# On the Magnet/II-29

CHAP. XXIX.
On Variety of Strength due to Quantity
or Mass.

The page and line references given in these notes are in all cases first to the Latin edition of 1600, and secondly to the English edition of 1900.

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.

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."