rgency is 2/3. But the compasses which are
made at Seville, Lisbon, Rochelle, Bordeaux, Rouen, and throughout all
England have an interval of 1/2 a rumbe. From those differences most
serious errors have arisen in navigation, and in the marine science. For as
soon as the bearings of maritime places (such as promontories, havens,
islands) have been first found by the aid of the mariners' compass, and the
times of sea-tide or high water determined from the position of the moon
over this or that point (as they say) of the compass, it must be further
inquired in what region or according to the custom of what region that
compass was made by which the bearings of those places and the times of the
sea-tides were first observed and discovered. For one who should use the
British compass and should follow the directions of the marine charts of
the Mediterranean Sea would necessarily wander very much out of the
straight course. So also he that should use the Italian compass in the
British, German, or Baltic Sea, together with marine charts that are made
use of in those parts, will often stray from the right way. These different
constructions have been made on account of the dissimilar variations, so
that they might avoid somewhat serious errors in those parts of the world.
But Pedro Nunez seeks the meridian by the mariners' compass, or versorium
(which the Spanish call the needle), without taking account of the
variation: and he adduces many geometrical demonstrations which (because of
his slight use and experience in matters magnetical) rest on utterly
vicious foundations. In the same manner Pedro de Medina, since he did not
admit variation, has disfigured his _Arte de Navegar_ with many errors.

* * * * *

CHAP. IX.

Whether the terrestrial longitude can be found from
_the variation_.

Grateful would be this work to seamen, and would bring the greatest advance
to Geography. But B. Porta in chap. 38 of book 7 is mocked by a vain hope
and fruitless opinion. For when he supposes that the magnetick needle would
follow order and proportion in moving along meridians, so that "the neerer
it is to the east, the more it will decline from the Meridian line, toward
the east; and the neerer it comes to the west, the {167} point of the
needle will decline the more to the west" (which is totally untrue), he
thinks that he has discovered a true index of longitude. But he is
mistaken. Nevertheless, admitting a