Q. Why does vapour in the air make the mercury sink?
A. Because vaporized air is lighter than dry air, and therefore its pressure is less on the mercury of the barometer.
Q. What is the 10TH SPECIAL RULE in regard to the barometer?
A. If (in frosty weather) it begins to snow, the barometer generally rises to 32°, where it remains as long as the snow continues to fall; if, after this, the weather clear up, you may expect very severe cold.
Q. How can you know if the mercury of the barometer be rising?
A. If it be convex (i. e. higher in the middle than at the sides;) it is in a rising state.
Q. How can you tell if the mercury of the barometer be about to fall?
A. If it be concave (i. e. hollow in the middle) it is in a falling state.
Q. Why is the mercury convex when it is rising?