On the Magnet/IV-8

[ 165 ]
On the construction of the common mariners'
compass[225], and on the diversity of the compasses
of different nations.

Gilbert De Magnete IlloI.jpg
n a round[226] hollow wooden bowl, all the upper part of which is closed with glass, a versorium is placed upon a rather long pin which is fixed in the middle. The covering prevents the wind, and the motion of air from any external cause. Through the glass everything within can be discerned. The versorium is circular, consisting of some light material (as card), to the under part of which the magnetick pieces of iron are attached. On the upper part 32 spaces (which are commonly called points) are assigned to the same number of mathematical intervals in the horizon or winds which are distinguished by certain marks and by a lily indicating the north. The bowl is suspended in the plane of the horizon in æquilibrium in a brass ring which also is itself suspended transversely in another ring within a box sufficiently wide with a leaden weight attached; hence it conforms to the plane of the horizon even though the ship be tossed to and fro by the waves. The iron works are either a pair with their ends united, or else a single one of a nearly oval shape with projecting ends, which does its work more certainly and more quickly. This is to be fitted to the cardboard circle so that the centre of the circle may be in the middle of the magnetick iron. But inasmuch as variation arises horizontally from the point of the meridian which cuts the horizon at right angles, therefore on account of the variation the makers in different regions and cities mark out the mariners' compass in different ways, and also attach in different ways the magnetick needles to the cardboard circle on which are placed the 32 divisions or points. Hence there are commonly in Europe 4 different constructions and forms. First that of the States on the Mediterranean Sea, Sicily, Genoa, and the Republick of Venice. In all these the needles are attached under the rose or lily on the cardboard versorium, so that (where there is no variation) they are directed to the true north and south points. Wherefore the north part marked with the lily always shows exactly the point of variation when the apex itself of the lily on the movable circle, together with the ends of the magnetick wires attached below, rests at the point of variation. Yet another is that of Dantzig, and throughout the Baltic Sea, and the Belgian provinces; [ 166 ] in which the iron works fixed below the circle diverge from the lily ¼ of a rumbe to the east. For navigation to Russia the divergency is ⅔. But the compasses which are made at Seville, Lisbon, Rochelle, Bordeaux, Rouen, and throughout all England have an interval of ½ a rumbe. From those differences most serious errors have arisen in navigation, and in the marine science. For as soon as the bearings of maritime places (such as promontories, havens, islands) have been first found by the aid of the mariners' compass, and the times of sea-tide or high water determined from the position of the moon over this or that point (as they say) of the compass, it must be further inquired in what region or according to the custom of what region that compass was made by which the bearings of those places and the times of the sea-tides were first observed and discovered. For one who should use the British compass and should follow the directions of the marine charts of the Mediterranean Sea would necessarily wander very much out of the straight course. So also he that should use the Italian compass in the British, German, or Baltic Sea, together with marine charts that are made use of in those parts, will often stray from the right way. These different constructions have been made on account of the dissimilar variations, so that they might avoid somewhat serious errors in those parts of the world. But Pedro Nuñez seeks the meridian by the mariners' compass, or versorium (which the Spanish call the needle), without taking account of the variation: and he adduces many geometrical demonstrations which (because of his slight use and experience in matters magnetical) rest on utterly vicious foundations. In the same manner Pedro de Medina, since he did not admit variation, has disfigured his Arte de Navegar with many errors.
The page and line references given in these notes are in all cases first to the Latin edition of 1600, and secondly to the English edition of 1900.

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,

Gilbert De Magnete Illo249a.jpg (or T).     G.     Gilbert De Magnete Illo249b.jpg (or L)     S.     O.     A (or L).     P.     M.

The letters correspond to the Italian names of the principal winds:

Tramontano North.
Greco North-East.
Levante East.
Sirocco South-East.
Ostro South.
Africo or Libeccio           South-West.
Ponente West.
Maestro North-West.

