94 MECHANICAL RECREATIONS [CH. V 



the ignis fatuus of self-taught mechanicians in much the same 

 way as the quadrature of the circle has been that of self-taught 

 geometricians. 



Now the obvious meaning of the third law of motion is 

 that a force is only one aspect of a stress, and that whenever a 

 force is caused another equal and opposite one is brought also 

 into existence — though it may act upon a different body, and 

 thus be immaterial for the particular problem considered. The 

 law however is capable of another interpretation*, namely, 

 that the rate at which an agent does work (that is, its action) 

 is equal to the rate at which work is done against it (that is, 

 its reaction). If it is allowable to include in the reaction the 

 rate at which kinetic energy is being produced, and if work is 

 taken to include that done against molecular forces, then it 

 follows from this interpretation that the work done by an 

 agent on a system is equivalent to the total increase of energy, 

 that is, the power of doing work. Hence in an isolated system 

 the total amount of energy is constant. If this is granted, 

 then since friction and some molecular dissipation of energy 

 cannot be wholly prevented, it must be impossible to construct 

 in an isolated system a machine capable of perpetual motion. 



I do not propose to describe in detail the various machines 

 for producing perpetual motion which have been suggested - )-, 

 but the machine described below will serve to illustrate one of 

 the assumptions commonly made by these inventors. 



The machine to which I refer consists of two concentric 

 vertical wheels in the same plane, and mounted on a horizontal 

 axle through their centre, C. The space between the wheels is 

 divided into compartments by spokes inclined at a constant 

 angle to the radii to the points whence they are drawn, and 

 each compartment contains a heavy bullet. This will be clear 

 from the diagram. Apart from these bullets, the wheels would 

 be in equilibrium. Each bullet tends to turn the wheels round 

 their axle, and the moment which measures this tendency is 



* Newton's Principles, last paragraph of the Scholium to the LawB of Motion. 

 + Several of them have been described in H. Dirck's Perpetuum Mobile, 

 London, 1861, 2nd edition, 1870. 



