A1A2N2N1. Again, during the third operation a certain amount of work is spent on the piston, which will be represented by the area A2A3N3N2; and lastly, during the fourth operation, work is spent in pushing the piston to an amount represented by the area A3AON3.
19. Hence the mechanical effect (represented by the area OAA A2N2) which was obtained during the first and second operations, exceeds the work (represented by N2A2A3AO) spent during the third and fourth, by an amount represented by the area of the quadrilateral figure AA1A2A3; and, consequently, it only remains for us to evaluate this area, that we may determine the total mechanical effect gained in a complete cycle of operations. Now, from experimental data, at present nearly complete, as will be explained below, we may determine the length of the line AA1 for the given temperature S, and a given absorption H, of heat, during the first operation; and the length of A2A3 for the given lower temperature T, and the evolution of the same quantity of heat during the fourth operation: and the curves A1PA2, A3P'A may be drawn as graphical representations of actual observations. The figure being thus constructed, its area may be measured, and we are, therefore, in possession of a graphical
