Book 8 of his Paraphrase on Aristotle's _Physica_.

[162] PAGE 71, LINE 9. Page 71, line 14. _Quod verò Fracastorius._--_Op. citat._, lib. i., cap. 7, p. 62 _verso_.

[163] PAGE 73, LINE 2. Page 73, line 2. _si A borealis._--The editions of 1628 and 1633 omit the twelve words next following.

[164] PAGE 73, LINE 9. Page 73, line 11. _ex minera._--_Minera_ is not a recognized word, even in late Latin. It occurs again, p. 97, line 12.

[165] PAGE 77, LINE 2. Page 77, line 2. _multo magis._--This is an _à fortiori_ argument. It is interesting to find Gilbert comparing the velocity of propagation of magnetic forces in space with the velocity of light. The parallel is completed in line 13 by the consideration that as the rays of light require to fall upon an object in order that they may become visible, so the magnetic forces require a magnetic object in order to render their presence sensible.

[166] PAGE 78, LINE 14. Page 78, line 16. _Orbem terrarum distinguunt._--The editions of 1628 and 1633 here add a figure of a globe marked with meridians and parallels of latitude, but with an erroneous versorium pointing to the south. These editions also both read _existentiam_ for the word _existentium_ in line 20.

[167] PAGE 83, LINE 5. Page 83, line 5. _magnes longior maiora pondera ferri attollit._--Gilbert discovered the advantage, for an equal mass of loadstone, of an elongated shape. It is now well known that the specific amount of magnetism retained by elongated forms exceeds that in a short piece of the same material subjected to equal magnetizing forces.

[168] PAGE 83, LINE 24. Page 83, line 28. _Non obstant crassa tabulata._--Gilbert has several times referred (_e.g._, on p. 77) to the way in which magnetic forces penetrate solid bodies. The experimental investigation in this chapter {47} is the more interesting because it shows that Gilbert clearly perceived the shielding action of iron to be due to iron conducting aside or diverting the magnetic forces.

[169] PAGE 85, LINE 26. Page 85, line 31. _non conveniant._--The editions of 1628 and 1633 both read _et conveniant_.

[170] PAGE 86, LINE 3. Page 86, line 3. _illud quod exhalat._--Literally, _that which exhales_, in the sense of that which escapes: but in modern English the verb exhale in the active voice is now not used of the substance that escapes, but is used of the thing which emits it. It must therefore be rendered _that which is exhaled_ (_i.e._, breathed out).

[171] PAGE 86, LINE 13. Page 86, line 15. _Ita tota interposita moles terrestris._--Gilbert's notion that the gravitational force of the moon in producing the tides acts _through_ the substance of the earth may seem curiously expressed. But the underlying contention is essentially true to-day. The force of gravity is not cut off or screened off by the interposition of other masses. A recent investigation by Professor Poynting, F.R.S., has shown that so far as all evidence goes all bodies, even the densest, are transparent with respect to gravitational forces.

[172] PAGE 86, LINE 18. Page 86, line 20. _Sed de æstus ratione aliàs._--There is no further discussion of the tides in _De Magnete_. But a short account is to be found in Gilbert's posthumous work _De Mundo nostro Sublunari Philosophia nova_ (Amsterdam, Elzevir, 1651), in Lib. v., the part which in the manuscript was left in English, and was turned into Latin by his brother. It comprises about fifteen quarto pages, from Cap. X. to Cap. XIX. inclusive, beginning with a characteristic diatribe against Taisnier, Levinus Lemnius, and Scaliger. But in assigning causes he himself goes wide of the mark. Proceeding by a process of elimination he first shows that the moon's light cannot be the cause that impels the tides. "Luna," he says, "non radio, non lumine, maria impellit. quomodo igitur? Sane corporum conspiratione, acque (ut similitudine rem exponam) Magnetica attractione." This cryptic utterance he proceeds to explain by a diagram, and adds: "Quare Luna non tam attrahit mare, quàm humorem & spiritum subterraneum; nec plus resistit interposita terra, quàm mensa, aut quicquam aliud densum, aut crassum, magnetis viribus."

[173] PAGE 87, LINE 7. Page 87, line 9. _armatura._--Here this means the cap or snout of iron with which the loadstone was armed. This is apparently the first use of the term in this sense.

