2 4 o TEE POPULAR SCIENCE MONTHLY 



B can be carefully let down, and the havoc wrought by the sweep of 

 A can be undone. The result of this operation, however, would be to 

 leave the system B in the condition in which it would be degenerated 

 if the process B had been allowed to sweep instead of being let down. 

 This is very much as if, having two similar houses A and B, one of 

 which A has been burned, we could rig up a mechanism which would 

 let doivn B to ashes and cause A to be restored in the original actual 

 materials of which it was first constructed. This is of course impos- 

 sible in the case of the two houses, but it is possible in every known 

 case of thermodynamic degeneration. This general theorem is as 

 thoroughly established as any generalization in physics, and if it is 

 true and if we ever find a way to convert heat into work unconditionally 

 and without cost or compensation, then it will be proved indirectly 

 that a shaft can be driven by heating one of the bearings in which it 

 rotates, for direct conversion of work into heat by one process must be 

 according to this general theorem equivalent to and replaceable by the 

 reverse of any ordinary sweeping process which converts work into heat. 



(d) A gas cannot pass directly from a region of low pressure into 

 a region of high pressure, nor can a gas be transferred from a region 

 of low pressure to a region of high pressure by any means without 

 compensation. 



Imagine a gas squirting itself backwards through a nozzle into a 

 high-pressure reservoir ! The repeated statement of self-evident facts 

 concerning direct repair in these statements of the second law of 

 thermodynamics may seem ridiculous to the intelligent reader, but the 

 second law of thermodynamics is a statement of a fact which every 

 one knows coupled with a generalizing clause which, once thoroughly 

 understood, is almost if not quite self-evident. 



Here is one more statement of the second law of thermodynamics, 

 the oldest English version of it: 



Humpty Dumpty sat on a wall, 



Humpty Dumpty had a great fall, 



All the king's horses and all the king's men 



Can not put Humpty Dumpty together again. 



This is perhaps the most sensible of all the statements of the second 

 law, for which we will allow it to pass for the moment, inasmuch as it 

 ignores direct repair and refers at once to the most powerful of external 

 means. It is important, however, to remember that in Humpty 

 Dumpty's case we are concerned with structural degeneration, not with 

 the much simpler kind of degeneration in a structureless fluid due to 

 turbulence. 



Of all the generalizations of physics, the second law of thermo- 

 dynamics is certainly the most deeply seated in the common sense of 



