316 Royal Institution. 



If, instead of being allowed to communicate its force to the works 

 of the clock, the weight be allowed to descend suddenly, as by cutting 

 the string by which it is suspended, it strikes the floor with a force 

 which shakes the house ; and thus conveys, almost instantaneously, 

 the amount of force which would be gradually dissipated, though 

 not ultimately consumed, by the clock in a week or nine days. 



This idea, however, of the perpetuity of force, is not what is com- 

 monly understood by the term perpetual motion : that expression is 

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

 which is restored by tlie 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 without 

 transferring force, as by a permanent magnet. All sound philoso- 

 phers 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 extra- 

 neous 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 pro- 

 ducing force, cannot reproduce this; for then the weaker would con- 

 quer 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 recognized 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 investiga- 

 tion. 



After CErsted made his discovery of electro-magnetism, philoso- 

 phers of the highest attainments argued, that as a current of elec- 

 tricity, circulating in a wire round a bar of iron, produced magnetism, 

 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 per- 

 manent 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, 



