318 Royal Institution. 



and when the piston has returned to its first position, the original 

 50° will remain as at first. Suppose this experiment repeated up 

 to the rise of the piston, but when the piston is at its full elevation, 

 and the cold body is applied, let the weight be removed, so as to drop 

 upon a wheel, or to be used for other mechanical purposes, the de- 

 scending piston Mull not now reach its original point without more 

 heat being abstracted ; from the removal of the weight there will 

 not be the same force to restore the 1°, and the temperature will be 

 49°, or some fraction short of the original 50° ; if this were other- 

 wise, then as the ball in falling may be made to produce heat by fric- 

 tion, we should have more heat than at first, or a creation of heat 

 out of nothing; in other words, perpetual motion. When force is 

 abstracted from a thermal machine we ought to lose heat. 



If we suppose that the degrees of heat at the lower temperature 

 represent the same amount of force as the same number of degrees 

 at the higher temperature ; if, for instance, we suppose that a body 

 cooling from 120° to 100°, gives off the same force as a body cool- 

 ing from 20° to zero ; this seems to be tacitly assumed by Carnot, 

 but is probably not correct, the results of high-pressure steam and 

 other facts indicating a contrary conclusion. If then the 20° on 

 the lower scale do not represent an equivalent force to the 20° on 

 the higher, we 7nay gain the same heat in degrees in the condenser 

 as was lost from the furnace, and yet get derived power. There is 

 frequently a confusion between the work performed which returns to 

 the machine, and the derived work, or that which does not return, 

 and is used for other purposes. This is puzzling to the reader of 

 treatises on the steam-engine, and kindred subjects, and has led to 

 much obscurity of thought and expression. 



M. Seguin, in 1839, contraverted the position that derived power 

 could be got by the mere transfer of heat, and by calculation from 

 certain known data, such as the law of Mariotte, viz. that the elastic 

 force of gases and vapours increased directly with the pressure, and 

 assuming that for vapour between 100° and 150° Centigrade each 

 degree of elevation of temperature was produced by a thermal unit, 

 he deduced the equivalent of mechanical work capable of being 

 performed by a given decrement of heat ; and thus concluded that 

 for ordinary pressures about one gramme of water losing one degree 

 Centigrade would produce a force capable of raising a weight of 500 

 grammes through a space of one metre ; this estimate is a little 

 beyond that given by the more recent experiments of Mr. Joule. 

 M. Seguin has, however, since the accurate and elaborate experi- 

 ments of M. Regnault, necessarily varied his estimate, as by these 

 experiments it appears that, within certain limits, for elevating the 

 temperature of compressed vapour by one degree, no more than 

 about -^ths of a degree of total heat is required ; consequently, the 

 equivalent multiplied in this ratio would be 1666 grammes, instead 

 of 500. Other investigators have given numbers more or less dis- 

 cordant, so that without giving any opinion on their different 

 results, this question may be considered at present far from settled. 

 M. Regnault himself does not give the law by which the ratio of 