In the _Dialogues of Galileo_ (p. 369 of Salusbury's _Mathematical Collections_, Dialogue iii.), Sagredus and Salviatus discuss the arming of the loadstone, and the increased lifting power conferred by adding an iron cap. Salviatus mentions a loadstone in the Florentine Academy which, unarmed, weighed six ounces, lifting only two ounces, but which when armed took up 160 ounces. Whereupon Galileo makes Salviatus say: "I extreamly praise, admire, and envy this Authour, for that a conceit so stupendious should come into his minde. ... I think him [_i.e._, Gilbert] moreover worthy of extraordinary applause for the many new and true Observations that he made, to the disgrace of so many fabulous Authours, that write not only what they do not know, but whatever they hear spoken by the foolish vulgar, never seeking to assure themselves of the same by experience, perhaps, because they are unwilling to diminish the bulk of their Books."

[174] PAGE 87, LINE 12. Page 87, line 15. The reference to _lib._ 3 is {48} a misprint for _lib._ 2. It is corrected in the edition of 1633, but not in that of 1628.

[175] PAGE 87, LINE 17. Page 87, line 21. _conactu._--The editions of 1628 and 1633 read _conatu_.

[176] PAGE 88, LINE 2. Page 88, line 3. _Coitio verò non fortior._--This heading to chap. xix., taken with the seven lines that follow, and the contrast drawn between _unitio_ and _coitio_, throw much light on the fundamental sense attached by Gilbert to the term _coitio_. It is here clearly used in the sense of _mutual tendency toward union_. Note also the contrasted use in chap. xx. of the verbs _cohære_ and _adhære_. Adhærence connotes a one-sided force (an impossibility in physics), cohærence a mutual force.

[177] PAGE 90, LINE 9. Page 90, line 9. _nempè vt alter polus maius pondus arripiat._--This acute observation is even now not as well known as it ought to be. Only so recently as 1861 Siemens patented the device of fastening a mass of iron to one end of an electromagnet in order to increase the power of the other end. The fact, so far as it relates to permanent magnets was known to Servington Savery. See _Philos. Transactions_, 1729, p. 295.

[178] PAGE 92, LINE 3. Page 92, line 4. _Suspendit in aëre ferrum Baptista Porta._--Porta's experiment is thus described (_Natural Magick_, London, 1658, p. 204): "_Petrus Pellegrinus_ saith, he shewed in another work how that might be done: but that work is not to be found. Why I think it extream hard, I shall say afterwards. But I say it may be done, because I have now done it, to hold it fast by an invisible band, to hang in the air; onely so, that it be bound with a small thread beneath, that it may not rise higher: and then striving to catch hold of the stone above, it will hang in the air, and tremble and wag itself."

[179] PAGE 97, LINE 29. Page 97, line 33. _Sed quæri potest ..._--The question here raised by Gilbert is whether the lifting-power of magnets of equal quality is proportional to their weight. If a stone weighing a drachm will lift a drachm, would a stone that weighs an ounce lift an ounce? Gilbert erroneously answers that this is so, and that the lifting-power of a loadstone, whether armed or unarmed, is proportional to its mass.

The true law of the tractive force or lifting-power of magnets was first given in 1729 by James Hamilton (afterwards Earl of Abercorn) in a work entitled _Calculations and Tables Relating to the Attractive Virtue of Loadstones ... Printed_ [at London?] _in the Year_ 1729. (See also a paper in the _Philos. Transactions_, 1729-30, vol. xxxvi., p. 245). This work begins thus:

"The Principle upon which these Tables are formed, is this: That if Two _Loadstones_ are perfectly Homogeneous, that is, if their Matter be of the same Specifick Gravity, and of the same Virtue in all Parts of one Stone, as in the other; and that Like Parts of their Surfaces are Cap'd or Arm'd with Iron; then the Weights they sustain will be as the Squares of the Cube Roots of the Weights of the _Loadstones_; that is, as their Surfaces."

Upon lifting-power see also D. Bernoulli, _Acta Helvetica_, iii., p. 223, 1758; P. W. Haecker, _Zur Theorie des Magnetismus_, Nürnberg, 1856; Van der Willigen, _Arch. du Musée Teyler_, vol. iv., Haarlem, 1878 ; S. P. Thompson, _Philos. Magazine_, July, 1888.

