Direction has hitherto been spoken of as if in nature there were no
variation; for in the preceding natural history we wished to omit and
neglect this, inasmuch as in a terrestrial globe, perfect and in every
sense complete, there would be none. Since, however, in fact, the earth's
magnetick direction, owing to some fault and slip, deviates from its right
course and from the meridian, we must extract and demonstrate the obscure
and hidden cause of that variance which has troubled and sore racked in
vain the minds of many. Those who before us have written on the magnetick
movements have made no distinction between direction and variation, but
consider the motion of magnetick iron to be uniform and simple. Now true
direction is the motion of the magnetick body to the true meridian and its
continuance therein with its appropriate ends towards the poles. But it
very often happens at sea and on land that the magnetick iron does not
point to the true pole, and that not only a versorium and magnetick pieces
of iron, and the needle of a compass, or a mariners' compass, but also a
terrella in its boat, as well as * iron ore, iron stones, and magnetick
earths, properly prepared, are drawn aside and deviate towards some point
of the Horizon very near to the meridian. For they with their poles
frequently face termini away from the meridian. This variation {152}
(observed by means of instruments or a nautical variation compass) is
therefore the arc of the horizon between the common point of intersecion of
it with the true meridian, and the terminus of the deflecion on the horizon
or projection of the deviating needle. That arc varies and differs with
change of locality. To the terminus of the variation is commonly assigned a
great circle, called the circle of variation, and also a magnetick meridian
passing through the zenith and the point of variation on the horizon. In
the northern regions of the earth this variation is either from the north
toward the east or from the north toward the west: similarly in the
southern regions it is from the south toward the east or toward the west.
Wherefore one should observe in the northern regions of the earth * that
end of the versorium or compass which turns toward the North; but in the
southern regions the other end looking to the south--which seamen and
sciolists for the most part do not understand, for in both regions they
observe only the boreal lily of the compass (that which faces North). We
have before said that all the motions of the magnet and iron, all its
turning, its inclination, and its settlement, proceed from bodies
themselves magnetical and from their common mother the earth, which is the
source, the propagatrix, and the origin of all these qualities and
properties. Accordingly the earth is the cause of this variation and
inclination toward a different point of the horizon: but how and by what
powers must be more fully investigated. And here we must at the outset
reject that common opinion of recent writers concerning magnetick
mountains, or any magnetick rock, or any phantasmal pole distant from the
pole of the earth, by which the motion of the compass or versorium is
controlled. This opinion, previously invented by others, Fracastorio
himself adopted and developed; but it is entirely at variance with
experience. For in that case in different places at sea and on land the
point of variation would change toward the east or west in proportion and
geometrical symmetry, and the versorium would always respect the magnetick
pole: but experience teaches that there is no such definite pole or fixed
terminus on the earth to account for the variation. For the arcs of *
variation are changed variously and erratically, not only on different
meridians but on the same meridian; and when, according to this opinion of
the moderns, the deviation should be more and more toward the east, then
suddenly, with a small change of locality, the deviation is from the north
toward the west as in the northern regions near Nova Zembla. Moreover, in
the southern regions, and at sea at a great distance from the æquator
towards the antarctick pole, there are frequent and great variations, and
not only in the northern regions, from the magnetick mountains. But the
cogitations of others are still more vain and trifling, such as that of
Cortes about a moving influence beyond all the heavens; that of {153}
Marsilius Ficinus about a star in the Bear; that of Peter Peregrinus about
the pole of the world; that of Cardan, who derives it from the rising of a
star in the tail of the Bear[218]; of Bessardus, the Frenchman, from the
pole of the Zodiack; that of Livio Sanuto from some magnetick meridian;
that of Franciscus Maurolycus from a magnetical island; that of Scaliger
from the heavens and mountains; that of Robert Norman, the Englishman, from
a point respective. Leaving therefore these opinions, which are at variance
with common experience or by no means proved, let us seek the true cause of
the variation. The great magnet or terrestrial globe directs iron (as I
have said) toward the north and south; and excited iron quickly settles
itself toward those termini. Since, however, the globe of the earth is
defective and uneven on its surface and marred by its diverse composition,
and since it has parts very high and convex (to the height of some miles),
and those uniform neither in composition nor body, but opposite and
dissimilar: it comes to pass that the whole of that force of the earth
diverts magnetical bodies in its periphery toward the stronger and more
prominent connected magnetick parts. Hence on the outermost surface of the
earth magnetical bodies are slightly perverted from the true meridian.
