42 EEPORT— 1881. 



position at which it rests, and to its capability of falling again when the 

 support is removed. 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 moi'e complicated the more thoroughly it is examined. 

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

 have been the result of the successive action of many minds. Attempts 

 •more or less successful in the same direction are frequently traced out ; 

 and even unsuccessful efibrts 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 tiltimate fruit. The history of the law 

 of the conservation of energy forms no exception 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 I am now speaking, the names of Seguin, 

 Clausius, Helmholtz, Mayer, and Colding on the Continent, and those 

 of Grove, Joule, Rankine, and Thomson in this country, will always be 

 associated with this great work. 



I must not, however, quit this subject without a passing notice of a 

 conclusion to which Sir William Thomson has come, and in which he is 

 followed by others who have pursued the transformation of energy to some 

 of its ultimate consequences. The nature of this will perhaps be most 

 easily apprehended by reference to a single instance. In a steam engine, 

 or other engine, in which the motive power depends upon heat, it is well 

 known that the source of power lies not in the general temperature of the 

 whole, but merely on the difference of temperature between that of the 

 boiler and that of the condenser. And the effect of the condenser is to 

 reduce the steam issuing from the boiler to the same temperature as that of 

 the condenser. "When this is once done, no more work can be got out of 

 the engine, unless fresh heat be supplied from an outside source to the 

 boiler. The heat originally communicated to the boiler has become 

 uniformly diffused, and the energy due to that difference is said to have 

 been dissipated. The energy remains in a potential condition as regards 

 other bodies ; but as regards the engine, it is of no further use. Now 

 suppose that we regard the entire material universe as a gigantic engine, 

 and that after long use we have exhausted all the fuel (in its most general 



