A. Because after the warm air of the room has ascended to the ceiling, or made its escape into the hall, &c., a partial vacuum is made at the bottom of the room; and cold air (from the hall) rushes under the door to supply the void.
Q. What is meant by a “partial vacuum being made, at the bottom of the room?”
A. A vacuum means a place from which the air has been taken: and a “partial vacuum” means, a place from which a part of its air has been taken away. Thus when the air on the floor ascends to the ceiling, a partial vacuum is made on the floor.
Q. And how is the vacuum filled UP again?
A. It is filled up by colder air, which rushes (under the door, and through the window crevices) into the room.
Q. Give me an illustration.
A. If I dip a pail into a pond and fill it with water, a hole (or vacuum) is made in the pond as big as the pail; but the moment I draw the pail out, the hole is filled up by the water around.
Q. Show how this illustration applies.
A. The heated air which ascends from the bottom of a room, is as much taken away, as the water in the pail; and (as the void was instantly supplied by other water in the pond) so the void of air is supplied by a current from without.
Q. What is the cause of wind?