Also
[35]
where
= specific volume of saturated steam (volume of 1 lb.) at a given temperature,
, Pressure 
, = specific volume of air at the same pressure and temperature
= density of saturated steam in lb. per cu. ft. at temperature 
But we have for air
[36]
Therefore
[37]
Hence, substituting in [34]
[38]
in. mercury =
lb. per sq. ft.
Hence
[39]