A ship is steered on a straight course by means of a magnetic compass. This consists of a freely suspended magnet balanced on a fine point of hard metal, to which is attached a light compass card marked with the points of the compass, and with degrees. The whole is encased in a sealed bowl of non-magnetic metal which is both air-tight and watertight. On the fore edge of the compass bowl a line, the "lubber" line, is marked, representing the ship's head. Thus the course the ship is steering can easily be read, as it is in line with the lubber line on the compass bowl. Remember the compass card remains stationary, it is the lubber line which moves with the ship. The compass does not point to true north, but to magnetic north, one of the poles of the earth's magnetic field. Allowance is made for this variation.