of a Mixture of Air and Steam upon Cold Surfaces. 



73 



Ratio of the pressure of air to that of steam. 



14. Conclusions. — The conclusions to be drawn from these expe- 

 riments are as follows : — 



1. That a small quantity of air in steam does very much retard 

 its condensation upon a cold surface ; that, in fact, there is no 

 limit to the rate at which pure steam will condense but the power 

 of the surface to carry off the heat. 



2. That the rate of condensation diminishes rapidly and nearly 

 uniformly as the pressure of air increases from two to ten per cent. 

 that of the steam, and then less and less rapidly until thirty per 

 cent, is reached, after which the rate of condensation remains nearly 

 constant. 



3. That in consequence of this effect of air the necessary size of a 

 surface-condenser for a steam-engine increases very rapidly with 

 the quantity of air allowed to be present within it. 



4. That by mixing air with the steam before it is used, the con- 

 densation at the surface of a cylinder may be greatly diminished, 

 and consequently the efficiency of the engine increased. 



5. That the maximum effect, or nearly so, will be obtained when 

 the pressure of the air is one tenth that of the steam, or when 

 about two cubic feet of air at the pressure of the atmosphere and 

 the temperature 60° R are mixed with each pound of steam. 



15. Remarks. As this investigation was nearly completed my 