In the book of James Hamilton, p. 5, he mentions a small terrella weighing 139 English grains, which would sustain no less than 23,760 grains, and was valued at £21 13s. 10¾d.

{49} In the _Musæum Septalianum_ of Terzagus (Dertonæ, 1664, p. 42) is mentioned a loadstone weighing twelve ounces which would lift sixty pounds of iron.

Sir Isaac Newton had a loadstone weighing 3 grains, which he wore in a ring. It would lift 746 grains.

Thomson's _British Annual_, 1837, p. 354, gives the following reference: "In the _Records of General Science_, vol. iii., p. 272, there is an interesting description of a very powerful magnet which was sent from Virginia in 1776 by the celebrated Dr. Franklin to Professor Anderson, of Glasgow. It is now in the possession of Mr. Crichton. It weighs 2½ grains, and is capable of supporting a load of 783 grains, which is equivalent to 313 times its own weight."

[180] PAGE 99, LINE 10. Page 99, line 11. _Manifestum est._--In this, as in many other passages, Gilbert uses this expression in the sense that _it is demonstrable_ rather than meaning that _it is obvious_: for the fact here described is one that is not at all self-evident, but one which would become plain when the experiment had been tried. For other instances of this use of _manifestum_ see pages 144, line 20; 158, line 19; 162, line 10.

[181] PAGE 100, LINE 20. Page 100, line 24. _si per impedim[=e]ta ... pervenire possunt._--All editions agree in this reading, but the sense undoubtedly requires _non possint_. Compare p. 91, line 21.

[182] PAGE 102, LINE 4. Page 102, line 4. _capite_ 4.--This is a misprint for _capite_ 40, and is retained in the later editions. In the quotation from Baptista Porta, where the English version of 1658 is adhæred to, the words "& deturbat eam" have been omitted by the translator.

[183] PAGE 107, LINE 16. Page 107, line 18. _Cardanus scribit._--The alleged perpetual motion machine is mentioned in _De rerum varietate_, _lib._ 9, cap. xlviii. (Basil., 1581, p. 641). See also the Note to p. 223. For Peregrinus and for Taisnier, see the note to p. 5, lines 8 and 12.

[184] PAGE 107, LINE 19. Page 107, line 21. _Antonij de Fantis._--His work is: _Tabula generalis scotice subtilitatis octo Sectionibus vniuersam Doctoris Subtilis Periti[=a] c[=o]plect[=e]s: ab excellentissimo doctore Antonio de F[=a]tis taruisino edita ..._ Lugd., 1530.

[185] PAGE 108, LINE 26. Page 108, line 31. _Cusani in staticis._--See the note to p. 64, line 16.

[186] PAGE 108, LINE 33. Page 108, line 41. _Languidi ... tardiùs acquiescunt._--The editions of 1628 and 1633 omit these seven words.

[187] PAGE 109, LINE 11. Page 109, line 13. _halinitro._--Either native carbonate of soda or native carbonate of potash might be meant, but not saltpetre. Scaliger, in his _De Subtilitate ad Cardanum_ (Lutet., 1557, p. 164), _Exercitatio_ CIII., 15, under the title, _Nitrum non est Salpetræ_, says: "More tuo te, tuaque confundis. Salpetræ inter salis fossilis ponis hîc. Mox Halinitrum inter salis, & nitri naturam, speciem obtinere."

"_Sal nitrum_ is salt which is boiled out of the earth, especially fat earth, as in stables, or any place of excrements." (_A Chymicall Dictionary explaining Hard Places and Words met withall in the Writings of Paracelsus ..._, Lond., 1650.)

[188] PAGE 109, LINE 20. Page 109, line 23. _arte ioculatoriâ._--Edition 1628, _joculatoriâ_; edition 1633, _jaculatoriâ_.

[189] PAGE 110, LINE 11. Page 110, line 12. _qualis fuit Antonij denarius._--The Elizabethan version of Pliny (book xxxiii., ch. ix., p. 479) runs thus: {50} "To come now unto those that counterfeit money. _Antonius_ whiles hee was one of the three usurping Triumvirs, mixed yron with the Romane silver denier. He tempered it also with the brasen coine, and so sent abroad false and counterfeit money."

