Book IV
The Fourth Book treats of the variation at different places; says that it is due to inequality among the earth’s elevations;[35] shows that variation and direction are due to the controlling force of the earth and the rotatory magnetic nature, not by an attraction or a coition or by other occult cause; explains the different modes of constructing the mariner’s compass, in vogue at the time,[36] and how the deviation of the needle is greater or less according to the distance of place.