130 THOMSON ON CARNOT'8 



mental science enables us to do, a complete solu- 

 tion of the question. 



I. On the nature of Thermal agency, considered 

 as a motive power. 



4. There are [at present known] two, and only 

 two, distinct ways in which mechanical effect can 

 be obtained from heat. One of these is by means 

 of the alterations of volume, which bodies may ex- 

 perience through the action of heat ; the other is 

 through the medium of electric agency. Seebeck's 

 discovery of thermo-electric currents enables us at 

 present to conceive of an electro-magnetic engine 

 supplied from a thermal origin, being used as a 

 motive power ; but this discovery was not made 

 until 1821, and the subject of thermo-electricity 

 can only have been generally known in a few iso- 

 lated facts, with reference to the electrical effects 

 of heat upon certain crystals, at the time when 

 Carnot wrote. He makes no allusion to it, but 

 confines himself to the method for rendering 

 thermal agency available as a source of mechanical 

 effect, by means of the expansions and contrac- 

 tions of bodies. 



5. A body expanding or contracting under the 

 action of force may, in general, either produce 

 mechanical effect by overcoming resistance, or re- 

 ceive mechanical effect by yielding to the action 