Georgius Agricola (_De Natura Fossilium_, p. 646) says:

"Sed ea fraus capitalis est, non aliter ac eorum qui adulterinas monetas cudunt, argento miscentes multam plumbi candidi portionem, aut etiam ferri, qualis fuit Antonii denarius, ut Plinius memoriæ tradidit. Nunc dicam de candido plumbo, nam majoris pretii est quàm aes. In quod plumbum album, inquit Plinius, addita aeris tertia portione candidi adulteratur stannum."

[190] PAGE 111, LINE 3. Page 111, line 3. _Meminerunt Chatochitis lapis Plinius, atque Iulius Solinus._--The passage in Pliny (English version of 1601, book xxxvii., ch. x., p. 625) runs:

"Catochitis is a stone proper unto the Island Corsica: in bignesse it exceedeth ordinarie pretious stones: a wonderfull stone, if all be true that is reported thereof, and namely, That if a man lay his hand thereon, it will hold it fast in manner of a glewie gum."

[191] PAGE 111, LINE 7. Page 111, line 7. _Sagda vel Sagdo._--Albertus Magnus in _De Mineralibus_ (Venet., 1542, p. 202) says:

"Sarda quem alij dicunt Sardo lapis est qui se habet ad tabulas ligni sicut magnes ad ferr[=u], et ideo adhæret ita fortiter tabulis nauium quòd euelli n[=o] possit, nisi abscindatur cum ipso ea pars tabulæ cui inhæserit, est aut[=e] in colore purissimus nitens."

And Pliny (_op. citat._, p. 629):

"Sagda is a stone, which the Chaldeans find sticking to ships, and they say it is greene as Porrets or Leekes."

[192] PAGE 111, LINE 8. Page 111, line 8. _Euace._--Evax, king of the Arabs, is said to have written to Nero a treatise on the names, colours, and properties of stones. See the note on Marbodæus, p. 7, line 20.

[193] PAGE 113, LINE 14. Page 113, line 19. _repulsus sit._ The words read thus in all editions, but the sense requires _repulsa sint._

[194] PAGE 113, LINE 23. Page 113, line 29. _Electrica omnia alliciunt cuncta, nihil omninò fugant vnquam, aut propellunt._ This denial of electrical repulsion probably arose from the smallness of the pieces of electric material with which Gilbert worked. He could hardly have failed to notice it had he used large pieces of amber or of sealing-wax. Electrical repulsion was first observed by Nicolas Cabeus, _Philosophia Magnetica_, Ferrara, 1629; but first systematically announced by Otto von Guericke in his treatise _Experimenta Nova (ut vocantur) Magdeburgica, de Vacuo Spatio_ (Amstel., 1672).

[195] PAGE 113, LINE 29. Page 113, line 37. _cùm de calore quid sit disputabimus._--The discussion of the nature of heat is to be found in Gilbert's _De Mundo nostro Sublunari_ (Amstel., 1651), lib. i., cap. xxvi., pp. 77-88.

[196] PAGE 115, LINE 23. Page 115, line 23. _trium vel quatuor digitorum._--Here as in all other places in Gilbert, _digitus_ means a finger's breadth, so that three or four digits means a length of two or three inches, or from six to eight centimetres.

[197] PAGE 117, LINE 26. Page 117, line 25. _ille Thebit Bencoræ trepidationis motus._

"Trepidation in the ancient Astronomy denotes a motion which in the Ptolemaic system was attributed to the firmament, in order to account for {51} several changes and motions observed in the axis of the world, and for which they could not account on any other principle." (Barlow's _Mathematical Dictionary_.)

[198] PAGE 118, LINE 10. Page 118, line 8. _cuspis is aut lilium._--Gilbert uses _cuspis_ or _lilium_ always of the North-pointing end of the needle. Sir Thomas Browne speaks of "the lilly or northern point"; but he differs from Gilbert in saying "the _cuspis_ or Southern point" (_Pseudodoxia Epidemica_, 1650, p. 46). Only in one place (p. 101, line 5) does Gilbert speak of _cuspis meridionalis_. Everywhere else the south-pointing end is called the _crux_.

[199] PAGE 118, LINE 15. Page 118, line 13. _nam æquè potens est._--Later observation showed this view to be incorrect. The horizontal component of the earth's magnetic field is not equally strong all over the globe, and the sluggishness of the needle's return to its position of rest is not due to the supporting pin becoming blunt with wear. The value of the horizontal component is zero at the north magnetic pole, and increases toward the magnetic equator. It is greatest near Singapore and in Borneo, being there more than twice as great as it is at London. (See Captain Creak in _Report of Voyage of H.M.S. Challenger, Physics and Chemistry_, vol. ii., part vi., 1889.)

