THE PROBLEMS OF SOLUTION 103 



a thermodynamic theory has been developed, 

 which connects the phenomena directly, and 

 brings out their relations with similar phenomena 

 in other branches of science. 



The two methods may perhaps be illustrated 

 in some such way as the following. In looking 

 at the face of a watch, certain relations are 

 observed between the positions of the two hands 

 at different times. In order to explain these 

 phenomena we make hypotheses concerning the 

 structure of the inside of the watch. We imagine 

 various arrangements of springs, wheels, and 

 levers till we hit on one particular system which 

 consideration shows us will give the observed 

 result. Here we have an intimate picture of the 

 inside of the watch, which may or may not 

 represent the only possible arrangement, and 

 may or may not correspond with the reality. 

 Such a picture is analogous to a molecular theory 

 of a physical problem. 



One day, however, we notice, in the course of 

 our studies of the watch, that, whatever be the 

 position of the hands, one of them always moves 

 twelve times as fast as the other. We have 

 discovered a necessary relation between the 

 phenomena, which enables us, if we will, to 

 dispense with all hypotheses about the wheels 

 and springs which drive the mechanism. The 

 observed connection between the rates of motion 

 allows us to evade all such complications, and 

 to calculate directly the relative positions of the 

 two hands at any future time. 



So with thermodynamics. Lord Kelvin's great 

 principle of the dissipation of energy, especially 

 in its modern form, which states that the available 



