Specific Heat of Elastic Fluids. 475 



tity of heat possessed by the vapour at its entrance is, according 

 to my experiments,, about 653 units ; that which it retains at its 

 escape amounts to 637. According to the theory of which I am 

 speaking, the quantity of heat rendered available for mecha- 

 nical work would be 653 — 637=16 units, that is to say, only 

 ^\yth of the quantity of heat communicated to the boiler. In a 

 condensing engine, receiving vapour of maximum density under 

 a pressure of five atmospheres, and in which the condenser con- 

 stantly presents an elastic force of 55 millims. of mercury, the 

 quantity of heat of the vapour entering would be 653 units, and 

 that of the vapour at the moment of condensation, that is to say, 

 when it is lost for the mechanical action, is 619 units. The heat 

 rendered available would be 34 units, or little more than ^^^th 

 of the heat communicated to the boiler. 



A much larger fraction would be obtained for the heat ren- 

 dered available as mechanical work, either by overheating the 

 vapour before it entered the engine, or by lowering as much as 

 possible the temperature of the condenser. But this latter 

 method is difficult to realize in practice ; it would require, more- 

 over, a considerable increase in the quantity of cold water by 

 which the condensation is effected, which would waste the motive 

 power, and the water used to feed the boiler would not be very 

 warm. The same object might be more easily attained by sub- 

 jecting the vapour to a lower pressure, and then condensing this 

 vapour by injecting a very volatile liquid, such as chloroform or 

 aether. The heat possessed by the water-vapour at the moment 

 of this condensation, of which a very small quantity only could 

 be converted into mechanical work, passes into the volatile 

 liquid, which it converts into high-pressure vapour. By making 

 this vapour pass into a second engine, where it expands until its 

 elastic force is equal to that to which the injected water might 

 practically bring it in the condenser, one portion of the heat is 

 converted into motive work; and the theory based upon the 

 numerical data of my experiments shows that this quantity is 

 much greater than that which should be obtained by means of d, 

 more considerable pressure of the water-vapour in the first engine. 

 In this manner a perfect explanation may be given of the oeco- 

 nomic result obtained by employing two connected engines, the 

 one worked by water-vapour, the other by vapour of aether or 

 chloroform, and with which experiments have been made for 

 some time. 



In air-machines, where the motive force is produced by the 

 expansion of the air in the machine by heat, or by the increase 

 in its elastic force, the motive work produced at each stroke of 

 the piston should always be proportional to the difference be- 

 tween the quantities of heat possessed by the air on entering and 



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