124 



SCIENTIFIC THOUGHT. 



Helmholtz in Germany, and Thomson in England, heard 

 about Sadi Carnot himself. Sadi Carnot, so much earlier 

 and so unlike Mayer, had nevertheless one point in 

 common with him. This point seems to have given 

 a common anchorage to all those thinkers who, in the 

 course of a generation, gradually lifted the theory of heat 

 and energy out of twilight into clear thought. Sadi 

 Carnot, Mayer, Joule, Helmholtz, Thomson, all express 

 or imply the same idea — viz., the impossibility of a 



In one form or other this seems 



22. 

 Perpetual 

 motion . . 



impossible, perpetual motion.^ 



^ The conception of a "perpet- 

 ual motion," or, as it is termed 

 abroad, of a " perpetuum mobile," 

 and that of its impossibility, 

 have been changed and more 

 clearly defined in the course of 

 the hundred years which followed 

 the decision of the Paris Academy 

 of Sciences in 1775 not to receive 

 in future any scheme of perpetual 

 motion. Into the same class of 

 axiomatic impossibilities were also 

 thrown the "squaring of the 

 circle" and the "trisection of the 

 angle." Helmholtz (appendix to 

 his Lecture on ' Die Wechselwirk- 

 ung der Naturkraf te, ' 1853, dated 

 1883) remarks that the proof of 

 the impossibility did not then 

 exist, and that the resolution was 

 therefore based merely on the 

 experience of past failures. The 

 doctrine of Energy, the arithmet- 

 ical discoveries of Gauss, and the 

 elegant researches of Hermite and 

 Lindemann, have thrown much 

 light on these celebrated prob- 

 lems. In the last chapter of 

 this volume I shall revert to the 

 two latter ; as to the first, the 

 "perpetual motion," what follows 

 may tend to clear the popular 

 conceptions. Tait has correctly 

 remarked that "perpetual motion 

 is simply a statement of Newton's 



first law of Motion " ( ' Recent 

 Advances,' 3rd ed., p. 74). He 

 might have added that it took 

 probably as much ingenuity on 

 the part of Galileo to arrive at the 

 principle of inertia — viz., that "all 

 motion is perpetual until force in- 

 terferes to alter and modify it " — as 

 it took to formulate correctly the 

 other principle that such a per- 

 petual motion is of no use, because 

 you cannot do any work with it, 

 except by using it up or anni- 

 hilating it. In the beginning of 

 the nineteenth century the im- 

 possibility of a mechanical device 

 for the so-called perpetual motion 

 was universally admitted, though 

 — as Rosenberger (' Geschichte der 

 Physik,' vol. iii. p. 229, note) 

 remarks — this was not also ex- 

 tended to physical processes, it 

 being taught that the processes of 

 nature represented a ' ' perpetual 

 cycle which uninterruptedly re- 

 newed itself." In fact, the truth 

 was beginning to dawn that if 

 motive power or energy could not 

 be obtained out of nothing neither 

 could it be destroyed. Carnot in 

 1824, and Mayer in 1842, both take 

 it as an axiom that power cannot 

 be created ; Mohr in 1837, and 

 Joule in 1843 and 1845, are equally 

 convinced that power cannot be 



