Fig. 19.—A WATER SPOUT.
679. Why does the chimney smoke when the fire is first lighted?
Because the air in the chimney is of the same temperature as that in the room, and therefore will not ascend.
680. Why does the smoking (into the room) cease, after the fire has been lighted a little while?
Because the air in the chimney, being warmed by the fire beneath, becomes lighter and ascends rapidly.
681. Why does a long chimney create a greater draught than a short one?
Because the short chimney contains less air than the long one; there is, consequently, less difference of weight between the warm air of the short chimney and the external air; it therefore has not so great an ascensive power.