24 



Report S.A.A. Advancement of Science. 



of the pressures of the separate gases ? Is the pressure of the aqueous 

 vapour in the air the same as it would be in an atmosphere of aqueous 

 vapour independently existing ? And, if all these things are provable 

 say under the receiver of an air pump, are they necessarily true at 

 large ? To all these relatively important side issues it is not necessary 

 to-day to volunteer a direct answer. We may confine ourselves to 

 the simple question (i) whether it is legitimate to subtract the 

 observed vapour tension' from the barometric pressure ; and (2), if so. 





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whether the remaining " pressure of dry air " maybe shewn to vary 

 throughout the day in some manner depending upon the temperature ? 

 The answer to the first part of the question is that it is not legitimate. 

 It has been shewn more than once that the pressure on the barometer 

 of the total quantity of aqueous vapour in the air is only perhaps a 

 quarter of what is called the vapour tension. INIoreover, it varies 

 considerably from place to place over areas throughout which the 

 total barometric pressure may not be much affected. And even if we 



