APPENDIX 327 



second there exists only the possibiUty of its manifestation. 

 We have just seen that bodies possess energy either as a 

 consequence of motion, or by virtue of their position {e.g. a 

 weight raised, or our separated balls). The first kind of 

 energy is called actual or kinetic energy, whereas energy depend- 

 ing on position has been called potential energy, i.e. energy 

 existing as tension. Therefore, energy becojjies manifested in 

 motion, and is concealed in a state of tension. Both kinds 

 are mutually transformable ; our balls present an obvious 

 illustration of this fact. Actual energy exerted in separating 

 these balls has not disappeared, but has transformed itself into 

 potential energy, into the tension of the springs. In that state 

 it can be stored and kept, and then used again whenever 

 wanted, retransformed into actual energy, into motion, and 

 moreover at pleasure either directly or gradually. Every day 

 of our lives we make such a storage of energy in winding 

 our watches : the kinetic energy of the hand which does the 

 winding transforms itself into the potential energy of the 

 watch-spring, which in the course of twenty-four hours 

 gradually passes into the state of actual energy, shown by 

 the moving hands. Something analogous to this happens 

 when a man saves money for his old age : he transforms the 

 superfluity of his actual energy, mechanical or intellectual, 

 into potential energy, so that he may use it when his actual 

 energy comes to an end. On all sides in Nature we see similar 

 transformations of motion into tension and vice versa. Keeping 

 this transformation in view we soon arrive at the conclusion 

 that energy as a rule does not arise anew nor disappear, that it 

 is eternal ; in other words we become convinced that aU the 

 work done, or which can be done in the universe by the forces of 

 Nature at any given moment, does not increase, nor decrease, 

 but remains the same. This broadest physical generalisation, 

 called the law of the ' conservation of energy,' is ' the highest 

 law in physical science which our faculties permit us to perceive ' 

 (Faraday) . 



There are, however, cases in which this law seems not to 

 hold ; energy seems sometimes to use itself up, and motion 

 instead of changing into tension seems to vanish altogether. 

 We have exactly such a case in our balls. I separate them and 

 let go. The balls knock against each other, and seem to lose 