[200] PAGE 119, LINE 5. Page 119, line 2. _lapis._--Both Stettin editions read _lapidis._

[201] PAGE 119, LINES 9-11. Page 119, lines 7-9. The gist of the whole book is summarized in these lines. They furnish a cardinal example of that inductive reasoning which was practist by Gilbert, and of which Bacon subsequently posed as the apostle. Compare pages 41 and 211.

[202] PAGE 120, LINE 8. Page 120, line 5. _dicturi sumus_.--Change of verticity is treated of in book iii., chap. x., pp. 137 to 140.

[203] PAGE 125, LINE 24. Page 125, line 29. _appositam._--All editions give this word, though the sense requires _appositum._

[204] PAGE 128, LINE 9. Page 128, line 11. _non nimis longum._--The editions of 1628 and 1633 read (wrongly) _minus_ instead of _nimis_.

[205] PAGE 130, LINE 12. Page 130, line 14. The word _hunc_ in the folio of 1600 is corrected in ink to _tunc_, and the Stettin editions both read _tunc_.

[206] PAGE 132, LINE 9. Page 132, line 10. _minimus & nullius ponderis._--The editions of 1628 and 1633 both wrongly read _est_ for _&_.

[207] PAGE 132, LINE 28. Page 133, line 1. _nutat._--The editions of 1628 and 1633 both wrongly read _mutat_.

[208] PAGE 134, LINE 22. Page 134, line 25. _in rectâ sphærâ._--The meaning of the terms a _right_ or _direct sphere_, an _oblique sphere_ and a _parallel sphere_ are explained by Moxon on pages 29 to 31 of his book _A Tutor to Astronomy and Geography_ (Lond., 1686):

"A _Direct Sphere_ hath both the _Poles_ of the _World_ in the Horizon ... It is called a _Direct Sphere_, because all the _Celestial_ Bodies, as _Sun_, _Moon_, and _Stars_, &c. By the _Diurnal_ Motion of the _Primum Mobile_, ascend directly Above, and descend directly Below the _Horizon_. They that Inhabit under the _Equator_ have the _Sphere_ thus posited."

"An _Oblique Sphere_ hath the _Axis_ of the _World_ neither _Direct_ nor _Parallel_ to the _Horizon_, but lies aslope from it."

"A _Parallel Sphere_ hath one _Pole_ of the _World_ in the _Zenith_, the other in the _Nadir_, and the _Equinoctial_ Line in the _Horizon_."

[209] PAGE 136, LINE 1. Page 136, line 1. _præsenti._--The editions of 1628 and 1633 read _sequenti_, to suit the altered position of the figure.

[210] PAGE 137, LINE 24. {52} Page 137, line 28. _atque ille statim._--The Stettin editions both wrongly read illi.

[211] PAGE 139. There is a curious history to this picture of the blacksmith in his smithy striking the iron while it lies north and south, and so magnetizing it under the influence of the earth's magnetism. Woodcuts containing human figures are comparatively rare in English art of the sixteenth century; a notable exception being Foxe's _Acts and Monuments_ with its many crude cuts of martyrdoms. The artist who prepared this cut of the smith took the design from an illustrated book of Fables by one Cornelius Kiliani or Cornelius van Kiel entitled _Viridarium Moralis Philosophiæ, per Fabulas Animalibus brutis attributas traditæ, etc._ (Coloniæ, 1594). This rare work, of which there is no copy in the British Museum, is illustrated by some 120 fine copper-plate etchings printed in the text. On p. 133 of this work is an etching to illustrate the fable _Ferrarii fabri et canis_, representing the smith smiting iron on the anvil, whilst his lazy dog sleeps beneath the bellows. The cut on p. 139 of Gilbert gives, as will be seen by a comparison of the pictures just the same general detail of forge and tools; but the position of the smith is reversed right for left, the dog is omitted, and the words _Septrenio_ and _Auster_ have been added.

In the Stettin edition of 1628 the picture has again been turned into a copper-plate etching separately printed, is reversed back again left for right, while a compass-card is introduced in the corner to mark the north-south direction.

