![]() In the early 19th-century calibrated hygrometers were coming into use, but they probably were too delicate to bang around in a Missouri River canoe. There were many well-known ways of watching changes in the air’s water vapor content a ball of wool that grew heavier in moist air had done the trick 300 years before for Leonardo da Vinci, no less. The saucer became an impromptu hygrometer, an instrument for measuring humidity. ![]() “I found by several experiments that a table spoon full of water exposed to the air in a saucer would avaporate in 36 hours,” wrote Lewis in his journal for. Well, how dry? What was the rate of evaporation? Captain Meriwether Lewis noticed cracks and shrinkage in his wooden sextant case, and he had trouble keeping his brass inkwell filled. To the Easterners of the Lewis and Clark Expedition, the air also felt remarkably dry. ![]() Afternoon temperatures were in the 80s, and at night one blanket was enough. In late May 1805, it was warm in what is today the state of Montana. ![]()
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