May I ask the principle of this simple barometer? The picture shows a glass tube and a glass bottle. The main reason is that the upper end of the glass tube is not vacuum. Why can it be measured?

The working principle of this simple barometer is that a certain mass of gas is sealed in a bottle. When the external atmospheric pressure decreases, the air pressure in the bottle is high, and the red water in the bottle is pressed into the thin tube. The smaller the external atmospheric pressure, the higher the red water column in the thin tube. On the contrary, when the external atmospheric pressure increases, the red water column in the thin tube will drop.

The disadvantage of this simple barometer is that it is affected by temperature. When the temperature rises, the volume of enclosed gas in the bottle expands, the pressure increases, and the red water column in the thin tube rises. Therefore, if the temperature rises, it will be mistaken for a decrease in atmospheric pressure.

In fact, this simple device is similar to Galileo thermometer. It measures the temperature by using the principle that the enclosed gas in the bottle expands with heat and contracts with cold. When measuring temperature, it will also be affected by the change of atmospheric pressure.