In the Stettin edition of 1633 the artist has gone back to Kiliani's original {53} plate, and has re-etched the design very carefully, but reversing it all right for left. As in the London version of 1600, the dog is omitted, and the words _Septentrio_ and _Auster_ are added. Some of the original details--for example, the vice and one pair of pincers--are left out, but other details, for instance, the cracks in the blocks that support the water-tub, and the dress of the blacksmith, are rendered with slavish fidelity.

It is perhaps needless to remark that the twelve copper-plate etchings in the edition of 1628, and the twelve completely different ones in that of 1633, replace certain of the woodcuts of the folio of 1600. For example, take the woodcut on p. 203 of the 1600 edition, which represents a simple dipping-needle made by thrusting a versorium through a bit of cork and floating it, immersed, in a goblet of water. In the 1633 edition this appears, slightly reduced, as a small inserted copper-plate, with nothing added; but in the 1628 edition it is elaborated into a full-page plate (No. xi.) representing the interior, with shelves of books, of a library on the floor of which stands the goblet--apparently three feet high--with a globe and an armillary sphere; while beside the goblet, with his back to the spectator, is seated an aged man, reading, in a carved armchair. This figure and the view of the library are unquestionably copied--reversed--from a well-known plate in the work _Le Diverse & Artificiose Machine_ of Agostino Ramelli (Paris, 1558).

In the Emblems of Jacob Cats (_Alle de Wercken_, Amsterdam, 1665, p. 65) is given an engraved plate of a smith's forge, which is also copied--omitting the smith--from Kiliani's _Viridarium_.

[212] PAGE 140, LINE 2.. Page 140, line 2. _præcedenti._--This is so spelled in all editions, though the sense requires _præcedente_.

[213] PAGE 141, LINE 21. Page 141, line 24. _quod in epistolâ quâdam Italicâ scribitur._--The tale told by Filippo Costa of Mantua about the magnetism acquired by the iron rod on the tower of the church of St. Augustine in Rimini is historical. The church was dedicated to St. John, but in the custody of the Augustinian monks. The following is the account of it given by Aldrovandi, _Musæum Metallicum_ (1648, p. 134), on which page also two figures of it are given:

"Aliquando etiam ferrum suam mutat substantiam, dum in magnetem conuertitur, & hoc experientia constat, nam Arimini supra turrim templi S. Ioannis erat Crux a baculo ferreo ponderis centum librarum sustentata, quod tractu temporis adeò naturam Magnetis est adeptum, vt, illivs instar, ferrum traheret: hinc magna admiratione multi tenentur, qua ratione ferrum, quod est metallum in Magnetem, qui est lapis transmutari possit; Animaduertendum est id à maxima familiaritate & sympathia ferri, & magnetis dimanare cum Aristoteles in habentibus symbolum facilem transitum semper admiserit. Hoc in loco damus imaginem frusti ferri in Magnetem transmutati, quod clarissimo viro Vlyssi Aldrouando Iulius Caesar Moderatus diligens rerum naturalium inquisitor communicauit; erat hoc frustum ferri colore nigro, & ferrugineo, crusta exteriori quodammodo albicante." And further on p. 557.

"Preterea id manifestissimum est; quoniam Arimini, in templo Sancti Ioannis, fuit Crux ferrea, quæ tractu temporis in magnetem conuersa est, & ab vno latere ferrum trahebat, & ab altero respuebat." See also Sir T. Browne's _Pseudodoxia Epidemica_ (edition of 1650, p. 48), and Boyle's tract, _Experiments and Notes about the Mechanical Production of Magnetism_ (London, 1676, p. 12).

{54} Another case is mentioned in Dr. Martin Lister's _A Journey to Paris_ (Lond., 1699, p. 83). "He [Mr. Butterfield] shewed us a Loadstone sawed off that piece of the Iron Bar which held the Stones together at the very top of the Steeple of _Chartres_. This was a thick Crust of Rust, part of which was turned into a strong Loadstone, and had all the properties of a Stone dug out of the Mine. _Mons. de la Hire_ has Printed a Memoir of it; also Mons. _de Vallemont_ a Treatise. The very outward Rust had no Magnetic Virtue, but the inward had a strong one, as to take up a third part more than its weight unshod." Gassendi and Grimaldi have given other cases.

