The Magnetick axis of the Earth
_persists invariable_.
As in the very first beginnings of the moving world, the earth's magnetick
axis passed through the midst of the earth: so now it tends through the
centre to the same points of the superficies; the circle and plane of the
æquinoctial line also persisting. For not without the vastest overthrow of
the terrene mass can these natural boundaries be changed, as it is easy to
gather from magnetick demonstrations. Wherfore the opinion of Dominicus
Maria of Ferrara, a most talented man, who was the teacher of Nicolas
Copernicus, must be cancelled; a view {213} which, according to certain
observations of his own, is as follows.[241] "I," he says, "in former years
while studying Ptolemy's _Geographia_ discovered that the elevations of the
North pole placed by him in the several regions, fall short of what they
are in our time by one degree and ten minutes: which divergence can by no
means be ascribed to an error of the tables: For it is not credible that
the whole series in the book is equally wrong in the figures of the tables:
Hence it is necessary to allow that the North pole has been tilted toward
the vertical point. Accordingly a lengthy observation has already begun to
disclose to us things hidden from our forefathers; not indeed through any
sloth of theirs, but because they lacked the prolonged observation of their
predecessors: For before Ptolemy very few places were observed with regard
to the elevations of the pole, as he himself also bears witness at the
beginning of his _Cosmographia_: (For, says he) Hipparchus alone hath
handed down to us the latitudes of a few places, but a good many have noted
those of distances; especially those which lie toward sunrise or sunset
were received by some general tradition, not owing to any sloth on the part
of authors themselves, but to the fact that there was as yet no practice of
more exact mathematicks. 'Tis accordingly no wonder, if our predecessors
did not mark this very slow motion: For in one thousand and seventy years
it shows itself to be displaced scarce one degree toward the apex of
dwellers upon the earth. The strait of Gibraltar shows this, where in
Ptolemy's time the North pole appears elevated 36 degrees and a quarter
from the Horizon: whereas now it is 37 and two-fifths. The like divergence
is also shown at Leucopetra in Calabria, and at particular spots in Italy,
namely those which have not changed from Ptolemy's time to our own. And so
by reason of this movement, places now inhabited will some day become
deserted, while those regions which are now parched at the torrid zone
will, though long hence, be reduced to our temper of climate. Thus, as in a
course of three hundred and ninety five thousands of years, is that very
slow movement completed." Thus, according to these observations of
Dominicus Maria, the North pole is at a higher elevation, and the latitudes
of places are greater than formerly; whence he argues a change of
latitudes. Now, however, Stadius, taking just the contrary view, proves by
observations that the latitudes have decreased. For he says: "The latitude
of Rome in Ptolemy's _Geographia_ is 41 degrees 2/3: and that you may not
suppose any error of reckoning to have crept in on the part of Ptolemy, on
the day of the Æquinox in the city of Rome, the ninth part of the gnomon of
the sun-dial is lacking in shadow, as Pliny relates and Vitruvius
witnesseth in his ninth book." But the observation of moderns (according to
Erasmus Rheinholdus) gives the same in our time as 41 degrees with a sixth:
so that you are in doubt as to half of one degree in {214} the centre of
the world, whether you show it to have decreased by the earth's obliquity
of motion. One may see then how from inexact observations men rashly
conceive new and contradictory opinions and imagine absurd motions of the
mechanism of the earth. For since Ptolemy only received certain latitudes
from Hipparchus, and did not in very many places make the observations
himself; it is likely that he himself, knowing the position of the places,
formed his estimate of the latitude of cities from probable conjecture
only, and then placed it in the maps. Thus one may see, in the case of our
own Britain, that the latitudes of cities are wrong by two or three
degrees, as experience teaches. Wherefore all the less should we from those
mistakes infer a new motion, or let the noble magnetick nature of the earth
be debased for an opinion so lightly conceived. Moreover, those mistakes
crept the more readily into geography, from the fact that the magnetick
virtue was utterly unknown to those geographers. Besides, observations of
latitudes cannot be made sufficiently exactly, except by experts, using
also finer instruments, and taking into account the refraction of the
lights.
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