1856.] the Negation of Perpetual Motion. 155 



by expansion ; 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 descending piston will 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 otherwise, then as the ball in falling may 

 be made to produce heat by friction, we should have more heat than 

 at first, or a creation of heat out of nothing, in other words, per- 

 petual motion. 



When force is abstracted from a thermal machine we ought to 

 lose heat, if we suppose degrees of heat at a lower temperature to 

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

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

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

 cooling 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 may 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, controverted 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 i^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- 



