四、請以英文作答：（20 分） The accuracy of magnetic compasses onboard ships can be influenced by both the terrestrial and ship magnetism. Such a physical phenomenon gives rise to either permanent or induced deviations to ships. Please discuss in English how the terrestrial and ship magnetism affect the magnetic compasses onboard vessels.

⑵Position line is any straight or curve line drawn on the chart. The ship position is known to lie on it. When you see a light house or other conspicuous objects, you can take the bearing of the light house or the objects with compass and azimuth circle. After obtaining the degrees of the specified light house and drawing the bearing line on the chart related to the target, a position line is obtained consequently. It shows that the vessel is located on that position line but without definite location at the particular time. As two or more position lines are obtained at the same time or only a few seconds apart, the intersection of the position lines becomes the FIX position of the vessel.（20 分）

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