Pascal · Puy de Dôme experiment
Does air weigh less up a mountain?
That time, that place
When news of Torricelli reached France, scholars split. One camp said the weight of air presses down; the other still said nature abhors a vacuum.
Both accounts explained mercury stopping at seventy-six centimeters. So the dispute would not resolve.
Why this question
What Pascal found was the place where the two accounts disagree.
If it is the weight of air, then climbing a mountain leaves less air overhead and the mercury column must fall. If it is nature's abhorrence, height should make no difference at all.
He had located the point where the predictions part.
What was found
He asked his brother-in-law Périer to run the experiment on the Puy de Dôme.
The design was excellent. Two identical instruments: one left at a monastery at the foot of the mountain with someone watching it all day, the other carried to the summit.
At about a thousand meters the mercury had dropped more than eight centimeters, and it rose again on the way down — while the instrument below had not budged all day.
So the change came from altitude, not from weather or the hour. Predict first, design the experiment where predictions diverge, and keep a control to compare against.
The abhorrence of a vacuum was an equally plausible account
Eight centimeters of drop at the summit, against a control below that never moved
The unit of pressure carries his name