108 MECHANICAL RECREATIONS [CH. V 



some of Captain Parry's experiments the report of a cannon, 

 when fired, travelled so much more rapidly than the sound 

 of the human voice that observers heard the report of the 

 cannon when fired before that of the order to fire it*: in the 

 Kinetic Theory of Gases, the complications in our universe that 

 might be produced by "Maxwell's demon "f : in the Theory of 

 Optics, the explanation of the Japanese "magic mirrors," J 

 which reflect the pattern on the back of the mirror, on which 

 the light does not fall ; various physiological paradoxes ; and the 

 theory of the " spectrum top," by means of which a white surface, 

 on which some black lines are drawn, can be moved so as to give 

 the impression § that the lines are coloured (red, green, blue, slate, 

 or drab), while the colours change with the direction of rotation : 

 on wave motion it has also been recently shown that if two 

 trains of waves, whose lengths are in the ratio m — 1 : m + 1, be 

 superposed, then every mth wave in the system will be big — 

 thus the current opinion that every ninth wave in the open sea 

 is bigger than the other waves may receive scientific confirma- 

 tion. There is no lack of interesting and curious phenomena 

 in physics, and in some branches, notably in electricity and 

 magnetism, the difficulty is rather one of selection, but I felt 

 that the connection with mathematics was in general either too 

 remote or too technical to justify the insertion of such a 

 collection in a work on elementary mathematical recreations, 

 and therefore I struck out the chapter. I mention the fact now 

 partly to express the hope that some physicist will one day give 

 us a collection of the kind, partly to suggest these questions to 

 those who are interested in such matters. 



* The fact is well authenticated. Mr Earnshaw (Philosophical Transactions, 

 London, 1860, pp. 133 — 1 48) explained it by the acceleration of a wave oaused by 

 the formation of a kind of bore, a view accepted by Clerk Maxwell and most 

 physicists, but Sir George Airy thought that the explanation was to be found in 

 physiology; see Airy's Sound, second edition, London, 1871, pp. 141, 142. 



+ See Theory of Heat, by J. Clerk Maxwell, second edition, London, 1872, 

 p. 308. 



J See a memoir by W. E. Ayrton and J. Perry, Proceedings of the Royal 

 Society of London, part i, 1879, vol. xxvm, pp. 127 — 148. 



§ See letters from Mr C. E. Benham and others in Nature, 1894-5; and 

 a paper read by Prof. G. D. Liveing before the Cambridge Philosophical Society, 

 November 26, 1894. 



