[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 chapter xii. of book iv., with the Table of Stars, was written by Edward Wright, the author of the Prefatory Epistle of De Magnete. Wright was Lecturer on Navigation to the East India Company, and author of sundry treatises on Navigation.
[231] Page 187, line 14. Page 187, line 16. hic qui versus boream constitit ... meridionalis est, non borealis, quem antè nos omnes existimabant esse borealem.—Earlier on, on pages 15 and 125, Gilbert had mentioned this point. His insistence caused Barlowe (Magneticall Aduertisements, 1616, p. 4) to speak of the south-pointing end of the needle as the "true North," and thereby drew on himself the animadversions of Marke Ridley.
[232] Page 188, line 15. Page 188, line 16. in rectâ sphærâ.—See [note] to p. [134].
[233] Page 190, line 14. Page 190, line 19. declinans in Borealibus.—Dipping as it does in northern regions; that is, with the north-seeking or true-south pole downward.
[234] Page 195, line 20. Page 195, line 24. multa maiora pondera.—Many greater weights. All editions read multa, but the sense requires multo: "much greater weights."
[235] Page 196, line 10. Page 196, line 12. constans est.—This must not be read "is constant," for it is constant only in any given latitude.
[236] Page 196, line 15. Page 196, line 18. De proportione declinationis pro latitudinis ratione.—Gilbert here announces, and proceeds in the next seven pages to develop, the proposition that to each latitude there corresponds a constant dip to a particular number of degrees. If this were accurately so, then a traveller by merely measuring the dip would be able to ascertain, by calculation, by reference to tables, or by aid of some geometrical appliance,
the latitude of the place. In this hope Gilbert fought to perfect the dipping-needle; and he also worked out, on pages [199] and [200], an empirical theory, and a diagram. This theory was still further developed by him, and given to Thomas Blundevile (see the [Note] to p. [240]). Briggs of Gresham College, on Gilbert's suggestion, calculated a table of Dip and Latitude on this theory. It was found, however, that the observed facts deviated more or less widely from the theory. Kircher (Magnes, 1643, p. 368) gives a comparative table of the computed and observed values. Further discovery showed the method to be impracticable, and Gilbert's hope remained unfulfilled.
[237] Page 197, line 18. Page 197, line 21. progressionis centri.—Note Gilbert's precision of phrase.