In a posting about Old Potrero Rye (in the Rye topic), Chuck Cowdery wrote, "...Certainly on the frontier, distillers struggled to make proof (i.e., 50% alc/vol). His high wines are like 140 proof. How many distillers in the 18th century could regularly achieve 140 proof?"
That brings up an intriguing point we tend to overlook, namely, that the trick to getting high proof isn't in "fir'in up thet thar still" but in keeping it from boiling. The idea is that, once the still reaches about 178-180, every degree hotter it gets, the LOWER the proof's going to be.
It's a little ironic that (at least before modern equipment) you could only get a good, safe, dependable fermentation in the cold months. If you could ferment in the summertime, you could probably get pretty near the right temperature for 140 proof whiskey by just letting the mash sit there in that big ol' all-copper still in the sunlight!