542 PROFESSOR WILLIAM THOMSON'S ACCOUNT OF 



(2.) How may the amount of this thermal agency necessary for performing 

 a given quantity of worli be estimated ? 



3. In the following paper I shall commence by giving a short abstract of the 

 reasoning by which Carnot is led to an answer to the fli'st of these questions : I 

 shall then explain the investigation by which, in accordance with his theory, 

 the experimental elements necessar}^ for answering the second question are indi- 

 cated; and, in conclusion, I shall state the data supplied by Regnault's recent 

 observations on steam, and apply them to obtain, as approximately as the pre- 

 sent state of experimental science enables us to do, a complete solution 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 experience 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 thi s 

 discovery was not made until 1821, and the subject of thermo-electricity can 

 only have been generally known in a few isolated 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 ex- 

 pansions and contractions of bodies. 



5. A body expanding or contracting imder the action of force, may, in gene- 

 ral, either produce mechanical effect by overcoming resistance, or receive mecha- 

 nical effect by yielding to the action of force. The amount of mechanical effect 

 thus developed will depend not only on the calorific agency concerned, but also 

 on the alteration in the physical condition of the body. Hence, after allowing the 

 volume and temperature of the body to change, Ave must restore it to its original 

 temperature and volume ; and then we may estimate the aggregate amount of 

 mechanical effect developed as due solely to the thermal origin. 



6. Now the ordinarily-received, and almost universally-acknowledged, prin- 

 ciples with reference to " quantities of caloric" and " latent heat," lead us to con- 

 ceive that, at the end of a cycle of operations, when a body is left in precisely its 

 primitive physical condition, if it has absorbed any heat during one part of the 

 operations, it must have given out again exactly the same amount during the 

 remainder of the cycle. The truth of tliis principle is considered as axiomatic by 

 Carnot, who admits it as the foundation of his theory ; and expresses himself in 

 the following terms regarding it, in a note on one of the passages of his treatise.* 



» Carnot, p. 37. 