Wind-roses marked with the names of the minor winds are found in Nautonier's Mécometrie de l'Eyman (Vennes, 1602-1604, pp. 151-152), and Kircher's Magnes Siue de Arte Magnetica (Colon. Agripp., 1643, p. 432). The description above given of the early Venetian wind-roses exactly describes the compass-card as depicted by Pedro de Medina in his Arte de Nauegar (Valladolid, 1545, folio lxxx.), in the sixth book entitled "las aguias de navegar"; while in the Breve compendio de la sphera of Martin Cortes (Sevilla, 1551, cap. iii., de la piedrayman) a similar wind-rose, without the letters, is found.

In the De Ventis et navigatione of Michaele Angelo Blondo (Venet., 1546, p. 15) is given a wind-rose, described as "Pixis uel Buxolus instrumentum et dux nauigantium," having twenty-six points inscribed with the names of the winds, there being six between north and east, and six between south and west, and only five in each of the other quadrants. In the middle is a smaller wind-rose exactly like the early Italian ones just mentioned.

In the Della Guerra di Rhodi of Jacobo Fontano (Venet., 1545, pages 71-74) is a chapter Dei Venti, e della Bvssola di nauicare di Giovanni Quintino, giving a wind-rose, and a table of the names of the winds, the north being indicated by a pointer, at the cusp of which are seven stars, and the west by an image of the sun. The other cardinal points are marked with letters.

Barlowe, in The Navigators Supply (Lond., 1597), speaks thus:

"The merueilous and diuine Instrument, called the Sayling Compasse (being one of the greatest wonders that this World hath) is a Circle diuided commonly into 32. partes, tearmed by our Seamen Windes, Rumbes, or Points of Compasse."

It is a disputed point with whom the method of naming the winds originated. Some ascribe it to Charlemagne. Michiel Coignet (Instruction novvelle ... touchant l'art de naviguer, Anvers, 1581, p. 7) ascribes it to Andronicus Cyrrhestes. See Varro, De Re Rustica, iii., 5, 17, and Vitruvius, i., 6, 4.

Gilbert's complaint of the evil practice of setting the needles obliquely beneath the card, with the intention of allowing for the variation, is an echo of a similar complaint in Norman's Newe Attractiue. In chapter x. of this work Norman thus enumerates the different kinds of compasses:

"Of these common Sayling Compasses, I find heere (in Europa) five sundry sortes or sets. The first is of Levant, made in Scicile, Genoüa, and Venice: And these are all (for the most parte) made Meridionally, with the Wyers directlye sette under the South, and North of the Compasse: And therefore, duely shewing the poynt Respective, in all places, as the bare Needle. And by this Compasse are the Plats made, for the most part of all the Levants Seas.
"Secondly, there are made in Danske, in the Sound of Denmarke, and in Flanders, that have the Wyers set at 3 quarters of a point to the Eastwards of the North of the compasse, and also some at a whole point: and by these Compasses they make both the Plats and Rutters for the Sound.
"Thirdly, there hath beene made in this Countrey particulary, for Saint Nicholas and Ruscia, Compasses set at 3 seconds of a point, and the first Plats of that Discoverie were made by this Compasse.
"Fourthly the Compasse made at Sevill, Lisbone, Rochell, Bourdeaux, Roan, and heere in England, are moste commonly set at halfe a point: And by this Compasse are the Plats of the East and West Indies made for their Pylotes, and also for our Coastes neere hereby, as France, Spayne, Portugall, and England: and therefore best of these Nations to bee used, because it is the most common sorte that is generally used in these Coastes."

Bessard (op. citat., pages 22 and 48) gives cuts of compasses showing the needle displaced one rumbe to the East.

Gallucci, in his Ratio fabricandi horaria mobilia et permanentia cum magnetica acu (Venet., 1596), describes the needle as inclined 10 degrees from the south toward the south-west.

The frontispiece of the work of Pedro Nuñez, Instrumenta Artis Navigandi, Basil., 1592, depicts a compass with the lily set one point to the east.

Reibelt, De Physicis et Pragmaticis Magnetis Mysteriis (Herbipolis, 1731), depicts the compass with the needle set about 12 degrees to the East of North. See also Fournier, Hydrographie (Paris 1667); De Lanis, Magisterium Natvræ et Artis (Brixiæ, 1684); Milliet Deschales, Cursus seu Mundus Mathematicus (Lugd., 1674). Both the latter works give pictures of the compass-cards as used in South Europe, and in North Europe, and of the various known shapes of needles.