70 CORRELATION OF PHYSICAL FORCES. 



and the complex mechanical contrivances requisite to give 

 the measure of power yielded by matter in its less dilatable 

 forms, would be far beyond our present experimental re- 

 ources. It would also be difficult to prevent the interference 

 of molecular attractions, inertia, &c., the overcoming of 

 which expends a part of the mechanical power generated, 

 but which could hardly be made to appear in the result. 

 We could not, for instance, practically realise the above con- 

 ception by the construction of a machine which should act by 

 the expansion and contraction of a bar of 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 ex- 

 panded 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 ex- 

 panded 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 result ? 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 com- 

 municating heat to neighbouring bodies. That which we 

 should have called heat if the expansion of the air had been 



