This he never did. Perhaps his appointment (in February, 1601) as chief physician in personal attendance on the Queen interfered with the project; or his death, of the plague, in 1603, intervened before his intention had been carried into effect. But it is probable that the substance of the proposed additions is to be found in the chapter, publisht in Gilbert's lifetime, in Blundevile's _Theoriques of the seuen Planets_ (London, 1602), thus described in the title-page of the work: "There is also hereto added, { 63} The making, description, and vse, of two most ingenious and necessarie Instruments for Sea-men, to find out thereby the latitude of any Place vpon the Sea or Land, in the darkest night that is, without the helpe of Sunne, Moone, or Starre. First inuented by M. Doctor Gilbert, a most excellent Philosopher, and one of the ordinarie Physicians to her Maiestie: and now here plainely set downe in our mother tongue by Master Blundeuile."
Of these two instruments the first consists of a mechanical device, with movable quadrants, to be cut out in cardboard, to be used in connection with the diagram of spiral lines which Gilbert had given as a folding plate between pages 200 and 201 of _De Magnete_. The intention was that the Sea-man having found by experiment with a dipping-needle the amount of the dip at any place, should by applying this diagram and its moving quadrants, ascertain the latitude, according to the theory expounded in book V., chap. VII.
The second instrument is a simplified portable dipping-needle, having the degrees engraved on the inner face of a cylindrical brass ring.
Blundevile adds a Table, calculated by Briggs, and "annexed to the former Treatise by _Edward Wright_, at the motion of the right Worshipful M. Doctor _Gilbert_." This gives the values of the dip for different latitudes, as calculated from Gilbert's empirical theory.
The other work, _De Mundo nostro Sublunari Philosophia Nova_, which Gilbert left in manuscript at his death, does not contain any additional matter on the magnetical investigations. Though it contains several direct references to the _de Magnete_, and particularly to Book VI. on the rotation of the earth, it is doubtful whether it was written after or before the publication of _de Magnete_. On pages 137 to 144 of the posthumous edition (Amsterdam, 1651) Gilbert refers to Peregrinus's alleged perpetually revolving sphere, and denies its possibility. The greater part of the work is an anti-Aristotelian discussion on Air, Meteorology, Astronomy, the Winds, Tides, and Springs.