Variation of the Earth's magnetick direction

Direction has hitherto been spoken of as if in nature there were no variation.

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.

The earth’s magnetick direction, due to some fault and slip, deviates from its right course and from the meridian.

What is the 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.

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.

This variation is therefore the arc of the horizon between:

• the common point of intersecion of it with the true meridian
• 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.

All the motions of the magnet and iron, all its turning, its inclination, and its settlement, come from:

• magnetical bodies
• their common mother the earth

The Earth is the source of all these qualities and properties.

Accordingly, the earth is the cause of this variation and inclination.*

Recent writers say that the cause is magnetick mountains or magnetick rocks or any phantasmal pole distant from the pole of the earth.

• I reject this.

Fracastorio adopted and developed this idea.

But it is entirely contradicts 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.

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.

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 reasons of others are still more trifling:

• Cortes points to a moving influence beyond all the heavens
• Marsilius Ficinus points to a star in the Bear
• Peter Peregrinus points to the pole of the world
• Cardan derives it from the rising of a star in the tail of the Bear of Bessardus
• the Frenchman from the pole of the Zodiack
• Livio Sanuto from some magnetick meridian
• Franciscus Maurolycus from a magnetical island
• Scaliger from the heavens and mountains
• Robert Norman, the Englishman, from a point respective

The great magnet or terrestrial globe directs iron toward the north and south and excited iron quickly settles itself toward those termini.

The globe of the earth:

• is defective and uneven on its surface
• is marred by its diverse composition
• has parts very high and convex (to the height of some miles)

This causes 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.

The Earth’s surface is divided into:

• high lands and deep seas
• great continental lands
• ocean and vastest seas

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.

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 obviously toward a stronger and more elevated magnetick part of the terrestrial globe.

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 4 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.

The variation happens 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.

From the coast of Guinea to Cape Verde, the Canary Isles, and the border of the kingdom of Morocco, and 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 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 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 near Cape Race than in the harbours of Norumbega or of Virginia

On the coast of Guinea, magnetick implements deviate by 1/3 of 1 rumbe to the East.

• In Cape Verde Islands by half
• on the coast of Morocco by 2/3
• in England at the mouth of the Thames by a whole rumbe

At London by nearly 11 degrees and 1/3.

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.