94 Professor W. Thomson on Mechanical 



A continuation of the inquiry raises the question, from 

 what source do the planets, large and small, derive the me- 

 chanical energy of their motions % This is a question to the 

 answering of which mechanical reasoning may legitimately be 

 applied. For we know that from age to age the potential 

 energy of the mutual gravitation of those bodies is gradually 

 expended, half in augmenting their motions, and half in gene- 

 rating heat ; and we may trace this kind of action either 

 backwards or forwards — backwards for a million of million of 

 years with as little presumption as forwards for a single day. 

 If we trace them forwards we find that the end of this world 

 as a habitation for man, or for any living creature or plant at 

 present existing in it, is mechanically inevitable ; and if we 

 trace them backwards according to the laws of matter and 

 motion — certainly fulfilled in all the actions of nature which 

 we have been allowed to observe — we find that a time must 

 have been when the earth, with no Sun to illuminate it, the 

 other bodies known to us as planets, and the countless smaller 

 planetary masses at present seen as the zodiacal light, must 

 have been indefinitely remote from one another, and from all 

 other solids in space. All such conclusions are subject to 

 limitation, as we do not know at what moment a creation of 

 matter or energy may have given a beginning, beyond which 

 mechanical speculations cannot lead us. If in purely mecha- 

 nical science we are ever liable to forget this limitation, we 

 ought to be reminded of it by considering that purely mecha- 

 nical reasoning shows a time when the earth must have been 

 tenantless, and teaches us that our own bodies, as well as all 

 living plants and animals, and all fossil organic remains, are 

 organized forms of matter to which science can point no ante- 

 cedent except the will of a Creator, a truth amply illustrated 

 by the evidence of geological history. But if duly impressed 

 with this limitation to the certainty of all speculations regard- 

 ing the future, and prehistorical periods of the past ; we may 

 legitimately push them into endless futurity, and we can be 

 stopped by no barrier of past time without ascertaining at some 

 finite epoch a state of matter not derivable from any antece- 

 dent by natural laws. Although we can conceive of such a state 

 of all matter, or of the matter within any limited space, and 



