THE CONSERVATION OF ENERGY. 75 



and all work done upon a system as an accession of energy. Conse- 

 quently, the energy lost by one system in performance of work will be 

 gained by another in having work done upon it, and the total energy, 

 as between the two systems, will remain unchanged. 



There are two cases, or conditions, of energy which, although sub- 

 stantially the same, are for convenience regarded separately. These 

 may be illustrated by the following example. Work may be done 

 upon a body, and energy communicated to it, by setting it in motion, 

 e.g. by lifting it against gravity. Suppose this to be done by a spring 

 and detent ; and suppose further the body, on reaching its highest 

 point, to be caught so as to rest at that level on a support. Then, 

 whether we consider the body at the moment of starting, or when 

 resting on the support, it has equally received an accession of energy 

 from the spring, and is therefore equally capable of communicating 

 energy to a third body. But in the one case this is due to the motion 

 which it has acquired, and in the other to the position at which it 

 rests, and to its capability of falling again when the support is re- 

 moved. Energy in the first of these states is called ' Energy of 

 Motion,' or Kinetic Energy,' and that in the second state, ' Energy 

 of Position,' or 'Potential Energy.' In the case supposed, at the 

 moment of starting, the whole of the energy is kinetic; as the 

 body rises, the energy becomes partly potential and partly kinetic ; 

 and when it reaches the highest point the energy has become wholly 

 potential. If the body be again dropped, the process is reversed. 



The history of a discovery, or invention, so simple at first sight, is 

 often found to be more complicated the more thoroughly it is ex- 

 amined. That which at first seems to have been due to a single mind 

 proves to have been the result of the successive actions of many minds. 

 Attempts more or less successful in the same direction are frequently 

 traced out j and even unsuccessful efforts may not have been without 

 influence on minds turned towards the same object. Lastly also, 

 germs of thought, originally not fully understood, sometimes prove in 

 the end to have been the first stages of growth towards ultimate fruit. 

 The history of the law of the conservation of energy forms no excep- 

 tion to this order of events. There are those who discern even in the 

 writings of Newton expressions which show that he was in possession 

 of some ideas which, if followed out in a direct line of thought, would 

 lead to those now entertained on the subjects of energy and of work. 

 But however this may be, and whosoever might be reckoned among 

 the earlier contributors to the general subject of energy, and to the 

 establishment of its laws, it is certain that within the period of which 



