Re: "How to use a barometer to measure the height of a building."
4) (with stopwatch) drop barometer - Newton is your friend.
A barometer's rate of fall is too prone to influence from air resistance and updrafts. There are two objects which can be used reliably:
A) A steel girder (dropped pointy end first, naturally)
B) A human body (everyone knows the terminal velocity of an unconscious person is 120mph)
Whichever one you choose, be careful where you're standing with the stopwatch. Also, if you choose B, make sure no-one ever knows you were on the roof.