Question

When air pressure decreases, the mercury in a barometer _____.

Answer 1

It goes down.

The level of mercury stops at 7.6 cm because that's where the both pressure come in equillibrium, so if atmospheric pressure decreased, less air particles will force the mercury in the container, and thus the mercury will slide down into the container from the barometer.

Answer 2

When air pressure decreases, the mercury in a barometer goes down.