during its Eocpamion under different circumstances. 403 



after the completion of work, if we assume that the vapour, as 

 long as it remains in connexion with the boiler, is completely 

 gaseous and at the same time completely saturated. In engines 

 where the expansive principle is applied, the first case becomes 

 applicable from the moment when the steam is shut oiF and the 

 piston is driven by expansion alone. Strictly speaking, the case 

 applies to those engines only in which the expansion continues 

 until the pressure within is equal to that of the atmosphere ; and 

 even here the correspondence would not be perfectly exact, inas- 

 much as the heat developed by the friction of the piston must 

 certainly be considerable*. 



We will finally apply ourselves to the 

 consideration of the third case, that is 

 to say, to the case to which the remark 

 of Mr. Thomson refers. Let the vessel 

 ABCD (see the accompanying figure) 

 be supposed to be filled with water to 

 EF, and from here upwards to be tilled 

 with steam. Let GJ be the orifice, 

 connected with which is a neck GJKM, 

 which Avidens slowly and renders the 

 expansion of the vapour more regular. 

 This is not essential, but is merely as- 

 sumed to render the conception of the 

 matter easier. By the application of 

 a proper source of heat, let the water be preserved at the con- 

 stant temperature t, so that the vapom* which escapes shall be 

 continually replaced by the newly developed, the state of things 

 as regards the issue of the vapour being thus preserved stationary. 



Let GH J represent a sui-face in which the vapour which passes 

 has throughout the expansive force p^, the temperature ^,, and 

 the volume r„ which exist within the vessel, and with which the 

 new vapour is developed. Let KLM, on the other hand, repi'e- 

 sent a surface in which the vapour which passes has throughout 

 the expansive force pc^, that is, of one atmosphere, the vapour 

 being supposed to be unmixed with air. During the passage of 

 the vapour from GHJ to KLM let heat be continually with- 



* Remark. — In connexion with the proposition which a])])lics to the first 

 ca.se, I cited in my former memoir the experiment matle Ijv Paml)our with 

 the steam procee(Un<r from a high-pressure engine after tlie completion of 

 work. I (leemed it sufficient to notice the fact, that Pambour did not find 

 a higlicr temperature than that which corresponded to tlie ju-cssure observed 

 at the same time, although according to the common theory lie nnist have 

 done so. To remiiro from such observations that they shall exhibit the 

 exact quantity or water mixed with tlie vapour which the theoiy gives, 

 would, for the reasons given above, and on account of many other simulta- 

 neous causes of disturbance, be uujustifialjle. 



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