In “Verhandel over de Nederduitsche spelling” (1805) in certain words the “s” sound is represented by an “f” character. Was there a spelling rule for this?

When comparing different spellings of the Dutch language, the document “Trade on the Nederduitsche spelling to promote unanimity in the same” (1805 by Matthias Siegenbeek) dropped the use of the symbol “f” ,in places where an “s” is expected becomes, on.

This is how the word “schoone” was written as “fschoone” (p3)

The text of the document can be consulted at the web addresses below.

http://www.archive.org/stream/verhandelove00repugoog

http://books.google.com/books?id=ideHBr9zNJIC&printsec=titlepage&source=gbs_summary_r&cad=0

In the other documents I examined, I did not come across this fact anywhere.