ass,
are to be rubbed, that they may acquire stronger verticity.

{viij} _Book 4._

Chap. 1. On Variation.

Chap. 2. That the variation is caused by the inaequality of the projecting
parts of the earth.

Chap. 3. The variation in any one place is constant.

Chap. 4. The arc of variation is not changed equally in proportion to the
distance of places.

Chap. 5. An island in Ocean does not change the variation, as neither do
mines of loadstone.

Chap. 6. The variation and direction arise from the disponent power of the
earth, and from the natural magnetick tendency to rotation, not from
attraction, or from coition, or from other occult cause.

Chap. 7. 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.

Chap. 8. On the construction of the common mariners' compass, and on the
diversity of the compasses of different nations.

Chap. 9. Whether the terrestrial longitude can be found from the variation.

Chap. 10. Why in various places near the pole the variations are much more
ample than in a lower latitude.

Chap. 11. Cardan's error when he seeks the distance of the centre of the
earth from the centre of the cosmos by the motion of the stone of
Hercules; in his book 5, _On Proportions_.

Chap. 12. On the finding of the amount of variation: how great is the arc
of the Horizon from its arctick to its antarctick intersection of the
meridian, to the point respective of the magnetick needle.

Chap. 13. The observations of variation by seamen vary, for the most part,
and are uncertain: partly from error and inexperience, and the
imperfections of the instruments: and partly from the sea being seldom
so calm that the shadows or lights can remain quite steady on the
instruments.

Chap. 14. On the variation under the aequinoctial line, and near it.

Chap. 15. The variation of the magnetick needle in the great Aethiopick and
American sea, beyond the aequator.

Chap. 16. On the variation in Nova Zembla.

Chap. 17. Variation in the Pacifick Ocean.

Chap. 18. On the variation in the Mediterranean Sea.

Chap. 19. The variation in the interior of large Continents.

Chap. 20. Variation in the Eastern Ocean.

Chap. 21. How the deviation of the versorium is augmented and diminished
by reason of the distance of places.

_Book 5._

Chap. 1. On Declination.

Chap. 2. D