424 rorULAR LECTURES AND ADDRESSES. 



(that is 772 times the amount of work required to 

 overcome a force equal to the weight of one pound 

 through a space of one foot) is required to generate 

 as much heat as will raise the temperature of a 

 pound of water by one degree. 



The reverse transformation of heat into mechan- 

 ical work was next considered, and the working 

 of a steam-engine was referred to as an illustration. 

 An original model of Stirling's air-engine was 

 shown in operation, developing motive power from 

 heat supplied to it by a spirit lamp, by means 

 of the alternate contractions and expansions of one 

 mass of air. Thermo-electric currents, and com- 

 mon mechanical action produced by them, were 

 referred to as illustrating another very distinct 

 class of means by which the same transformation 

 may be effected. It was pointed out that in each 

 case, while heat is taken in by the material 

 arrangement or machine, from the source of heat, 

 heat is always given out in another locality, which 

 is at a lower temperature than the locality at 

 which heat is taken in. But it was remarked that 

 the quantity of heat given out is not (as Carnot 



