76 THE SCIENTIFIC PAPERS OF 



In the discussion of the Paper 



"ON COMBINED STEAM," 

 By JOHN WITHERED, United States, 



MR. C. W. SIEMENS * said that he had, for nearly the last fifteen 

 years, occupied himself with this question. When he first turned 

 his attention to the subject, he endeavoured to ascertain the advan- 

 tage of superheating steam, and he made some rather elaborate 

 experiments to determine the rate of expansion. He found, that 

 Gay-Lussac's law of an uniform rate of expansion of elastic fluids 

 by heat, was not applicable, but that steam, near its point of 

 saturation, expanded in a greater ratio than afterwards. A curve 

 representing the rate of expansion was rounded at the beginning, 

 but gradually approached a straight line ; it was, in fact, a hyper- 

 bola, the asymptote of which ran parallel to a line representing 

 the uniform progression of expansion of air. So that at a tem- 

 perature considerably removed from the boiling point, air and 

 steam expanded, practically, at the same rate ; whereas at first, 

 steam expanded four times, or five times more than air, for the 

 same increase of temperature. He also found, that steam near 

 its point of saturation, possessed great capacity for heat ; and 

 therefore, by simply superheating steam, no great economy could 

 be produced. The results of recent practice showed, however, in 

 many instances, a very large saving, which might be ascribed to a 

 secondary cause. Steam, in expanding behind a working piston, 

 lost a portion of its heat, which was converted into mechanical 

 effect. This heat was entirely lost, and a portion of the steam 

 must condense, or form water. This water re-evaporated when 

 the pressure had become reduced by expansion, and cooled the 

 sides of the cylinder, thereby producing condensation in the fresh 

 steam from the boiler ; and thus an action was established on the 

 sides of the cylinder, (resembling that of a sponge, which, under 

 the influence of an alternating pressure, absorbed and emitted 

 water,) by which a certain amount of steam passed, at each stroke, 



* Excerpt Minutes of Proceedings of the Institution of Civil Engineers, Vol. 

 XIX. Session 1859-60, pp. 479-480. 



