" George Hutcheson, CIH (Certified Industrial Hygienist), John Deere Waterloo Works, points out that in the formula given "vapor pressure" should properly

   "refer to the partial pressure instead? The vapor pressure of a
    material depends primarily on the temperature and does not vary
    with the amount of a material present.  The partial pressure is
    the fraction of the total pressure due to a component and does
    vary with the amount of material present."

The following text is also unclear:

   "Although the R.H. factor varies with different kinds of organic
    fiber, it remains constant for the same kind of fiber. Since the
    moisture content of paper affects its weight and printing
    properties, its R.H. must be known. It may be measured by a
    sword hygrometer inserted in a pile of paper."

"R.H. factor" is not defined, but seems to refer to moisture content, probably equilibrium moisture content. While affected by R.H., moisture content is not the same as R.H., which the above text would suggest. [wh] (Thanks to Terry T. Schaeffer, Chemical Hygiene Officer, Los Angeles County Museum of Art, who provided thoughtful comments on these topics)