[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.