24 THE SCIENTIFIC PAPERS OF 



The present paper will be confined to showing the effect of the 

 above experiments upon the rate of expansion of steam within the 

 steam cylinder of an engine. It was demonstrated by the first- 

 named experiments, that expanded steam is super-heated steam; 

 and, by the second, it is shown what is the expansion of bulk due 

 to an increase of temperature. 



Supposing the results of the experiments to be correct, the 

 expansion curve as laid down by Tambour, and which is based 

 upon Watt's law, requires a modification due to the excess of 

 temperature in expanded steam ; and it will be observed that this 

 correction in the curve of expansion is in favour of working 

 engines expansively, as a greater average pressure is obtained 

 during expansion than would be the case if the expanded steam 

 were not thus super-heated. Its correctness is corroborated by 

 some actual observations by Mr. Edward A. Cowper in taking 

 diagrams of expansive engines, previous to his acquaintance with 

 the above experiments. It moreover appears that in Cornwall, 

 engineers have been practically acquainted with the fact, that 

 expanded steam is super-heated steam, and more economic in its 

 use than saturated steam ; for it is a practice with them to 

 generate the steam at very high pressure, and to expand it down to 

 the required pressure previous to its reaching the steam cylinder. 



Another remarkable practical observation is, that a jet of high- 

 pressure steam does not scald the naked hand, while a jet of low- 

 pressure steam does, although the high-pressure steam is the hotter 

 substance. The cooling effect of a jet of high-pressure steam is so 

 powerful, that, as the' author has been informed, ice has been 

 actually produced in the heat of summer in America, by blowing a 

 powerful jet of steam of 400 Ibs. pressure per square inch against 

 a damp cloth. This phenomenon may be explained by the perfectly 

 dry and under-saturated state of expanded steam, which, with a 

 strong tendency to re-saturate itself, produces a powerful evapora- 

 tion on moist surfaces with which it comes in contact. 



The rapid rate of expansion of steam by heat, when still near 

 its boiling point, proves the economy of heating the steam 

 cylinder either by a steam jacket, or by the application of fire. 

 It is, however, important to observe, that the specific heat of 

 steam seems to diminish, the more the temperature exceeds the 

 boiling point. The annexed table of observations gives the data 

 from which the curve A in Fig. 3 has been drawn. 



