8 Mr Parker, On Garnots principle [Oct. 31, 



This quantity, it is clear, has no connection with the fornoer 



quantity of work, f/ — U„- 6 (<^ — (h^ ; and it is also clear that 



6' — 6" 

 the efficiency of the Carnot's engine, — ^, — , which is independent 



of Z7j — C/g , is entirely different from the miscalled ' efficiency,' 



in the former case. In one case, it will be observed, the method 

 of obtaining work is entirely reversible ; in the other, partly re- 

 versible and partly irreversible. 



This example illustrates the well-known fact that it is more 

 profitable to give corn to horses than to use it as fuel in a steam- 

 engine. 



In applying Carnot's principle to the vegetable world, we 

 must again choose our system so that, while under consideration, 

 it neither gains nor loses matter. It must therefore contain, 

 besides the living vegetation, a sufficient quantity of air, car- 

 bonic acid, ammonia, water, soil, &c. Suppose now that the 

 only vegetation in the system at first is a twig ; then let it grow 

 into a tree ; and then let the whole tree, except a single twig, 

 be destroyed by fire. Suppose also, for simplicity, that the tem- 

 perature of the system is uniform and constantly equal to 6 

 during the growth of the tree, and that the pressure is uniform 

 and constant during the cycle. Then since water is one of the 

 products of the burning of wood, the air will contain more aqueous 

 vapour after the products of combustion are reduced to the 

 original temperature 6 than it did before the conflagration, un- 

 less it was saturated before the conflagration. It will therefore 

 be convenient to have the air saturated with aqueous vapour 

 and at the temperature 6 before the conflagration. The com- 

 plete cycle will then consist of the following operations at con- 

 stant pressure, at the end of each of which the temperature 

 is 6:— 



(1) The twig grows into a tree at the temperature 6. 



(2) The air is then saturated with aqueous vapour. 



(3) The conflagration takes place, and the products of com- 

 bustion are reduced to the original temperature 6. 



(4) The quantity of vapour in the air is made the same 

 as at the beginning of the cycle. 



In the third operation, a positive quantity of heat, q say, will 



be given out, and the corresponding value of S ^ may be written 

 — -^, , where > 6. If the air neither gains nor loses aqueous 



