why is the mercury separated?

Air sometimes gets trapped in the tube during transit. This breaks the solid column of mercury and causes inaccurate readings. Restoring the column means tipping the case carefully to coax the air bubble out. A specialist handles this task best.

how do I hang a wheel barometer safely?

Most have a brass hanging loop at the top. Secure a sturdy hook into a wall stud or use a heavy masonry fixing. Keep them away from direct heat sources. Radiators can warp the carcase and throw off the thermometer readings.

I have heard you are supposed to tap the glass dial before taking a reading but I don't know if that actually does anything or if it just breaks the glass.

A light tap overcomes surface tension in the mercury tube. It frees the internal float to settle at the current atmospheric pressure without damaging the instrument.

What does the little dial right at the top of my barometer do, the one that has damp and dry written on it?

That is a hygrometer for measuring humidity. It relies on a piece of prepared oat beard that twists and untwists as moisture in the air changes. The twisting moves a small needle across the dial. They rarely still work after two hundred years, but they finish the instrument panel nicely.

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Antique Mahogany Barometers

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An antique mahogany barometer is a wall-mounted scientific instrument built to measure atmospheric pressure and forecast weather, pairing a glass mercury tube with a protective wooden case.