108 MOTIVE POWER OF HEAT. 



above, regarding them as exact, to the examination 

 of the different methods proposed up tc date, for 

 the realization of the motive power of heat. 



It has sometimes been proposed to develop mo- 

 tive power by the action of heat on solid bodies. 

 The mode of procedure which naturally first occurs 

 to the mind is to fasten immovably a solid body 

 a metallic bar, for example by one of its extremi- 

 ties ; to attach the other extremity to a movable 

 part of the machine; then, by successive heating 

 and cooling, to cause the length of the bar to vary, 

 and so to produce motion. Let us try to decide 

 whether this method of developing motive power 

 can be advantageous. We have shown that the 

 condition of the most effective employment of heat 

 in the production of motion is, that all changes 

 of temperature occurring in the bodies should be 

 due to changes of volume. The nearer we come 

 to fulfilling this condition the more fully will the 

 heat be utilized. Now, working in the manner 

 just described, we are very far from fulfilling this 

 condition : change of tempeiYiture is not due here 

 to change of volume ; all the changes are due to 

 contact of bodies differently heated to the con- 

 tact of the metallic bar, either with the body 

 charged with furnishing heat to it, or with the 

 body charged with carrying it off. 



