Experience shows that, in all cases, like poles of magnets repel and unlike poles attract. This principle, called the law of magnets, is utilized in the device known as the mariner's compass (Fig. 70), which consists of a magnetized steel needle balanced on a point so that it will turn freely; the points of the horizon are marked on a compass card. The needle is acted upon by the earth, which is a magnet. The needle behaves the same way in all parts of the earth. The north magnetic pole is near Hudson Bay, the south magnetic pole in the Antarctic Ocean. As the true North Pole and the magnetic north pole are not the same, allowance must be made for this variation. Ships travel from point to point by the assistance of the mariner's compass.