You may see this by taking a
lighted candle, and putting it in the sun so as to get its shadow thrown
on a piece of paper. How remarkable it is that that thing which is light
enough to produce shadows of other objects, can be made to throw its own
shadow on a piece of white paper or card, so that you can actually see
streaming round the flame something which is not part of the flame, but is
ascending and drawing the flame upwards. Now, I am going to imitate the
sunlight, by applying the voltaic battery to the electric lamp. You now
see our sun, and its great luminosity; and by placing a candle between it
and the screen, we get the shadow of the flame.
[Illustration: Fig. 4.]
You observe the shadow of the candle and of the wick; then there is a
darkish part, as represented in the diagram, and then a part which is more
distinct. Curiously enough, however, what we see in the shadow as the
darkest part of the flame is, in reality, the brightest part; and here you
see streaming upwards the ascending current of hot air, as shewn by
Hooker, which draws out the flame, supplies it with air, and cools the
sides of the cup of melted fuel.
I can give you here a little further illustration, for the purpose of
shewing you how flame goes up or down; according to the current.
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