"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 cœlestia.—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
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."