34 The Realm of Nature CHAP. 



saved the advertiser much lost time and useless trouble. 

 We know that if a machine could run without resistance 

 it would go on for ever at the same rate in virtue of 

 inertia if energy is once imparted to it. But if a machine 

 could not only keep going but set looms in motion as 

 well, energy must be created at every turn, and experiment 

 proves that this has never taken place. If energy be a real 

 thing the Perpetual Motion is impossible. Energy is always 

 undergoing transformation, visible motion, magnetism, elec- 

 tricity, heat, and light being a few of the many forms which 

 it assumes. But Nature says sternly and unmistakably, 

 " Nothing for nothing." No form of energy can be obtained 

 without paying an exact equivalent in some other form. 



56. Invisible Energy. Work can be done and potential 

 energy stored in separating atoms ( 44) as well as in climb- 

 ing mountains ; and the union of the separated atoms recon- 

 verts potential to kinetic energy as truly as the downward 

 rush of an avalanche. When a stone strikes the ground its 

 energy of motion as a whole is changed into energy of 

 motion of its parts, which we recognise as heat. Three 

 kinds of motion occur both on the great scale, perceptible 

 to the eye, and on the small scale, discoverable by observa- 

 tion and reason. These are simple translation, like the 

 movement of falling stones or of the darting particles of 

 gases ; 'wave motion, like the undulations of the sea or the 

 vibrations producing light ; and vortex motion, like whirl- 

 pools in tidal streams or the disturbances we recognise as 

 magnetism. 



57. Wave -motion. Every elastic substance ( 35) 

 can propagate wave-motion. This motion consists in one 

 particle moving through a comparatively short path and 

 returning to its previous position, after passing on its energy 

 of motion to another particle which also moves a short 

 distance and returns. Waves of to -and -fro or up-and- 

 down motion occur in solids and liquids ; and waves of 

 alternate compression and expansion occur in gases. 

 Waves are measured by the distance between similar parts 

 of successive waves. The distance between crest and crest 

 (CC in Fig. 6) or between trough and trough (TT) of 



