TRANSACTIONS OF SECTION G. 769 



ture and pressure of the steam }>egin to run down, and if we could continue to 

 expand indefinitely, the entire heat would be exhausted, and the energy previously 

 expended m separating the water into steam would be wliolly given up in external 

 effect ; but this exhaustion would not be complete until the absolute zero of tem- 

 perature was reached (viz., 461° below the zero of Fahrenheit). I do not mean 

 to say that an ideally perfect engine necessarily involves unlimited expansion, 

 seeing that if instead of discharging the steam at the end of a given expansion, we 

 made the engine itself do work in compressing it, we might under the conditions 

 of Carnot's reversible cycle, so justly celebrated as the foundation of the theory 

 of the steam engine, recommence the action with all the unutilised heat in an 

 available form. But an engine upon this principle could only give an amount of 

 useful effect corresponding to the difference between the whole work done by the 

 engine, and that very large portion of it expended in the operation of compres- 

 sion, and this difference viewed in relation to the necessary size of the engine, 

 woidd be quite insignificant, and would in fact be wholly swallowed up in friction. 

 Carnot did not intend to suggest a real engine, and his hypothesis therefore takes 

 no cognizance of losses incident to the application of an actual fire to an actual 

 boiler. His ideal engine is also supposed to be frictionless, and impervious to heat 

 except at the point where heat has to be transmitted to the water, and there the 

 condition of perfect conduction is assumed. In short an engine which would even 

 approximately conform to the conditions of Carnot's cycle is an impossibility, and 

 a perfect steam engine is alike phantom whether it be sought for in the cyclical 

 process of Carnot, or under the condition of indefinite expansion. Practically we 

 have to deal with a machine which, like all other machines, is subject to friction, 

 and m expanding the steam we quickly arrive at a point at which the reduced 

 pressure on the piston is so little in excess of the friction of the machine, as to 

 render the steam not worth retaining, and at this point we reject it. In figurative 

 language we take the cream off the bowl and throw away tlie milk. We do save 

 a little by heating the feed water, but this gain is very small in comparison with 

 the whole loss. What happens in the condenser is that all the remaining energy 

 which has taken the form of internal work is reconverted into heat, but it is heat 

 of 80 low a grade that we cannot apply it to the vaporization of water. But 

 although the heat is too low to vaporize water it ia not too low to vaporize ether. 

 If instead of condensing by the external application of water, we did so by the 

 similar application of ether, as proposed and practised by M. du Trembley twenty-five 

 years ago, the ether would be vaporized, and we should be able to start airesh with 

 high tension vapour, which in its turn would be expanded until the frictional limit 

 was again reached. At that point the ether would have to be condensed by the 

 outward application of cold water and pumped back, in the liquid state, to act over 

 agam in a similar manner. This method of working was extensively tried in 

 France when introduced by M. du Trembley and the results were sufficiently en- 

 couraging to justify a resumption of the trials at the present time, when they could 

 be made under much more favourable conditions. There was no question as to the 

 economy effected, but in the discussions which took place on the subject, it was 

 contended that equally good results might be attained bv improved applications of 

 the steam, without resorting to an additional medium. " The compound engine of 

 the present da,y does in fact equal the efficiency of Du Trembley's combined steam 

 and ether engine, but there is no reason why the ether apparatus should not confer 

 the same advantage on the modern engine that attended its application to the older 

 form. The objections to its use are purely of a practical natiu-e and might very 

 possibly yield to persevering efforts at removal. 



I need scarcely notice the advantage to be derived from increasing the initial 

 pressure of the steam so as to widen the range of expansion by raising the upper 

 limit of temperature instead of reducing the lower one. It must be remembered, 

 however, that an increase of temperature is attended with the serious drawback of 

 increasing the quantity of heat carried off by the gases from the fire, and also tlie 

 loss by radiation, so that we have not so much to gain by increase of pressure as ia 

 commonly imagined. 



But even supposing the steam engine to be improved to the utmost extent that 

 1881. 3 D 



