[223] Page 161, line 2. Page 161, line 2. Insula in Oceano variationem non mutat.—The conclusions derived from the magnetic explorations of the Challenger expedition, 1873-1876, are briefly these: That in islands north of the magnetic equator there is a tendency to produce a local perturbation, attracting the north-seeking end of the needle downwards, and horizontally towards the higher parts of the land; while south of the magnetic equator, the opposite effects are observed. (See Challenger Reports, Physics and Chemistry, vol. ii., part vi., Report on the Magnetical Results by Staff-Commander Creak, F.R.S.)
[224] Page 162, line 2. Page 162, line 3. quarè & respectiuum punctum ... excogitauit.—The passage referred to is in The newe Attractiue of Robert Norman (Lond., 1581), chap. vi.
"Your reason towards the earth carrieth some probabilitie, but I prove that there be no Attractive, or drawing propertie in neyther of these two partes, then is the Attractive poynt lost, and falsly called the poynt Attractive, as shall be proved. But because there is a certayne point that the Needle alwayes respecteth or sheweth, being voide and without any Attractive propertie: in my judgment this poynt ought rather to bee called the point Respective ... This Poynt Respective, is a certayne poynt, which the touched Needle doth alwayes Respect or shew ..."
[225] Page 165, line 2. Page 165, line 2. De pyxidis nauticæ vsitatæ compositione.—Gilbert's description of the usual construction of the mariner's compass should be compared with those given by Levinus Lemnius in The Secret Miracles of Nature (London, 1658); by Lipenius in Navigatio Salomonis Ophiritica (Witteb., 1660, p. 333); and with that given in Barlowe's Navigators Supply (London, 1597). See also Robert Dudley's Dell' Arcano del Mare (Firenze, 1646).
[226] Page 165 deals with the construction; the process of magnetizing by the loadstone had already been discussed in pp. [147] to [149]. It is interesting to see that already the magnetized part attached below the compass-card was being specialized in form, being made either of two pieces bent to meet at their ends, or of a single oval piece with elongated ends. The marking of the compass-card is particularly described. It was divided into thirty-two points or "winds," precisely as the earlier "wind-rose" of the geographers, distinguisht by certain marks, and by a lily—or fleur-de-lys—indicating the North. Stevin in the Havenfinding Art (London, 1599), from which work the passage on p. [167] is quoted, speaking on p. 20 of "the Instrument which we call the Sea-directorie, some the nautical box, ... or the sea compasse," mentions the "Floure de luce" marking the North.
The legend which assigns the invention of the compass to one Goia or Gioja of Amalfi in 1302 has been already discussed in the [Note] to page [4]. Gilbert generously says that in spite of the adverse evidence he does not wish to deprive the Amalfians of the honour of the construction adopted in the compasses used in the Mediterranean. But Baptista Porta the Neapolitan, who wrote forty years before Gilbert, discredited the legend. "Flavius saith, an Italian found it out first, whose name was Amalphus, born in our
Campania. But he knew not the Mariners Card, but stuck the needle in a reed, or a piece of wood, cross over; and he put the needles into a vessel full of water that they might flote freely." (Porta's Natural Magick, English translation, London, 1658, p. 206.) See also Lipenius (op. citat. p. 390).
The pivotting of the needle is expressly described in the famous Epistle on the Magnet of Peter Peregrinus, which was written in 1269. Gasser's edition, Epistola Petri Peregrini ... de magnete, was printed in Augsburg in 1558. In Part II., cap. 2, of this letter, a form of instrument is described for directing one's course to towns and islands, and any places in fact on land or sea. This instrument consists of a vessel like a turned box (or pyxis) of wood, brass, or any solid material, not deep, but sufficiently wide, provided with a cover of glass or crystal. In its middle is arranged a slender axis of brass or silver, pivotted at its two ends into the top and the bottom of the box. This axis is pierced orthogonally with two holes, through one of which is passed the steel needle, while through the other is fixed square across the needle another stylus of silver or brass. The glass cover was to be marked with two cross lines north-south and east-west; and each quadrant was to be divided into ninety degrees. This the earliest described pivotted compass was therefore of the cross-needle type, a form claimed as a new invention by Barlowe in 1597. The first suggestion of suspending a magnetic needle by a thread appears to be in the Speculum Lapidum of Camillus Leonardus (Venet., 1502, fig. k ij, lines 25-31): "Nã tacto ferro ex una pte magnetis ex opposita eius pte appropinquato fugat: ut expiẽtia docet de acu appenso filo."
The earliest known examples of the "wind-rose" are those in certain parchment charts preserved in the Biblioteca Marciana in Venice. These go back to 1426 or 1436, the best being ascribed to Andrea Bianco. They have the North indicated by a fleur-de-lys, a trident, a simple triangle, or a letter T; while the East is distinguisht by a cross. The West is marked with a P. (see Fincati, op. citat.). The eight marks in order, clock wise, run thus,
