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 and assuming these things (as though they
were perfectly true), he makes a large compass indicating degrees and
minutes, by which these proportional changes of the versorium might be
observed. But those very principles are false, and ill conceived, and very
ill considered; for the versorium does not turn more to the east because a
journey is made toward the east: and although the variation in the more
westerly parts of Europe and the adjoining ocean is to the east and beyond
the Azores is changed a little to the west, yet the variation is, in
various ways, always uncertain, both on account of longitude and of
latitude, and because of the approach toward extensive tracts of land, and
also because of the form of the dominant terrestrial eminences; nor does
it, as we have before demonstrated, follow the rule of any particular
meridian. It is with the same vanity also that Livio Sanuto so greatly
torments himself and his readers. As for the fact that the crowd of
philosophizers and sailors suppose that the meridian passing through the
Azores marks the limits of variation, so that on the other and opposite
side of that meridian a magnetick body necessarily respects the poles
exactly, which is also the opinion of Joannes Baptista Benedictus and of
many other writers on navigation, it is by no means true. Stevinus (on the
authority of Hugo Grotius) in his _Havenfinding Art_ distinguishes the
variation according to the meridians: "It may be seene in the Table of
variations, that in _Coruo_ the Magneticall needle pointeth due North: but
after that, the more a man shal goe towards the East, so much the more also
shall he see the needle varie towards the East [[Greek: anatolizein]], till
he come one mile to the Eastward from _Plimouth_, where the variation
comming to the greatest is 13 degr. 24 min. From hence the Northeasting
[Anatolismus] beginneth to decrease, til you come to _Helmshude_ (which
place is Westward from the North Cape of Finmark) where againe the needle
pointeth due North. Now the longitude from _Coruo_ to _Helmshude_ is 60
degr. Which things being well weighed, it appeareth that the greatest
variation [Chalyboclysis] 13 degr. 24 minutes at _Plimmouth_ (the longitude
whereof is 30 degr.) is in the midst betweene the places where the needle
pointeth due North." But although this is in some part true in these
places, yet it is by no means true that along the whole of the meridian of
the island of Corvo the versorium looks truly to the north; nor on the
meridian of Plymouth is the variation in other places 13 deg. 24 min.--nor
again in other parts of the meridian of Helmshuda does it point to the true
pole. For on the meridian passing through Plymouth in Latitude 60 degrees
the North-easterly variation is greater: in Latitude 40 deg. much less; in
Latitude 20 deg. very small indeed. On the meridian of Corvo, although
there is no variation near the {168} island, yet in Latitude 55 degrees the
variation is about ½ a rumbe to the North-west; in Latitude 20 deg. the
versorium inclines ¼ of a rumbe toward the East. Consequently the limits of
variation are not conveniently determined by means of great circles and
meridians, and much less are the ratios of the increment or decrement
toward any part of the heavens properly investigated by them. Wherefore the
rules of the abatement or augmentation of Northeasting or Northwesting, or
of increasing or decreasing the magnetick deviation, can by no means be
discovered by such an artifice. The rules which follow later for variation
in southern parts of the earth investigated by the same method are
altogether vain and absurd. They were put forth by certain Portuguese
mariners, but they do not agree with the observations, and the observations
themselves are admitted to be bad. But the method of haven-finding in long
and distant voyages by carefully observed variation (such as was invented
by Stevinus, and mentioned by Grotius) is of great moment, if only proper
instruments are in readiness, by which the magnetick deviation can be
ascertained with certainty at sea.
* * * * *