SIR WILLIAM SIEMENS, F.R.S. 73 



though the result had not proved favourable in that case, he thought 

 that the failure might be fully accounted for by the imperfect 

 way in which the experiment had been tried : any single cases 

 however of want of success could not be admitted, he considered, 

 as permanent objections to the introduction of a system. There 

 was much difficulty in making an experiment complete and 

 arriving at the correct result, and the value of the result depended 

 upon the mode in which it had been conducted and a perfect 

 knowledge of all the circumstances involved. Amongst the cir- 

 cumstances to be considered in any experiments on superheated 

 steam were the actual size and construction of the engine, whether 

 the cylinders had steam jackets or were well or imperfectly 

 clothed, the degree of exposure and length of the steam pipes, and 

 the construction, form, and size of the boilers : the particulars of 

 all these circumstances should be carefully noted in any experi- 

 ments to determine the practical value of superheating the steam. 

 The advantages to be attained by superheating, most of which 

 were referred to in the paper, were firstly, entirely preventing the 

 passage of priming water with the steam, by completely evaporating 

 this water in passing the steam through the superheating apparatus ; 

 secondly, obtaining a greater bulk of steam from the same water, 

 by its expansion with the increase of temperature ; and thirdly, 

 preventing any condensation of the steam by contact with the 

 cooler sides of the cylinder, whereby the steam could be kept in a 

 perfectly dry state throughout the entire stroke. The evaporation 

 of the priming water was a clear gain, as it was difficult if not 

 impossible to prevent priming altogether, and in many cases the 

 amount of priming was very considerable, causing serious risk of 

 injury to the engine as well as waste of heat. In regard to the 

 advantage to be obtained by adding heat to the steam to increase 

 its bulk, this involved the theoretical question of the rate of 

 expansion of steam by heat. It had been generally assumed that 

 isolated steam expanded under all circumstances at the same rate 

 as air, namely l-490th of its bulk at 32 for each degree Fahr. : but 

 he had shown by the results of experiments given in a paper at a 

 former meeting* (Proceedings Inst. M.E. 1852, page 131) that its 

 rate of expansion was considerably greater when near the con- 



* See p. 17, ante. 



