A navigational device used on ships in Arcadia. Because there is no science in Arcadia, the Spirit Compass uses magic instead of magnetism, and points to the 'magical north pole', suggesting there is some powerful (and permanent) magical field somewhere to the north in Arcadia. Spirit Compasses are very accurate but can be confused by the presence of a strong magical field.
In order to reach Alais after Captain Nebevay turns the White Dragon around after spotting the Chaos Storm, April confuses the Spirit Compass using the powerful magic generated by the Talisman of the Balance.
The Spirit Compass works something like the compasses in Discworld; because the Disc has no magnetic field, if you enchant a needle and put it in a cup of water, it will point to the Hub.