542 PROFESSOR WILLIAM THOMSON’S ACCOUNT OF 
(2.) How may the amount of this thermal agency necessary for performing 
a given quantity of work 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 first of these questions; I 
shall then explain the investigation by which, in accordance with his theory, 
the experimental elements necessary for answering the second question are indi- 
cated; and, in conclusion, I shall state the data supplied by ReaNnautt’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. SrEBEcK’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 isolated facts, with reference to the 
electrical effects of heat upon certain crystals, at the time when Carnor 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 under the action of force, may, m 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, we 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 this principle is considered as axiomatic by 
CaRNOoT, 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. 


