[Footnote 9: The following charming extract, bearing upon this point, was discovered and written out for me by my deeply lamented friend Dr. Bence Jones, when Hon. Secretary to the Royal Institution:-- 'In every kind of magnitude there is a degree or sort to which our sense is proportioned, the perception and knowledge of which is of the greatest use to mankind. The same is the groundwork of philosophy; for, though all sorts and degrees are equally the object of philosophical speculation, yet it is from those which are proportioned to sense that a philosopher must set out in his inquiries, ascending or descending afterwards as his pursuits may require. He does well indeed to take his views from many points of sight, and supply the defects of sense by a well-regulated imagination; nor is he to be confined by any limit in space or time; but, as his knowledge of Nature is founded on the observation of sensible things, he must begin with these, and must often return to them to examine his progress by them. Here is his secure hold: and as he sets out from thence, so if he likewise trace not often his steps backwards with caution, he will be in hazard of losing his way in the labyrinths of Nature.'--(_Maclaurin: An Account of Sir I. Newton's Philosophical Discoveries. Written 1728; second edition_, 1750; pp. 18, 19.) ] [Footnote 10: I do not wish to encumber the conception here with the details of the motion, but I may draw attention to the beautiful model of Prof. Lyman, wherein waves are shown to be produced by the _circular_ motion of the particles. This, as proved by the brothers Weber, is the real motion in the case of water-waves.] [Footnote 11: Copied from Weber's _Wellenlehre_.] [Footnote 12: See _Lectures on Sound_, 1st and 2nd ed., Lecture VII.; and 3rd ed., Chap. VIII. Longmans.] [Footnote 13: _Boyle's Works_, Birch's edition, p. 675.] [Footnote 14: Page 743.] [Footnote 15: The beautiful plumes produced by water-crystallization have been successfully photographed by Professor Lockett.] [Footnote 16: In a little volume entitled 'Forms of Water,' I have mentioned that cold iron floats upon molten iron. In company with my friend Sir William Armstrong, I had repeated opportunities of witnessing this fact in his works at Elswick, 1863. Faraday, I remember, spoke to me subsequently of the perfection of iron castings as probably due to the swelling of the metal on solidification. Beyond this, I have given the subject no special attention; and I know that many intelligent iron-founders doubt the fact of expansion. It is quite possible that the solid floats because it is not _wetted_ by the molten iron, its volume being virtually augmented by capillary repulsion. Certain flies walk freely upon water in virtue of an action of this kind. With bismuth, however, it is easy to burst iron bottles by the force of solidification.]