Theory of Heat to the Steam-engine, 243 



3. These two examples will suffice to show, that the principles 

 upon which our former theory of steam-engines was founded have 

 suffered such essential modifications through the mechanical 

 theory of heat, as to render a new investigation of the subject 

 necessary. 



In the present memoir I have attempted to develope the prin- 

 ciples of the calculation of the work of the steam-engine in 

 accordance with the mechanical theory of heat. I have, how- 

 ever, limited myself to the steam-engines now in use, without at 

 present entering into a consideration of the more recent and cer- 

 tainly very interesting attempts to employ vapour in a super- 

 heated state. 



In recording the results of my investigation, I shall only 

 assume my last memoir, " On a modified Form of the Second 

 Fundamental Theorem in the Mechanical Theory of Heat*,^' as 

 known. This will of course necessitate the deduction, in a 

 somewhat different manner, of results which are no longer new, 

 but have already been found by myself or others; I believe, 

 however, that this repetition, by leading to greater unity and 

 facility of comprehension, will not be found superfluous. At the 

 proper places I shall, to the best of my ability, cite the works 

 wherein these results first appeared. 



4. The expression " a machine is driven by heat^^ is not of 

 course strictly accurate. By it we must understand, that, in 

 consequence of the changes produced by heat upon some kind 

 of matter in the machine, the parts of the latter are set in motion. 

 We shall refer to this matter as that which manifests the action 

 of heat. 



If a continuously-acting machine is in uniform action, all 

 accompanying changes occur periodically, so that the condition 

 which at a given time prevails in the machine and all its parts 

 returns at equal intervals. Hence the matter which manifests 

 the action of the applied heat must at such regularly-recurring 

 periods be present in the machine in equal quantity, and in the 

 same state. This condition can be fulfilled in two different ways. 

 First. One and the same quantity of matter may always remain 

 in the machine, when the modifications which this matter suffers 

 during the action of the machine will be such, that at the end of 

 each period it will regain its original condition and recommence 

 the same cycle of changes. 



Secondly, The machine may always expel the matter which 

 served to produce the effect during a period, and in its place 

 receive from an external source just as much matter of the same 

 kind. 



5. The last method is the one usually employed in most 

 * Poggendorff's Annalen, vol. xciii. p. 481 ; and Phil. Mag. August 1856. 



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