fttESIDfiKTtAL AbDRESS. 81 f 



Tills definitidh leads to an absolute scale of temperature which Is independent} 

 of the substance operated on, and Joule and Thomson's experiments have shown 

 that this scale differs but slightly from that of the ordinary air thermometer. 

 Joule had suggested to Thomson, in a letter to him in 1848, that tiie probable 

 value of Carnot's function is the reciprocal of the absolute temperature as measured 

 on a perfect gas thermometer. 



Thus Clausius appears to have anticipated Thomson, not in the suggestion of 

 an absolute scale o'temperatiuv, but in the idea of an absolute zero founded upon 

 the combination of Carnot's law and Joule's law. Thomson, in his papers, verv 

 modestly attributes the second law — the law of the transformation of heat— to 

 Carnot and Clausius ; but in this he undervalued his work, because Clausius 

 appears to have assumed what Thomson and Joule proved ; that is, the coinci- 

 dence of the absolute scale with the air thermometer scale. 



It will thus be seen that the position usually assumed by the engineer at 1850, 

 of the equality between heat given to the engine and heat given to the condenser, 

 was fundamentally untrue. Without this deduction, however, no determination 

 of values of the Carnot function could have led to the determination of an absolute 

 zero. According to the material theory, as seen in the light of Carnot's cycle, a 

 heat unit could give an indefinitely increased amount of work with lowering of 

 the temperature. Nothing in the theory sets a limit to this increase, and, accord- 

 ingly, there is nothing to suggest an absolute zero. Immediately, however, we 

 accept the dynamical theory of heat we find that a pound of water requires the 

 exertion of 1 ,390 foot-pounds of work to heat it through 1° C. We also know from 

 the Carnot cycle that under ordinary conditions of human existence only a portion 

 of this work can be returned; but as no conditions could conceivably exist in 

 wliich a greater amount of work could be obtained from a pound of water than the 

 1,390 foot-pounds put into it to heat it through 1° C, it follows that, inasmuch as 

 the Carnot function increases with diminishing temperature, the limit of tem- 

 perature is reached when, according to the Carnot cycle, the whole of that work, 

 put into the pound of water, can be got out again as work. This limit is the 

 absolute zero of temperature. No lower temperature is conceivable without in- 

 troducing the idea of the creation of energy. So far as human beings are con- 

 cerned, this idea is as inconceivable as the idea of the creat ion of matter. Tlio 

 determination of this limit with the close accuracy necessary for a well-founded 

 constant is to be entirely attributed to Thomson and Joule. In his 1 851 paper 

 Thomson thus succeeds in answering the questions which he put to himself in 

 his 1849 paper, and he supplies a quantitative method of connecting the amount 

 of the thermal agency necessary with the amount of work which can be performed 

 under varying conditions. 



Engineers dealing with motive power are thus deeply in debt to Thomson and 

 Joule for the secure position occupied by them to-day. 



The brilliant work of Meyer, published so early as 1842, is held by some to 

 have anticipated to a large extent both the work of Thomson and of Joule. 

 Undoubtedly Meyer formulated true ideas and carried his generalisations through 

 a wide range. Helmholtz also very early arrived at similar conclusions to those 

 of Joule and Thomson; but it has been thought better to discuss the work of 

 Thomson and Joule separately, in order to illustrate the transition period through 

 which many distinguished minds were passing about the time. Undoubtedly 

 great credit is due to Meyer, Helmholtz, Clausius, and Hirn, and Thomson himself 

 recognised this in the most generous way. 



The ideas of Thomson and Joule now form so much of the basis of all 

 reasoning upon motive-power engines that there is some little danger to the 

 present generation of forgetting what they owe to these two great men. To 

 appreciate the step made by them it Is necessary to consider the position of motivo 

 power produced by heat at about the middle of the last century. At that time 

 many attempts had been made to displace the steam engine as a heat engine bv 

 air engines in various forms— both engines heated externally and those heate'd 

 internally, now known as internal-combustion engines. Papers read at the 

 Institution of Civil Engineers in 1845 and 1853, and the discussion of tho4 

 1908. 3g ^ 



