[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. {49} 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. Thomson's _British Annual_, 1837, p. 354, gives the following reference: "In the _Records of General Science_, vol. iii., p. 272, there is an interesting description of a very powerful magnet which was sent from Virginia in 1776 by the celebrated Dr. Franklin to Professor Anderson, of Glasgow. It is now in the possession of Mr. Crichton. It weighs 2½ grains, and is capable of supporting a load of 783 grains, which is equivalent to 313 times its own weight." [180] PAGE 99, LINE 10. Page 99, line 11. _Manifestum est._--In this, as in many other passages, Gilbert uses this expression in the sense that _it is demonstrable_ rather than meaning that _it is obvious_: for the fact here described is one that is not at all self-evident, but one which would become plain when the experiment had been tried. For other instances of this use of _manifestum_ see pages 144, line 20; 158, line 19; 162, line 10.