138 THE SCIENTIFIC PAPERS OF 



naturally induce priming, independently of the further circumstance 

 that the liberation of the steam from the water would no longer be 

 facilitated by the passage of bubbles of heated air through the mass 

 of the water. 



In reference to the other question why the mixture of steam 

 and air should do more duty in the engine than an equal volume of 

 steam alone at the same pressure he did not think this could be 

 because the air-compressing engine which was united with the 

 steam-engine was more economical than the steam-engine itself. 

 On the contrary, mere air-engines, wherever they had been tried, 

 had not given such favourable results as good steam-engines ; and 

 there were theoretical reasons why they could not give better results 

 than a proper steam-engine. It had however occurred to him as an 

 explanation which might be suggested of the economy found to 

 result from the use of mixed steam and air in the engine, that the 

 presence of air in the steam cylinder would certainly prevent the 

 condensation of steam against the sides of the cylinder : and the 

 loss from this cause he believed to be much greater than was usually 

 supposed. If the cylinder was uncovered, the loss by condensation 

 was very great, and was shown by the form of the indicator dia- 

 grams taken from such engines ; and even when the cylinder was 

 carefully clothed, there was a considerable condensation from the 

 high-pressure steam coming in contact with surfaces that had been 

 cooled by previous exposure to the condenser or to the exhaust steam 

 at atmospheric pressure. This condensation he believed took place 

 even when the steam cylinder was thoroughly steam-jacketed all 

 round and at the ends, although to a much less extent, and in that 

 case it arose from the slow conducting power of the thick metal sides 

 of the cylinder allowing the inner surface to continue considerably 

 cooler than the outer surface heated by the steam in the jacket ; 

 and though the steam thus condensed in the cylinder was afterwards 

 evaporated a \n in the latter part of the stroke, yet its mechanical 

 effect was aln ^<st entirely lost. The presence of air mixed with the 

 steam, it appeared to him, would have the effect of preventing 

 this condensation from taking place after the first moment of con- 

 tact of the steam with the cool metal, because at that moment the 

 condensation of the particles of steam which first touched the cool 

 metal would leave a lining of air in contact with the metal, consist- 

 ing of the portion of air that had been mixed with the steam before 



