The "octogésimale" scale is different from the Fahrenheit scale. Fourier and de Saussure were both using the Reaumur temperature scale introduced by the French physicist René Antoine Réaumur (1683-1757) which had the same fixed points as the "centésimale" scale (centigrade or Celsius), but it was divided in 80 instead of 100 (freezing at 0 and boiling at 80; hence the name octogésimale).
So I think that clears it up rather nicely. Had I thought, I might have managed to guess that "octogesimale" meant eighty. But I didn't.
There is a wikipedia page about the Reaumur scale: http://en.wikipedia.org/wiki/Reaumur but of course that didn't exist when I first wrote the translation.