Moreover, since the surface of the globe is divided into high lands and
deep seas, into great continental lands, into ocean and vastest seas, and
since the force of all magnetical motions is derived from the constant and
magnetick terrestrial nature which is more prevalent on the greater
continent and not in the aquæous or fluid or unstable part; [219]it follows
that in certain parts there would be a magnetick inclination from the true
pole east or west away from any meridian (whether passing through seas or
islands) toward a great land or continent rising higher, that is, obviously
toward a stronger and more elevated magnetick part of the terrestrial
globe. For since the diameter of the earth is more than 1,700 German miles,
those large lands can rise from the centre of the earth more than four
miles above the depth of the ocean bottom, and yet the earth will retain
the form of a globe although somewhat uneven at the top. Wherefore a
magnetical body is turned aside, so far as the true verticity, when
disturbed, admits, and departs from its right (the whole earth moving it)
toward a vast prominent mass of land as though toward what is stronger. But
the variation does really take place, not so much because of the more
prominent and imperfect terrestrial parts and continent lands as because of
the inæquality of the magnetick globe, and because of the real earth, which
stands out more under the continent lands than under the depths of the
seas. We must see, therefore, how the _apodixis_ of this theory can be
sustained by more definite observations. Since throughout all the course
from the coast of Guinea to Cape Verde, the Canary Isles, and the border of
the kingdom of Morocco, and {154} thence along the coasts of Spain, France,
England, Belgium, Germany, Denmark, and Norway, there lie on the right hand
and toward the east a continent and extensive connected regions, and on the
left extensive seas and a vast ocean lie open far and wide, it is consonant
with the theory (as has been carefully observed by many) that magnetical
bodies should turn slightly to the East from the true pole toward the
stronger and more remarkable elevations of the earth. But it is far
otherwise on the eastern shores of northern America; for from Florida by
Virginia and Norumbega to Cape Race and away to the north the versorium is
turned toward the west. But in the middle spaces, so to speak, as in the
more westerly Azores, it looks toward the true pole. That any magnetick
body turns itself similarly to the same regions of the earth is not,
however, because of that meridian or because of the concordancy of the
meridian with any magnetick pole, as the crowd of philosophizers reckon,
for it is not so throughout the whole of that meridian. For on the same
meridian * near Brazil something very different occurs, as we will show
further on. The variation (cæteris paribus) is always less near the
æquator, greater in higher latitudes, with the limitation that it be not
very near the pole itself. Hence the variation is greater on the coast of *
Norway and Belgium than on the coast of Morocco or Guinea: greater also
near Cape Race than in the harbours of Norumbega or of Virginia. On the
coast of Guinea magnetick implements deviate by a third part of one rumbe
to the East: in Cape Verde Islands by a half: on the coast of Morocco by
two thirds: in England at the mouth of the Thames by a whole rumbe: and at
London by nearly eleven degrees and one third. For indeed the moving
magnetick virtue is stronger in a higher latitude; and the larger regions
extending toward the poles dominate the more, as is easily apparent
anywhere on a terrella. For as in the case of true Direction magnetick
bodies tend toward the pole (namely, toward the stronger end, the whole
earth causing the motion), so also do they incline a little toward the
stronger and higher parts by the action of the whole along with the
conjoint action of iron bodies.
* * * * *
{155} CHAP. II.
That the variation is caused by the inæquality of the
_projecting parts of the earth_.
