[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."