A. Because the heated air is blown more rapidly away; in consequence of which, cold air more rapidly succeeds to absorb heat from the surface of the tea or broth.
Q. If a shutter be closed in the day-time, the stream of light (piercing through the crevice) seems in constant agitation. Why is this?
A. The air (in the sun-beam piercing through the shutter-crevice) is more heated, than that in its neighbourhood; the convective current, therefore, is distinctly seen, where little motes and particles of dust are thrown into agitation by the violence of the current.
Q. Why is the gallery of a church or theatre hotter than the aisle or pit?
A. The hot air ascends from the bottom to the top of the room, and cold air (from the doors and windows) flies to the bottom to supply its place.
Q. Why does a crowded room produce head-ache?
A. Because we breathe air vitiated by the crowd.
Q. How does a crowd vitiate the air of a room?
A. Whenever we breathe, the elements of the air are separated in the lungs, some of the oxygen is absorbed by the blood, and some of it is converted into carbonic acid gas, and exhaled with the nitrogen.