaff or quadrant, neither of which required the user to stare directly into the sun, and later by the octant and the sextant. Perhaps influenced by the backstaff, some navigators modified the cross-staff to operate more like the