Demonstration of this may manifestly be made [Illustration] * by means of a
terrella in the following way: let there be a round loadstone somewhat
imperfect in some part, and impaired by decay (such an one we had with a
certain part corroded to resemble the Atlantick or great Ocean): place upon
it some fine iron wire of the length of two barleycorns, as in the
following figure. A B, a Terrella in certain parts somewhat imperfect and
of unæqual virtue on the circumference. The versoria E, F, do not vary, but
look directly to the pole A; for they are placed in the middle of the firm
and sound part of the terrella and somewhat distant from the imperfect
part: that part of the surface which is distinguished by dots and
transverse lines is the weaker. The versorium O also does not vary (because
it is placed in the middle of the imperfect part), but is directed toward
the pole, {156} just as near the western Azores on the earth. The versoria
H and L do vary, for they incline toward the sounder parts very near them.
As this is manifest in a terrella whose surface is sensibly rather
imperfect, so also is it in others whole and perfect, when often one part
of the stone has stronger external parts, which nevertheless do not
disclose themselves manifestly to the senses. In such a terrella the
demonstration of the variation and the discovery of the stronger parts is
on this wise. [Illustration] * Let A be the pole, B the place of the
variation, C the stronger regions; then the horizontal versorium at B
varies from the pole A toward C: so that both the variation is shown and
the stronger places of the loadstone recognized. The stronger surface is
also found by a fine iron wire of the length of two barleycorns: for since
at the pole of the terrella it rears up perpendicularly, but in other
places inclines toward the æquator, if in one and the same parallel circle
it should be more erect in one place than in another; where the wire is
raised more upright, there the part and surface of the terrella is
stronger. Also when the iron wire placed over the pole inclines more to one
part than to another. [Illustration] * {157} Let the experiment be made by
means of a fine iron wire of three digits length placed over the pole A, so
that its middle lies over the pole. Then one end is turned away from B
toward C, and is not willing to lie quietly toward B; but on a terrella
which is perfect[220] all round and even it rests on the pole directed
toward any point of the æquator you please. Otherwise, let there be two *
[Illustration] meridians meeting in the poles A B, let iron wires be reared
just at the ends D and C of the equal arcs D A and C A; then the wire at D
(the stronger region) will be more raised up than that at C, the weaker.
And thus the sounder and stronger part of the loadstone is recognized,
which otherwise would not be perceived by the touch. In a terrella which is
perfect, and even, and similar in all its parts, there is, at equal
distances from the pole, no variation[221]. Variation is shown by means of
a terrella, a considerable part of which, forming a surface a little higher
than the rest, does, although it be not decayed and broken, allure the
versorium from the true * direction (the whole terrella co-operating).
A terrella uneven in surface.
[Illustration] {158} It is shown by a small spike placed over a terrella or
by a small versorium; for they are turned by the terrella toward the mass
that stands out and toward the large eminences. In the same way on the
earth the verticity is perturbed by great continents, which are mostly
elevated above the depths of the seas and make the versorium deviate
sometimes from the right tracks (that is, from the true meridians). On a
terrella it is thus demonstrated: the end of the versorium A is not
directed straight to the pole P, if there be a large protuberance B on the
terrella; so also the cusp C deviates from the pole because of the eminence
F. In the middle between the two eminences the versorium G collimates to
the true pole because, being at equal distances from the two eminences B
and F, it turns aside to neither, but observes the true meridian,
especially when the protuberances are of equal vigour. But the versorium N
on the other side varies from the pole M toward the eminences H, and is not
held back, stopped, or restrained by the small eminence O on the terrella
(as it were, some island of land in the ocean). L, however, being
unimpeded, is directed to the pole M. The variation is demonstrated in
another way on a terrella, just as on the earth. Let A be the pole of the
earth, B the equator, C the parallel circle of latitude of 30 degrees, D a
great eminence spread out toward the pole, E another eminence spread out
from the pole toward the æquator. It is manifest that in the middle of D
the versorium F {159} does not vary; while G is very greatly deflected: but
H very little, because it is further removed from D. Similarly also the
versorium I placed directly toward E does not deviate from the pole: but L
and M turn themselves away from the pole A toward the eminence E.
[Illustration]
* * * * *