A compass which suffers deviation due to magnetic material in the boat can be checked by swinging. That means to hold the boat approximately at one point, but with her head at successive directions around the horizon. When the boat is brought to rest at each point the bearing of some fIxed point ashore is read from the compass. Since any magnetic material in the boat will have been moved around the compass during this process the deviation it causes on each heading can be noted. These errors can be noted on a Deviation card or they may be plotted as a curve on graph paper.