Manchester Memoirs, Vol. xli. (1896), No. 3. 3 



and weight, a great deal more steam is often generated in 

 proportion to the space than was the case in the experi- 

 ments. Also the velocity of the steam after entering the 

 steam pipes is, in practice, often so great that, even where 

 these are ascending, any water that may have been drawn 

 in with the steam, or produced by condensation owing to 

 the radiation of heat from the pipes, is swept along with 

 the steam ; and where, as in cases like the locomotive, 

 the engine is under the boiler, so that the pipes are 

 descending, this must be so. Under such conditions the 

 steam as it enters the boiler will be accompanied by some 

 water, and is then variously called " wet steam," " nearly 

 dry steam," or "super-saturated" steam, though the last 

 name is apparently intended to imply that, notwith- 

 standing Regnault's experiments, the density of steam 

 after drainage is not necessarily a definite function of the 

 temperature or pressure. 



Whatever may be the cause of the water entering 

 the engine with the steam, its presence in unknown 

 quantity prevents Regnault's formula for the total heat 

 of evaporation from being used to form a correct estimate 

 of the quantity of heat received by the engine. For the 

 only measures of the steam supplied to the engine are 

 obtained from the measures of the feed-water supplied 

 to the boiler or the water discharged from a surface 

 condenser, so that, if an unknown quantity of water 

 enters with the steam, estimates so formed must be in 

 excess. 



This is a matter of very serious consideration in all 

 attempts to obtain a comparison of the actual performance 

 of an engine in work done as compared with the theoretical 

 performance under ideal conditions. And, as the modern 

 practice of steam engineering is largely guided by the 

 results of such attempts, methods of assuring dry steam 

 or, failing that, of in some way measuring the percentage 



