HEAT. 55 



iron, and produce a power equal to that of a steam engine 

 supplied with an equal quantity of heat. 



Carnot, who wrote in 1824 an essay on the motive power 

 of heat, regarded the mechanical power produced by heat as 

 resulting from a transfer of heat from one point to another, 

 without any ultimate loss of heat. Thus, in the action of an 

 ordinary steam-engine, the heat from the furnace having 

 expanded the water of the boiler and raised the piston, a 

 mechanical motion is produced ; but this cannot be continued 

 without the removal of the heat, or the contraction of the 

 expanded water. This is done by the condenser, and the 

 piston descends. But then we have apparently transferred the 

 heat from the furnace to the condenser, and in the transfer 

 effected mechanical motion. 



Should the mechanical motion produced by heat be con- 

 sidered as the effect of a simple transference of heat from 

 one point to another, or as the result of a conversion of heat 

 into the mechanical force of which this motion is the evidence ? 

 This question leads to the following : does the heat which 

 generates the mechanical power return to the thermal machine 

 as heat, or is it conveyed away by the work performed ? 



If a definite quantity of air be heated it is expanded, and 

 by its expansion it cools or loses some of its power of commu- 

 nicating heat to neighbouring bodies. That which we should 

 have called heat if the expansion of the air had been pre- 

 vented, we call mechanical effect, or may regard as converted 

 into mechanical effect ceasing to be heat ; but, throwing out 

 of the question nervous sensation, this expansion or mecha- 

 nical effect is all the evidence we have of heat, for if the 

 air is allowed to expand freely, this expansion becomes the 

 index of the heat ; if the air be confined, the expansion of 

 the matter of the vessel confining it, or of the mercury of a 

 thermometer in contact with it, &c., are the indices of the 

 heat. 



If, again, the air which has been expanded be, by mecha- 

 nical pressure or by other means, restored to its original bulk, 

 it is capable of heating or expanding other substances to a 

 degree to which it would not be equal, if it had remained 

 in its expanded state. To produce continuous motion, or the. 



