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which this is the seed, may be capable of being worked out completely, 

 and that it may be possible to determine how much heat will be 

 represented by so much energy ; or, on the other hand, how much 

 energy will correspond to so much heat. 



Let me illustrate this by an example. The sun gives out heat at a 

 certain rate. The earth is moving in her orbit with great velocity, about 

 eighty times as fast as a cannon ball : suppose she should impinge upon 

 some immovable obstacle, or, what would be the same thing, upon a 

 body exactly like herself coming the other way, then the heat supplied 

 by this awful collision would be equivalent to that given out by the sun 

 in about eighty days. If we imagine a still more strange catastrophe, and 

 conceive the earth to be brought to rest, and then allowed to fall as she 

 would into the sun, the result of her tremendous fall of more than 

 90,000,000 of miles would be a generation of heat which would keep the 

 sun supplied for more than ninety years. And if the same catastrophe 

 should happen to the planet Jupiter, which is immensely larger in mass 

 than the earth, and much further from the sun, the result would be to 

 produce as much heat as would last the sun, at its present rate of 

 expenditure, for no less a time than 32,000 years. 



The result of this law of the Conservation and Transformation of 

 Energy is to give us quite a new insight into the economy of Nature. 

 All kinds of forces are subject to it, whether they arise from gravitation, 

 electricity, magnetism, or what not. Energy can, in all cases, only be 

 transformed : we can no more create it than we can create matter. We 

 burn coal, and we change the heat which we produce into steam power : 

 the power of the engine is merely the heat of the coal in another form. 



And it is not difficult to see how vast are the fields of inquiry and 

 speculation which are opened by a theory of this kind : it helps us to 

 speculate upon the possible past, and throws also some light upon the 

 possible future. There is just one such speculation to which I will call 

 your attention, in consequence of its great interest. 



I refer to the question of the probable extent of time, during which 

 something like the present state of things has existed upon our globe. 

 Sir William Thomson has discussed this question in three different ways; 



