The normal section of the cone at that point is equal to dS cosO, and the solid angle dw is equal to dS cos0/x 2.
He therefore employed the corresponding expression for a cycle of infinitesimal range dt at the temperature t in which the work dW obtainable from a quantity of heat H would be represented by the equation dW =HF'(t)dt, where F'(t) is the derived function of F(t), or dF(t)/dt, and represents the work obtainable per unit of heat per degree fall of temperature at a temperature t.
If dW is the external work done, dH the heat absorbed from external sources, and dE the increase of intrinsic energy, we have in all cases by the first law, dH-dE=dW.
Since Od4 cannot be less than dH, the difference (61d4-dE) cannot be less than dW.
The condition in this form can be readily applied provided that the external work dW can be measured.