This experiment is even more important than the preceding one, and you
should by all means do it, for it is going to prove more conclusively
than anything else what causes the wind, and in miniature it is a real
storm.
Place a little alcohol lamp on the table, or a wax candle will do. Over
this place an ordinary lamp chimney, lifting it a short distance off the
table, and it can be held in position by any little object. (See Fig.
10.) Over the chimney hold some smoke paper. (Smoke paper is nothing
more than filter paper, or brown wrapping paper of a soft texture.) From
the experiments already visualized to you, you should know what to
expect. You will again see that the heated air is rising; it has
expanded and become light. Now what becomes of the air that is rising
and where does it go? In doing this experiment be careful not to make
any unnatural movements that will change the current of wind. Stand
perfectly still so that the experiment will be perfect, because you are
now producing in miniature a real storm, or demonstrating the cause of
wind.
The next observation—what happens at the bottom of the chimney? Here you
will find the outside air is coming in, the same as it did in the window
experiment. Particularly, notice however, that the smoke enters
underneath the chimney from all directions, and the smoke paper should
be moved away from the glass chimney to determine the distance at which
the smoke flames will still be drawn into the chimney.
You now produced for yourself in miniature a storm and wind. The air
that has been heated rises over a heated area, and cooler air from all
directions around is passing into the space underneath the chimney and
taking the place of the heated air that has gone up. This experiment
illustrates what takes place, except on a smaller scale, out in the
atmosphere when a portion of the earth becomes heated. If this is clear
to you, it will help you to understand the main principles underlying
storms and winds, which will be given later on.