The weatherglass is a very simple yet effective indicator of changes in barometric pressure. It consists of a blown glass vessel with a narrow spout, with water inside separating the air inside the sealed chamber from the atmosphere outside.
Here's an ASCII art version of a typical weatherglass:
( ) bend- / _
/ \ /#/
/ \ /#/
| | |#| not quite to scale
| | |#|
\##########/ # = water
To use a weatherglass, one fills the vessel with water. This usually requires holding it at various odd angles under a slow stream of water from a faucet. The spout is only about 3/16 inch in diameter, whereas the rest of the container is larger (on mine, it's about the size of a small papaya).
The ratio of the small surface area in the spout to the large one in the container allows a minor change in the volume of air within (due to changing pressure) to cause a perceptible movement of the water level.
Generally, a decrease in air pressure indicates bad weather approaching. This will be indicated by the water level in the spout rising. The water may even overflow. Local storms tend to cause a rapid drop in air pressure, which usually does not bring the water past the bend at the end of the spout. The approach of a large front will often bring it all the way up. Nearby tornadoes will cause water to drip rapidly down the outside of the spout and off the ball at the bottom - it might be a good idea to seek shelter at this point...
Rising air pressure usually indicates improving weather conditions. The water will lower in the spout. Mine usually settles about one and a half inches below the bend in the spout, but this will depend on the design (and just how much water spills out from approaching storms).
Although it cannot be calibrated to read in millibars or inches of mercury, the weatherglass does a very good job of indicating the changes in air pressure that precede storms and fronts.