ON THE DISSIPATION OF ENERGY. 471 



radiation or conduction, and of tne efficiency of 

 heat as a motor, discovered by several thousand 

 years of observation and several hundred years of 

 experiment and dynamical theory, suffices to show 

 that when the weight is run down, and the potential 

 energy (or capacity to do work), which it had in 

 the beginning, has been all spent in heat, this heat 

 is not available for raising the weight and giving the 

 clockwork a renewed lease of motivity. The solar 

 system, according to the best of modern scientific 

 belief, is dynamically analogous to the clockwork, 

 in all the essentials of our consideration. Not 

 going back in thought to a beginning of which 

 science knows nothing, let us compare the solar 

 system as it was three thousand years ago with the 

 solar system as it is now. Let our analogue be a 

 clockwork which three hours ago was known to be 

 going with its weight partially run down, and 

 which is still going with its weight not yet wholly 

 run down. 



During these three thousand years the sun has 

 been giving out radiant heat (light being included 

 in the designation " radiant heat ") in all directions, 



