A. By convection, thus:—The sun heats the earth, and the earth heats the air resting upon it; the air thus heated rises, and is succeeded by other air, which is heated in a similar way, till all is warmed by “convective currents.”
Q. What is meant by “convective currents of air?”
A. Streams of air heated by the earth, which rise upwards and carry heat with them, are called “convective currents” of hot air.
Q. Is the air in a room in perpetual motion, as the air abroad is?
A. Yes; there are always two currents of air in the room we occupy, one of hot air flowing out of the room, and another of colder air flowing into the room.
Q. How do you know, that there are these two currents of air in every occupied room?
A. If I hold a lighted candle near the crevice at the top of the door, the flame will be blown outward (towards the hall); but if I hold the candle at the bottom of the door, the flame will be blown inwards (into the room).
Q. Why would the flame be blown outwards (towards the hall), if the candle were held at the top of the door?
A. Because as the air of the room is warmed by the fire, &c., it ascends; and (floating about the upper part of the room) some of it escapes through the crevice at the top of the door, and thus produces a current of air outwards (into the hall).
Q. Why would the flame be blown inwards (into the room), if the candle were held at the bottom of the door?