Actually it is not exactly a barometer - it is a hygrometer, but anyway it can help to predict the weather that is coming. Apart from other designs it is the simplest one. It is very easy to build this device, so this is instruction how to build it:
Cut off a branch of a pine or fur tree, 10 cm long. Cut off all needles, except one long needle at the side of the branch (see Figure). Take a plank with size of 16x11 cm, and fasten the branch to the plank with nails. Be sure that the needle can move freely in vertical direction. The "barometer" is ready to use.
Now we need to calibrate this "barometer". To do this, we need a heat source - a stove or a fire. Bring the "barometer" close to the heat source - the needle will straighten out and rise up as well because of the heat. Mark a place where the needle stopped. Now we need a boiling kettle. Bring the device close to the steam - the needle will lower down. Make here another mark. Connect this marks with an arc and divide the arc into some equal parts and write the text as shown in the Figure.
Put this barometer in a place with no direct sunlight. The position of the needle will predict the weather. That's all!