I bought this old antique thing....What is Barometric Pressure Anyways?

Isn't it cool!  It is an old anchor!  And a barometer! 

Uhmmm, yeah, and what does it do . . . .  Exactly?
This is an "aneroid" barometer, as opposed to a mercury barometer.  It measures atmospheric pressure.  It did not come with any directions!!!

Ok, I have to admit that this post is mostly lifted from wikipedia, but the drama is to unfold: the needle is supposed to move from 29 up to 30 or down to 28, depending on the weather!
Reader beware: THIS POST IS PLAGERIZED FROM MANY SOURCES!

Average sea-level pressure is 101.325 kPa (1013.25 mbar, or hPa) or 29.921 inches of mercury (inHg) or 760 millimeters (mmHg).  Apparently, I am supposed to call the local radio station or weather station or coast guard and see what the local barometric pressure is, then set my barometer to that reading.  There is a screw on the back in order to do this.

It is all mechanical, so no batteries are needed. If the metal chamber cracks then the barometer will no longer work.  These barometers usually have words such as “Stormy” for low pressures (980 to 1000 hPa) and “Dry” for high pressures (1020 to1030 hPa). These words are at best only a first guide to the weather and date back to Vice-Admiral (Royal Navy) Robert Fitzroy (1805-65) who first visited New Zealand with Charles Darwin on the Beagle in 1835.

Will it rain? Tap the (aneroid) barometer!

It is OK to tap the outer glass of an aneroid barometer (firmly but lightly). By so doing you shift any recent pressure change stored in the mechanical linkage to the measuring needle. The resulting slight movement indicates whether the pressure is rising, steady, or falling. If the measuring needle goes to the left then there has been a drop in pressure recently.