CHAP. IX.
On the anomaly of the Præcession of the Equinoxes,
and of the obliquity of the Zodiack.
Copernicus' contorted circlet in the Arctick circle of the Zodiack.
Let F B G be the half of the Arctick circle described round the pole of the Zodiack: A B C the solstitial colure: A the pole of the Zodiack; D E the anomaly of longitude 140 minutes at either side on both ends: B C the anomaly of obliquity 24 minutes: B the greater obliquity of 23 degrees 52 minutes: D the mean obliquity of 23 degrees 40 minutes: C the minimum obliquity of 23 degrees 28 minutes.
The period of motion of the præcession of the æquinoxes is 25,816 Ægyptian years; the period of the obliquity of the Zodiack is 3434 years, and a little more. The period of the anomaly of the præcession of the æquinoxes is 1717 years, and a little more. If the whole time of the motion AI were divided into eight equal parts: in the first eighth the pole is borne somewhat swiftly from A to B; in the second eighth, more slowly from B to C; in the third, with the same slowness from C to D; in the fourth, more swiftly again from D to E; in the fifth, with the same swiftness from E to F; again more slowly from F to G; and with the same slowness from G to H; in the last eighth, somewhat swiftly again from H to I. And this is the contorted circlet of Copernicus, fused with the mean motion into the curved line which is the path of the true motion. And thus the pole attains the period of the anomaly of the præcession of the æquinoxes twice; and that of the declination or obliquity once only. It is thus that by later astronomers, but especially by Copernicus (the Restorer of Astronomy)[252], the anomalies of the motion of the Earth's axis are described, so far as the observations of the ancients down to our own times admit; but there are still needed more and exact observations for anyone to establish aught certain about the anomaly of the motion of the præcessions, and at the same time that also of the obliquity of the Zodiack. For ever since the time at which, by means of various observations, this anomaly was first observed, we have only arrived at half a period of the obliquity. So that all the more all these matters about the unequal motion both of the præcession and of the obliquity are uncertain and not well known: wherefore neither can we ourselves assign any natural causes for it, and establish it for certain. Wherefore also do we to our reasonings and experiments magnetical here set an end and period.[253]
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.
251 ↑ Page 237, line 19. Page 237, line 22. vt motus quidem obscuri saluarentur.—It has been conjectured that quidem is here a misprint for quidam, but the adverb quidem adds a satirical flavour to his argument against the folly of those who held the doctrine of the moving spheres. The verb salvare does not occur in classical Latin.
252 ↑ Page 240, line 13. Page 240, line 17. à Copernico (Astronomiæ instauratore).—Gilbert was the first in England to uphold the doctrines of Copernicus as to the motion of the earth on its axis and its revolution around the sun. He considered that his magnetic observations brought new support to that theory, and his views are quoted with approbation by Kepler, Epitome Astronomiæ Copernicanæ ... Authore Ioanne Keplero ... (Francofurti, 1635); and by Galileo, Dialogus de Systemate Mundi (Augustæ Treboc., 1635), an English translation of which appeared in Salusbury's Mathematical Collections and Translations (London, 1661, pp. 364 to 377).
For this the book De Magnete was considered by many as heretical. Many of the copies existing in Italy are found to be either mutilated or else branded with a cross. For example, the copy in the library of the Collegio Romano in Rome has book VI. torn out. Galileo states that the Book of Gilbert would possibly never have come into his hands "if a Peripatetick Philosopher, of great fame, as I believe to free his Library from its contagion, had not given it me." In England Barlowe, in his Magneticall Aduertisements (1616), expressly repudiated Gilbert's Copernican notions, while praising his discoveries in magnetism. Marke Ridley, while upholding Gilbert's views, in his Magneticall Animadversions (1617) did not consider him "skilfull in Copernicus." The Jesuit writers, Cabeus, Kircher, Fonseca, Grandamicus, Schott, Leotaudus, Millietus, and De Lanis, one and all, who followed Gilbert in their magnetic writings, repudiated the idea that the magnetism of the globe gave support to the heretical modern Astronomy.
The works referred to are:
Cabeus, Philosophia Magnetica, in qua Magnetis natura penitus explicatur ... auctore Nicolao Cabeo Ferrarensi Soc. Jesv. (Ferrariæ, 1629).
Kircher, Magnes, Siue de Arte Magnetica, Libri tres, Authore Athanasio Kirchero ... e Soc. Iesv. (Romæ, 1641).
Grandamicus, Nova Demonstratio immobilitatis terræ petita ex virtute magnetica (Flexiæ, 1645). This work is most beautifully illustrated with copper-plate etchings of cupids making experiments with terrellas.
Schott, Gaspar, Thaumaturgus Physicus (Herbipolis, 1659).
Leotaudus, R. P. Vincentinii Leotavdi Delphinatis, Societ. Iesv., Magnetologia; in qva exponitvr Nova de Magneticis Philosophia, (Lvgdvni, 1668).
Millietus (Milliet Deschales), Cursus seu Mundus Mathematicus (Lugd., 1674), Tomus Primus, Tractatus de Magnete.
De Lanis, Magisterium Natvræ et Artis. Opus Physico-Mathematicvm P. Francisci Tertii de Lanis, Soc. Jesv. (Brixiæ, 1684).
253 ↑ Page 240, line 24. Page 240, line 31. hic finem & periodum imponimus.
On February 13 [1601] Gilbert wrote to Barlowe (see Magneticall Aduertisements, p. 88):
"I purpose to adioyne an appendix of six or eight sheets of paper to my booke after a while, I am in hand with it of some new inventions, and I would haue some of your experiments, in your name and inuention put into it, if you please, that you may be knowen for an augmenter of that arte."
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, 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.