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.