Other examples of iron acquiring strong permanent magnetism from the earth are not wanting. The following is from Sir W. Snow Harris's _Rudimentary Magnetism_ (London, 1872, p. 10).

"In the _Memoirs of the Academy of Sciences_ for 1731, we find an account of a large bell at Marseilles having an axis of iron: this axis rested on stone blocks, and threw off from time to time great quantities of rust, which, mixing with the particles of stone and the oil used to facilitate the motion, became conglomerated into a hardened mass: this mass had all the properties of the native magnet. The bell is supposed to have been in the same position for 400 years."

[214] PAGE 142, LINE 13. Page 142, line 15. _tunc planetæ & corpora coelestia._--Gilbert's extraordinary detachment from all metaphysical and ultra-physical explanations of physical facts, and his continual appeal to the test of experimental evidence, enabled him to lift the science of the magnet out of the slough of the dark ages. This passage, however, reveals that he still gave credence to the _nativities_ of judicial Astrology, and to the supposed influence of the planets on human destiny.

[215] PAGE 144, LINE 14. Page 144, line 14. _ijdem._--The editions of 1628 and 1633 erroneously read _iisdem_.

[216] PAGE 147, LINE 27. Page 147, line 29. _ex optimo aciario._--Gilbert recommended that the compass-needle should be of the best steel. Though the distinction between iron and steel was not at this time well established, there is no reason to doubt that by _aciarium_ was meant edge-steel as used for blades. Barlowe, in his _Magneticall Advertisements_ (Lond., 1616), p. 66, gives minute instructions for the fashioning of the compass-needle. He gives the preference to a pointed oval form, and describes how the steel must be hardened by heating to whiteness and quenching in water, so that it is "brickle in a manner as glass it selfe," and then be tempered by reheating it over a bar of red hot iron until it is let down to a blue tint. Savery (_Philos. Trans._, 1729) appears to have been the first to make a systematic examination of the magnetic differences between hard steel and soft iron.

Instructions for touching the needle are given in the _Arte de Nauegar_ of Pedro de Medina (Valladolid, 1545, lib. vi., cap. 1).

[217] PAGE 149, LINE 8. Page 149, line 9. _per multa sæcula._--Compare Porta's assertion (p. 208, English edition) "iron once rubbed will hold the vertue a hundred years." Clearly not a matter within the actual experience of either Porta or Gilbert.

[218] PAGE 153, LINE 2. Page 153, line 2. _Cardani ab ortu stellæ in cauda vrsæ._--What Cardan said (_De Subtilitate_, _Edit. citat._, p. 187) was: "ortum stellæ in cauda ursæ minoris, quæ quinque partibus orientalior est polo mundi, respicit."

[219] PAGE 153, LINE 21. Page 153, line 26. _sequitur quod versus terram magnam, siue continentem ... à vero polo inclinatio magnetica fiat._--Gilbert {55} goes on to point out how, at that date, all the way up the west European coast from Morocco to Norway, the compass is deflected eastward, or toward the elevated land. He argued that this was a universal law.

In _Purchas his Pilgrimes_ (Lond., 1625), in the Narrative, in vol. iii., of Bylot and Baffin's Voyage of 1616, there is mentioned an island between Whale-Sound and Smith's Sound, where there had been observed a larger variation than in any other part of the world. Purchas, in a marginal note, comments on this as follows: "Variation of the Compass 56° to the West, which may make questionable D. Gilbert's rule, tom. 1., l. 2, c. 1, that where more Earth is more attraction of the Compass happeneth by variation towards it. Now the known Continents of Asia, &c., must be unspeakably more than here there can be, & yet here is more variation then about Jepan, Brasil, or Peru, &c."

