CHAP, x.] COMBINED ENGINES. 503 



CHAPTER X. 



COMBINED ENGINES, HOT-AIR, AND GAS-ENGINES. 



I. COMBINED ENGINES. 



THE principles of the mechanical theory of heat show that the value 

 of a heat-engine, its effective power, or, which is the same thing, its 

 economical coefficient, depends, other things being the same, on the 

 difference of the extreme temperatures between which it works. It 

 is of little consequence from this point of view, whether one liquid or 

 another is employed to obtain the vapour whose elastic force is made 

 use of as prime mover. The quantity of heat expended being the same, 

 since it is this heat that is converted into work, the work done by the 

 engine remains the same. 



It may therefore be advantageous to employ a liquid which 

 vaporizes at a temperature below that at which water boils : sulphuric 

 ether, for example, boils at 37. The steam, which at its departure 

 from the cylinder passes on to be liquefied in the condenser, leaves 

 there a quantity of heat sufficient to vaporize ether. The vapour from 

 this latter liquid may then serve to drive a second engine annexed to 

 the first, and whose condenser may thus be kept at a lower temperature 

 than that of the steam condenser. This combination tends to increase 

 the difference of the extreme temperatures between which the elastic 

 fluid works, from its entry into the cylinder to its exit to the atmo- 

 sphere or its condensations. The quantity of heat converted into 

 mechanical work will thus be increased in the same proportion. 



Such is the principle on which several combined engines are made, 

 on which we will say a few words. 



A French engineer, M. du Trembley, invented and had constructed 

 in 1840 a combined engine for steam and ether vapour which was 



