A. The heat of the air passing through the glass tube into the mercury, causes it to expand more or less, and rise in the tube accordingly.
Q. Why is the tube of a barometer left open?
A. That the air may press upon it freely; and as this pressure is more or less, the mercury rises or falls in the tube.
Q. How can weather be affected by the weight of the air?
A. When air is warm or moist, it is lighter than usual:
When it is cold or dry, it is heavier: and as a barometer marks whether the air be light or heavy, it indicates these changes.
Q. How can you tell (by looking at a barometer) what kind of weather it will be?
A. Because the mercury in the tube rises and falls, as the air becomes lighter or heavier: and we can generally tell by the weight of the air, what kind of weather to expect.
Q. Does the weight of the air vary much?