A. Because the cold air (condensed in the upper part of the flue), will sink from its own weight, and sweep the ascending smoke back with it into the room.
Q. What is the use of a cowl upon a chimney-pot?
A. The cowl acts as a screen against the wind, to prevent it from blowing into the chimney.
Q. What harm would the wind do, if it were to blow into a chimney?
A. 1st—It would prevent the smoke from getting out: and
2ndly—The cold air (introduced into the chimney by the wind) would fall down the flue, and drive the smoke with it back into the room.
Q. Why does a smoke-jack turn round in a chimney?
A. The current of hot air up the chimney, striking against the oblique vanes of the smoke-jack, drives them round and round; in the same way as the sails of a wind-mill are driven round by the wind.
Q. Why are some things solid, others liquid, and others gaseous?
A. As heat enters any substance, it drives its particles further asunder; and a solid (like ice) becomes a liquid; and a liquid (like water) becomes a gas.