Use cursor to rotate and open compass cover.
This instrument was normally mounted on the end of a staff that could be rested on the ground. The compass housing and the cross piece with the vertical metal "sights" on either end rotate. To take a bearing, Lewis or Clark would rotate the cross piece and upright sights while looking through the holes for a fixed visible point. When that fixed point was found, the bearing or azimuth between it and the observer could be read from the compass dial.