108 
HAROLD’S DISCUSSIONS, 
One day I held a candle near the top of the door¬ 
way between a hot and a cold room; the dame was 
bent toward the cold room. The more I lowered it 
the less it was bent, until the flame was perfectly 
erect about the middle of the doorway. Below that 
the flame was bent inward toward the warm room. 
This proved to me that the warm air moved outward 
at the top of the door 
and the cold air inward 
at the bottom of the 
door. 
One day as I was 
watching a bonfire, I 
kicked into it some of 
the ashes of burnt pa¬ 
per that were about the 
edge. The heated air 
above the fire at once 
carried tliem up for 
some distance, and as 
there was no wind they 
gradual floated off to 
the side and down to 
fcu- , . ^ 
tne earth again. 1 per- 
Fig. 57.—Bonfire, showing circu¬ 
lating air currents. 
ceived immediately that 
the heat of the fire warmed the air and expanded it. 
and that the colder air around it pushed the lighter 
air upward. This in turn became heated and was 
pushed up in like manner by other cold air. Just 
above the fire there was a vertical circulation caused 
by the horizontal inflow of the cold air near the 
