[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.
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.