I get an impression from this enyclopedia entry
that the primary source of the Boltzmann entropy equation $S = k \log W$ might be
1866, Über die Mechanische Bedeutung des Zweiten Hauptsatzes der Wärmetheorie, Wiener Berichte, 53: 195–220; in WA I, paper 2.
I cannot read German. The only PDF I was able to obtain did not have a text-layer, so I could not copy-paste parts into Google Translate. Eventually I might go through and hand-type the whole thing into Google translate, but in the interim I would appreciate assistance expediting my search.
What is the original source of the Boltzmann entropy equation?