You're expecting a high degree of accuracy from a barometer wouldn't you say?
It seems that we have a competition between the two world leading authorities in zeteticismical szienze.
The objective of this exciting competition is to create the sillyest experiment to prove the flatness of the globe.
Here is my experiment... With a pan a few inches deep, and perhaps an inch of water in it, place a level across the top. Once it is level, have two pieces of straight metal wire ready. Tape one piece of wire hanging straight down from the level at the edge of the inside of the pan until just touches the water. Tape the other piece in the middle straight down until it touches the water.
Mark the wires where they are even with the bottom of the level. Snip them off with wire cutters, and using a bathroom scale, weigh each piece. If Earth is flat, they'll weigh the same. If curved, the middle piece should be lighter as it's shorter.