Gilbert's view was in truth founded on an incomplete set of facts. At that time, as he tells us, the variation of the compass at London was 11-1/3 degrees eastward. But he did not know of the secular change which would in about fifty-seven years reduce that variation to zero. Still less did he imagine that there would then begin a westward variation which in the year 1816 should reach 24° 30', and which should then steadily diminish so that in the year 1900 it should stand at 16° 16' westward. For an early discussion of the changes of the variation see vol. i. of the _Philosophical Transactions_ (Abridged), p. 188. Still earlier is the classical volume of Henry Gellibrand, _A Discovrse Mathematical on the Variation of the Magneticall Needle_ (Lond., 1635). Gilbert heads chapter iii. of book iiii. (p. 159) with the assertion _Variatio uniuscuiusque loci constans est_, declaring that to change it would require the upheaval of a continent. Gellibrand combats this on p. 7 of the work mentioned. He says:

"Thus hitherto (according to the Tenents of all our _Magneticall_ Philosophers) we have supposed the variations of all particular places to continue one and the same. So that when a Seaman shall happly returne to a place where formerly he found the same variation, he may hence conclude he is in the same former _Longitude_. For it is the Assertion of _Mr. Dr. Gilberts_. _Variatio vnicuiusq; Loci constans est_, that is to say, the same place doth alwayes retaine the same variation. Neither hath this Assertion (for ought I ever heard) been questioned by any man. But most diligent magneticall observations have plainely offred violence to the same, and proved the contrary, namely that the variation is accompanied with a variation."

In 1637 Henry Bond wrote in the _Sea-Mans Kalendar_ that in the year 1657 the variation would be zero at London. Compare Bond's _Longitude Found_ (Lond., 1676, p. 3).

As to inconstancy of the variation in one place see further Fournier's _Hydrographie_ (Paris, 1667, liv. xi., ch. 12, p. 413), and Kircher, _Magnes_ (Colon. Agripp., 1643, p. 418).

[220] PAGE 157, LINE 4. Page 157, line 5. _perfecto._--Though this word is thus in all editions, it ought to stand _perfectâ_, as in line 10 below.

[221] PAGE 157, LINE 11. Page 157, line 13. _varietas_, for _variatio_.

[222] PAGE 160, LINE 20. Page 160, line 23. _in Borrholybicum._--This name for the North-west, or North-North-West, is rarely used. It is found on the chart or windrose of the names of the winds on pp. 151 and 152 of the _Mécometrie de l'Eyman_ of G. Nautonier (1602). Here the name _Borrolybicus_ is given as a synonym for _Nortouest Galerne_, or [Greek: Olumpias], while the two winds on the points next on the western and northern sides respectively are called _Upocorus_ and _Upocircius_.

{56} In Swan's _Specvlvm Mundi_ (Camb., 1643, p. 174) is this explanation: "Borrholybicus is the North-west wind."

In Kircher's _Magnes_ (Colon. Agripp., 1643, p. 434) is a table of the names of the thirty-two winds in six languages, where _Borrolybicus_ is given as the equivalent of _Maestro_ or _North-West_.

[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 {57} 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 [=p]te magnetis ex opposita eius [=p]te appropinquato fugat: ut ex[=p]i[~e]tia 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,

[Lily] (or T). G. [Cross] (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.

{58} 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 {59} 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.

[227] PAGE 168, LINE 29. Page 168, line 33. _Directio igitur inualidior est propè polos._ Here as in many passages _direction_ means _the force which directs_. A similar usage prevails with the nouns _variation_ and _declination_, meaning frequently the force causing variation or declination respectively.

PAGE 172, LINE 13. _perquirere._ The edition of 1633 reads _perquirero_, in error.

[228] PAGE 172, LINE 29. Page 172, line 33. _Ad pyxidis nauticæ veræ & meridionalis formam ... fiat instrumentum._--An excellent form of portable meridian compass, provided with sights for taking astronomical observations, is described by Barlowe (_The Navigators Supply_, London, 1597), and is depicted in an etched engraving. An identical engraving is repeated in Dudley's _Arcano del Mare_ (Firenze, 1646). Gilbert's new instrument was considerably larger.

[229] PAGE 174, LINE 19. Page 174, line 21. _addendo vel detrahendo prostaphæresin._--"Prosthaphæresis, conflata dictione, ex additione et subtractione speciebus logistices, nomen habet ab officio, quia vt in semicirculo altero ad æquabilem motum adijcitur, ita in altero subtrahitur, vt adparens motus ex æquabili taxetur: atque hinc fit, quòd quæ Prosthaphæresis dicitur Ptolemæo, ea vulgò æquatio vocetur." (Stadius, _Tabulæ Bergenses_, Colon. Agripp., 1560, p. 37.)

[230] PAGE 174, LINE 28. Page 174, line 31. _Stellæ Lucidæ._--According to Dr. Marke Ridley (_Magneticall Animadversions_, London, 1617, p. 9), this