Why the variation from that lateral cause is not
_greater than has hitherto been observed, having been_
rarely seen to reach two points of the mariners'
_compass, except near the pole_.
The earth, by reason of lateral eminences of the stronger globe, diverts
iron and loadstone by some degrees from the true pole, or true meridian.
As, for example, with us English at London it varies eleven degrees and
1/3: in some other places the variation is a little greater, but in no
other region is the end of the iron ever moved aside very much more from
the meridian. For as the iron is always directed by the true verticity of
the earth, so the polar nature of the continent land (just as of the whole
terrene globe) acts toward the poles: and even if that mass divert
magnetick bodies from the meridian, yet the verticity of those lands (as
also of the whole earth) controls and disposes them so that they do not
turn toward the East by any greater arc. But it is not easy to determine by
any general method how great the arc of variation is in all places, and how
many degrees and minutes it subtends on the horizon, since it becomes
greater or less {164} from diverse causes. For both the strength of true
verticity of the place and of the elevated regions, as well as their
distances from the given place and from the poles of the world, must be
considered and compared; which indeed cannot be done exactly: nevertheless
by our method the variation becomes so known that no grave error will
perturb the course at sea. If the positions of the lands were uniform and
straight along meridians, and not defective and rugged, the variations near
lands would be simple; such as appear in the following figure.
[Illustration]
This is demonstrated by a long loadstone the poles of which are in the ends
A B; let C D be the middle line and the æquinoctial, and let G H and E F
(the lines) be for meridians on which versoria are disposed, the variations
of which are greater at a greater distance from the æquator. But the
inequalities of the maritime parts of the habitable earth, the enormous
promontories, the very wide gulfs, the mountainous and more elevated
regions, render the variations more unequal, or sudden, or more obscure;
and, moreover, less certain and more inconstant in the higher latitude.
* * * * *
{165} CHAP. VIII.
On the construction of the common mariners'
compass[225], and on the diversity of the compasses
_of different nations_.
In a round[226] hollow wooden bowl, all the upper part of which is closed
with glass, a versorium is placed upon a rather long pin which is fixed in
the middle. The covering prevents the wind, and the motion of air from any
external cause. Through the glass everything within can be discerned. The
versorium is circular, consisting of some light material (as card), to the
under part of which the magnetick pieces of iron are attached. On the upper
part 32 spaces (which are commonly called _points_) are assigned to the
same number of mathematical intervals in the horizon or winds which are
distinguished by certain marks and by a lily indicating the north. The bowl
is suspended in the plane of the horizon in æquilibrium in a brass ring
which also is itself suspended transversely in another ring within a box
sufficiently wide with a leaden weight attached; hence it conforms to the
plane of the horizon even though the ship be tossed to and fro by the
waves. The iron works are either a pair with their ends united, or else a
single one of a nearly oval shape with projecting ends, which does its work
more certainly and more quickly. This is to be fitted to the cardboard
circle so that the centre of the circle may be in the middle of the
magnetick iron. But inasmuch as variation arises horizontally from the
point of the meridian which cuts the horizon at right angles, therefore on
account of the variation the makers in different regions and cities mark
out the mariners' compass in different ways, and also attach in different
ways the magnetick needles to the cardboard circle on which are placed the
32 divisions or points. Hence there are commonly in Europe 4 different
constructions and forms. First that of the States on the Mediterranean Sea,
Sicily, Genoa, and the Republick of Venice. In all these the needles are
attached under the rose or lily on the cardboard versorium, so that (where
there is no variation) they are directed to the true north and south
points. Wherefore the north part marked with the lily always shows exactly
the point of variation when the apex itself of the lily on the movable
circle, together with the ends of the magnetick wires attached below, rests
at the point of variation. Yet another is that of Dantzig, and throughout
the Baltic Sea, and the Belgian provinces; {166} in which the iron works
fixed below the circle diverge from the lily ¼ of a rumbe to the east. For
navigation to Russia the divergency is 2/3. But the compasses which are
made at Seville, Lisbon, Rochelle, Bordeaux, Rouen, and throughout all
England have an interval of ½ 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 Nuñez 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.
* * * * *