8 Mr. J. Gill on the Dynamical Theory of Heat. 



sequently, as regards tangible proof from direct experiment, M. 

 Hirn's earlier opinion that heat did not disappear from the 

 steam in the production of work may have been really in accord- 

 ance with facts as far as they were perceived, though at variance 

 with the whole truth ; and I would submit that the conclusions 

 arrived at from his more recent experiments, as above quoted, 

 though in accordance with truth, may be questioned as to their 

 satisfactory proof by experiment. In short, I now imagine that 

 our investigations on the subject by experiments on the actual 

 working of steam-engines have not been conducted on really 

 correct grounds, and that the results have therefore been ano- 

 malous. It is no doubt true that the heat furnished by the fire 

 is in all cases more than the heat which passes into the con- 

 denser; and if no heat were otherwise lost, this excess would 

 correspond to Joule's equivalent ; but when the engine works 

 with the full boiler-pressure throughout the whole stroke, it 

 seems evident that the whole work is in effect done in the boiler 

 by the expansion of the water into steam, and is merely trans- 

 mitted by the steam to the piston ; so that the heat equivalent 

 to the work done disappears from the boiler, and is locally made 

 up by the fire directly without any indication of these pheno- 

 mena being perceptible in the subsequent parts of the process 

 of working the engine. If this view of the subject is correct, 

 it is difficult to perceive how M. Hirn's deduction of an exact 

 proportionality between heat lost and work done could be fairly 

 proved from the experiments (though doubtless the law is true); 

 for with a cut-off at one-sixth of the stroke, about two-thirds of the 

 work would be done by the expansion of the isolated steam in the 

 cylinder, with a corresponding disappearance of heat in the con- 

 denser, which should be very perceptible ; while in proportion 

 as the engine worked with steam approaching to full pressure, 

 the proportion of work actually done by the steam in the 

 cylinder would become less and less, until the disappearance of 

 heat from the working steam might become inappreciable. 



In order to maintain the work of the engine constant with a 

 variable consumption of steam, throttling or wire-drawing was 

 used more as the expansion from cut-off became less ; and as 

 the process of wire-drawing increases the heat of the working 

 steam at the expense of the heat in the boiler, the quantity of heat 

 passing into the condenser would be affected also by this 

 cause. 



It is certain that the performance of work by the expansion of 

 isolated steam drawing solely on its own self-contained sources 

 of energy must cost to the steam a full equivalent of force in 

 some shape ; and as in this case no other form of force can be 

 directly detected except heat (or molecular motion, as it is now 



