THE CONSTITUTION OF NATURE. 27 



the facts of attraction as they stand, and affirms only 

 the constancy of working-power. That power may exist 

 in the form of MOTION ; or it may exist in the form of 

 FORCE, with distance to act through. The former is 

 dynamic energy, the latter is potential energy, the con- 

 stancy of the sum of both being affirmed by the law of 

 conservation. The convertibility of natural forces con- 

 sists solely in transformations of dynamic into potential, 

 and of potential into dynamic energy. In no other sense 

 has the convertibility of force any scientific meaning. 



Grave errors have been entertained as to what is 

 really intended to be conserved by the doctrine of con- 

 servation. This exposition I hope will tend to remove 

 them. 



