1856.] the Negation of Perpetual Motion. 158 



commonly understood by the terra perpetual motion : that expression 

 is used to convey the notion of a motive machine, the initial force 

 of which is restored by the motion produced by itself, — a clock, so 

 to speak, which winds itself up by its own wheels and pendulum, a 

 pump which keeps itself going by the weight of the water which it 

 has raised. Another notion, arising from a confusion between 

 static and dynamic forces, was, that motion might be obtained with- 

 out transferring force, as by a permanent magnet. All sound 

 philosophers are of opinion that such effects are impossible ; the 

 work done by a given force, even assuming there were no such 

 thing as friction, aerial resistance, &c., could never be more than 

 equal to the initial force ; the theoretical limit is equilibrium. The 

 weight raised at one end of a lever can never, without the fresh 

 application of extraneous force, raise the opposite weight which has 

 produced its own elevation. A force can only produce motion 

 when the resistance to it is less powerful than itself; if equal, it is 

 equilibrium : thus if motion be produced, the resistance, being less 

 than the initial or producing force, cannot reproduce this ; for then 

 the weaker would conquer the stronger force. 



The object of this evening's communication was not, however, to 

 adduce proofs that perpetual motion, in the sense above defined, 

 is impossible ; but assuming that as a recognised truth, to show 

 certain consequences which had resulted, and others which were 

 likely to result, from the negation of perpetual motion ; and how 

 this negation may be made a substantive and valuable aid to 

 scientific investigation. 



After Oersted made his discovery of electro-magnetism, philo- 

 sophers of the highest attainments argued, that as a current of 

 electricity, circulating in a wire round a bar of iron, produced mag- 

 netism, and as action and reaction are equal, and in contrary 

 directions, a magnet placed within a spiral of wire should produce in 

 the wire an electrical current : had it occurred to their minds that, 

 if a permanent magnet could so produce electricity, and thence 

 necessarily motion, they would thus get, in effect, perpetual motion, 

 they would probably have anticipated the discovery of Faraday, 

 and found that all that was required was to move the magnet with 

 reference to the wire, and thus electricity might have been expected 

 to be produced by a magnet without involving the supposed 

 absurdity. 



In a very different instance, viz. the expansion of water when 

 freezing, not only heat, or the expansive force given to other bodies 

 by a body cooling, would be given out by water freezing, but also 

 the force due to the converse expansion in the body itself; and 

 upon the argument that force would, in this case, be got out of 

 nothing, Mr. J. Thomson saw that this supposed impossibility 

 would not result if the freezing point of water were lowered by 

 pressure, which was exjierimentally proved to be the case, by his 

 brother. 



