Barometer Calibration?

I recieved a barometer as a gift a few years ago. It came with no instruction manuals, or anything suggesting how to set it up. It has two hands - a large black one, and a brass one that I can move using a knob on the front. There are no buttons, switches, dials, etc. anywhere on it except for that. So, my question is - how do I go about calibrating this thing? Does it even need to be calibrated, and if so, what am I supposed to do with the knob on the front?

You set the adjustable arm to match the position of the barometer. That way you can tell at a glance if the barometer has risen or fallen since last you checked.

Check carefully- there is often a small hole in the middle of the back for a watchmaker’s screwddriver. Turn this when you have very settled high pressure over your area and thus calibrate the barometer. If there is no calibration like this, take a reading during a spell of high pressure and then add/subtract the difference to get a true reading for any future reading.

In fact, it is changes in pressure that indicate weather patterns, not especially the exact pressure at an time.