CONCLUDING EEMARKS. 193 



which to the thinking mind is a conviction. Some of these 

 I have ventured to put forward in the present essay, but 

 many remain, and will crowd upon the mind of those who 

 pursue the subject. Does not, for instance, the impossibility 

 of perpetual motion, when thought out, involve the demon- 

 stration of the impossibility, to which I have previously allud- 

 ed, of any event identically recurring ? 



The pendulum in vacuo, at each beat leaves a portion of 

 the force which started it in the form of heat at its point of 

 suspension : this force, though ever existent, can never be re- 

 stored in its integrity to the ball of the pendulum, for in the 

 process of restoration it must affect other matter, and alter 

 the condition of the universe. To restore the initial force to its 

 integrity, everything as it existed at the moment of the first 

 beat of the pendulum must be restored in its integrity : but 

 how can this be for while the force was escaping from the 

 pendulum by radiating heat from the point of suspension, 

 surrounding matter has not stood still ; the very attraction 

 which caused the beat of the pendulum has changed in degree, 

 for the pendulum is nearer to or further from the sun, or 

 from some planet or fixed star. 



It might be an interesting and not profitless speculation 

 to follow out these and other consequences ; it would, I be- 

 lieve, lead us to the conviction that the universe is ever 

 changing, and that notwithstanding secular recurrences which 

 would prima facie seem to replace matter in its original posi- 

 tion, nothing in fact ever returns or can return to a state of 

 existence identical with a previous state. But the field is too 

 illimitable for me to venture further. 



The inevitable dissipation or throwing off a portion of 

 the initial force presents a great experimental difficulty in the 

 way of establishing the equivalents of the various natural 

 forces. In the steam-engine, for instance, the heat of the 

 furnace not only expands the water and thereby produces the 

 motion of the piston, but it also expands the iron of the boil- 

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