Here are some pictures of an old mechanical barometer.

The drum is driven by a clockwork mechanism. The silver circular object is a pressure sensor made by evacuating the air from inside the metal bellows.

The ink arm shown in front was connect via a series of levers to the pressure bellows so that as air pressure changed the arm moved up and down against the paper on the drum. The tip of the pen has a hollow section that held a small amount of ink.