287 



evaporateJ in unity of time gives the gross effect of the engine in 

 unity of time. The result affords the means of calculating all the 

 circumstances connected with the working of a steam-engine ac- 

 cording to the principle of the conservation of vis viva, or, in other 

 words, of the equality of power and effect, which regulates the 

 action of all machines that move with a uniform or periodical velo- 

 city. This principle was first applied to the steam*engine by the 

 Count de Pambour, and, accordingly, the formulae of this paper only 

 differ from those of his work in the expressions for the pressure and 

 expansive action of the steam, which are results peculiar to the au- 

 thor's theory. As an illustration of the use of the formulae, the 

 maximum useful effect of a double-acting Cornish engine is computed, 

 and compared with the result of the calculation of M. de Pambour 

 for the same engine, shewing the latter to be too large by about one- 

 fifteenth. 



In an Appendix are given two tables; one for calculating the vo- 

 lume of steam from its pressure, and vice versa, and its mechanical 

 action at full pressure, the other for computing the amount of its ac- 

 tion in expansive engines. 



In order to shew the limit of the possible eflFect from the expen- 

 diture of a given quantity of heat in evaporating water under given 

 circumstances, the maximum gross effect of unity of weight of steam, 

 evaporated at a higher temperature, and liquefied at a lower, is com- 

 puted in two examples, and compared with the heat which disappears 

 during the action of the steam, as calculated directly. In the first 

 example, the water is supposed to be evaporated at the pressure of 

 four atmospheres, and condensed at that of half an atmosphere ; in 

 the second, to be evaporated at eight atmospheres, and condensed at 

 one atmosphere. 



In both these examples, the direct calculation of the heat rendered 

 effective, agrees with the calculation from the power developed, 

 thus verifying the methods of computation founded on the author's 

 theory. 



The heat converted, in those examples, into engine-power amounts 

 to only about one-sixth part of the heat expended in evaporatino- the 

 water, the remainder being carried off by the steam and liquid water 

 which escape from the cylinder. In practice, the proportion of heat 

 rendered effective is still smaller, and in some unexpansive engines 

 amounts to only one twenty-fourth part, or even loss. It is thus 



